You are halfway through an activation map. The tachycardia started with a cycle length of 280 ms. The reference looks stable, the coronary sinus activation sequence is consistent, and the map is beginning to explain the rhythm.

Then the TCL becomes 290 ms.

A little later, 300 ms.

Maybe it returns to 285 ms, or perhaps it remains around 310 ms.

At some point, the mapping system begins rejecting beats because they no longer satisfy the cycle length acceptance criteria.

Increasing the tolerance and continuing the acquisition may be appropriate, but that decision cannot be based only on the number displayed by the system. In some tachycardias, cycle length varies while the activation circuit remains essentially unchanged. In others, even a relatively small TCL change can accompany a meaningful change in activation pattern.

Published high-resolution mapping protocols commonly use relatively narrow cycle-length stability windows, together with reference and catheter-stability criteria. These settings are useful for controlling automatic acquisition, but they were not validated as biological boundaries defining whether two beats belong to the same tachycardia. [1,2]

So when the cycle changes during mapping, the relevant problem is not simply how many milliseconds changed.

We need to understand what changed with it.

Figure 1. The Cycle Length Dilemma.
EP Master original educational illustration. Conceptual representation, not patient-specific data.

What does a change in TCL actually mean?

In a sustained reentrant tachycardia, the cycle length reflects the time required for activation to travel through the functional circuit and return to tissue that is again capable of being activated.

If a tachycardia changes from 280 to 310 ms, the additional 30 ms must arise somewhere within that activation process.

Conduction may have slowed within the same circuit. A functional line of block may have changed. The wavefront may now be using a slightly different route around scar. The relative contribution of two loops may have changed. In another situation, the rhythm may actually have transitioned to a different tachycardia.

For that reason, when I see a TCL change during mapping, I would not begin by asking how much I can increase the acceptance window. I would first try to understand where that difference in time appeared and whether the rest of the activation pattern still supports the same mechanism.

There is a particularly useful example in the literature.

Su et al. described a postoperative scar-related right atrial tachycardia in which the cycle length alternated between approximately 280 and 320 ms. High-density mapping demonstrated that the reentrant pathway remained the same, while the difference in TCL was localized to changes in conduction through a slow-conduction zone. [3]

For the mapper, this case is useful because it shows why TCL variation should not be interpreted in isolation. The cycle length changed by approximately 40 ms, but the mapping demonstrated that the reentrant pathway was preserved. The difference was explained by a change in conduction behavior within a specific slow-conduction region.

That does not mean that a 40 ms variation should generally be accepted. It means that the magnitude of TCL variation cannot define tachycardia identity by itself.

Figure 2. Same Circuit, Different Cycle Length.
EP Master original educational illustration. Conceptual representation, not patient-specific data. For the original clinical maps and data, review Su et al. [3] in the references.

Is there an acceptable amount of TCL variation?

This is probably the practical question most mappers would like to answer first.

Can we accept 10 ms? Twenty? Thirty?

The literature does not provide a universal cutoff that separates “same tachycardia” from “different tachycardia.”

Published high-resolution mapping studies often use narrow beat-acceptance windows because automatic acquisition needs to avoid mixing clearly dissimilar cycles. The important point, however, is that these settings are acquisition criteria. They are not physiological definitions of tachycardia identity.

The 2019 APHRS expert consensus on 3D mapping systems describes contemporary platforms as using combinations of parameters rather than cycle length alone, including reference timing, catheter position, morphology or secondary-reference relationships. [2]

That architecture itself tells us something.

If TCL alone were enough to define rhythm identity, there would be little reason to combine it with several other stability parameters.

Commercial systems may also allow the operator to select a much wider TCL tolerance than what is commonly used in published mapping protocols. That flexibility is operational. It does not mean that every beat inside a broader range represents the same mechanism.

The mapper still has to decide what those beats represent.

Why the same 30 ms can mean different things?

An absolute difference also needs context.

A 30 ms variation represents 15% of a 200 ms tachycardia, 10% of a 300 ms tachycardia and only 6% of a 500 ms tachycardia.

Percentage alone does not solve the problem either.

Takigawa et al. studied 138 high-resolution atrial tachycardia maps and evaluated cycle-length behavior as a predictor of mechanism. The cohort included 8 focal ATs, 36 localized reentries and 94 macroreentrant ATs. Focal ATs showed greater CL variability than reentrant ATs. In that specific cohort, a CL variation below 24 ms favored a reentrant mechanism with high diagnostic performance, while beat-to-beat CL alternans was observed only in reentrant ATs. [4]

That result is valuable, but only if we use it for the question the study actually addressed.

The investigators were asking whether TCL characteristics could help predict AT mechanism and localization.

They were not testing whether 24 ms should be used as a beat-acceptance cutoff during LAT acquisition.

I would not transform their result into a rule saying that beats separated by less than 24 ms can safely coexist in the same activation map.

That conclusion is not supported by the study.

The activation pattern may be more informative than the amount of TCL change

Consider two situations.

In the first, the TCL varies from 280 to 305 ms. The CS activation sequence remains essentially unchanged, relative timing between important CS bipoles is preserved, the reference electrogram retains comparable morphology, key local EGMs remain in the same order and activation continues through the same anatomical regions.

That may be compatible with the same tachycardia undergoing variable conduction.

Now imagine another rhythm that changes from 280 to 288 ms. The numerical difference is small, but the earliest CS region changes, the relationship between reference and local EGMs changes and the major propagation pattern no longer follows the same route.

The second situation deserves more concern even though the TCL changed less.

Complex ATs after previous atrial interventions frequently contain multiple and interacting circuits. Derval et al. mapped 214 ATs in 132 patients. Multiple ATs were present in 58 patients, corresponding to 45% of the cohort, and dual-loop reentry was identified in 27 patients, approximately 20%. Sequential tachycardias were also common. [1]

This is why I would treat TCL as a stability marker rather than a complete rhythm fingerprint.

When it changes, the next step is to determine whether the activation pattern changed with it.

Figure 3. Assessing the Shift: Variable Conduction or Circuit Change?
EP Master original educational illustration. Conceptual representation, not patient-specific data.

Before changing the filter, look at the rhythm

When the mapping system begins rejecting beats, I would first look at four areas:

1. the reference

2. the global activation sequence

3. important local electrograms

4. the TCL trend over several seconds

The idea is not to delay the procedure by watching the tachycardia for several minutes. Usually, several seconds are enough to identify whether we are dealing with an isolated cycle, continuous variability, progressive drift, alternans or a sustained shift to another cycle length.

Each of these patterns leads to a different question.

The reference may be the hidden problem

Every LAT point is timed relative to a reference event.

If the reference event changes, the temporal position of points acquired against that reference can change as well.

This creates a different problem from incorrect local annotation.

If one local electrogram is annotated incorrectly, one point or one region becomes wrong.

If the temporal reference changes, a whole population of subsequently collected points may become temporally inconsistent with points acquired earlier.

For that reason, reference stability cannot be reduced to checking whether the catheter icon still appears in approximately the same place.

The coronary sinus can look stable while the reference electrogram changes

The coronary sinus is frequently used as a temporal reference during atrial mapping because it provides continuous multipolar atrial recordings and also gives us a useful view of the activation sequence.

But not every coronary sinus has the same anatomy, and catheter position inside the vessel is not always as mechanically stable as it appears.

This is something I see in daily mapping practice.

Imagine a relatively large CS.

At the beginning of the map, CS 3-4 sits close to one wall and records a large, sharp and reproducible electrogram. If our reference annotation uses a prominent peak, the signal is excellent.

Later, the catheter may move subtly within the vessel lumen without obviously migrating proximally or distally. Fluoroscopy or the 3D geometry may still suggest that the catheter remains in essentially the same anatomical region.

Its relationship with the wall, however, may have changed.

The bipolar electrogram can become smaller. A component that was previously dominant may lose amplitude while another becomes relatively more prominent. The reference signal can therefore become more difficult for automatic detection even without obvious catheter displacement.

I have not found a study specifically designed to quantify this scenario as a cause of LAT reference error in a large coronary sinus.

What the literature does clearly demonstrate is the underlying electrophysiological principle.

Bipolar electrograms are directionally sensitive. Their amplitude and morphology change according to electrode orientation, interelectrode spacing and the relationship between the bipolar pair and the propagating wavefront. Takigawa et al. demonstrated significant voltage and electrogram differences when the same site was evaluated using orthogonal bipolar orientations. [5]

Gaeta et al., using computational modeling together with 18,740 clinical left atrial electrograms, further characterized how wavefront orientation and electrode geometry influence bipolar amplitude. [6]

Those studies do not prove the specific “floating CS catheter” scenario. They support the electrophysiological mechanism that makes the observation plausible.

Reference stability has more than one component

For practical purposes, I think reference stability can be separated into three layers.

The first is spatial stability. Does the catheter appear to remain in the same anatomical region?

The second is electrogram stability. Does the reference EGM retain similar morphology and relative components?

The third is fiducial stability. Is the system still marking the same physiological component as the temporal reference?

These three conditions can diverge.

A catheter may remain in approximately the same anatomical location while the bipolar morphology changes. The EGM can also remain broadly recognizable while the relative amplitude of its components changes enough for the detection algorithm to choose another peak.

Imagine a reference electrogram with two atrial components separated by 12 ms.

At the beginning of the map, the second component is larger and is consistently selected by the chosen reference detection method.

Later, because the morphology changes, the first component becomes dominant.

If the detection rule now moves to that component, the reference timestamp changes by approximately 12 ms even though the tachycardia itself may not have changed.

Points collected after that shift are now being timed against a different temporal zero.

This is one reason why looking only at catheter position is insufficient.

Figure 4. Dissecting Reference Stability.
EP Master original educational illustration. Conceptual representation, not patient-specific data.

Figure 5. The Risk of Fiducial Shift.
EP Master original educational illustration. Conceptual representation, not patient-specific data.

What if the reference becomes poor halfway through a large map?

This becomes uncomfortable when several thousand points have already been acquired.

Imagine that CS 3-4 was an excellent reference at the beginning but now records a low-amplitude or inconsistent signal.

Before doing anything with the reference, I would first determine what changed.

Did the catheter move?

Did the bipolar morphology change without clear displacement?

Did the tachycardia itself change?

Is the annotation algorithm still marking the same component?

Can the original reference quality be restored?

The idea of simply moving the reference to another CS bipole should be treated carefully.

If CS 3-4 becomes unreliable, moving to CS 5-6 is not merely choosing a better version of the same reference. CS 5-6 records another anatomical location and therefore another activation time.

Even when the bipoles are close, we cannot assume that their temporal relationship remained perfectly constant throughout the entire map, especially if the catheter itself has changed position.

Changing the reference bipole therefore means changing the temporal reference.

If that becomes necessary, the mapper needs to understand exactly how the specific system handles previously acquired points before continuing the same LAT reconstruction.

Modern mapping platforms differ in how they store and retrospectively process electrogram data, and some permit reannotation or reconstruction from stored signals. The 3D mapping consensus describes several retrospective capabilities, but the exact behavior depends on platform and software generation. [2]

I would therefore avoid a universal instruction such as “change the reference and continue.”

The first goal should be to preserve temporal comparability between the points already collected and whatever comes next.

What happens when the WOI changes?

A change in TCL often creates another practical temptation.

The tachycardia was 280 ms and is now 320 ms. The current window no longer covers the activation you want to see, so you enlarge the WOI.

This may be necessary, but we need to understand what it actually changes.

The WOI defines the temporal region available to the system for local activation analysis. Depending on the platform and workflow, changing it may also allow previously recorded electrograms to be reprocessed or reannotated.

What it does not do is transform a point acquired during a 280 ms beat into a point acquired during a 320 ms beat.

The recorded electrophysiological event remains the original event.

Consider an EGM with two components:

component A at +120 ms

component B at +165 ms.

If the original WOI ends at +140 ms, component B cannot participate in annotation because it is outside the selected temporal region.

If the WOI is extended to +180 ms, both components are now available.

Depending on the annotation method, the system may choose a different component.

The LAT changes even though the original electrogram did not.

TCL, WOI and annotation therefore need to remain separate in our reasoning.

TCL describes the cycle of the rhythm.

WOI defines the temporal region in which the system can evaluate activation.

The annotation method determines which component inside that region is assigned as the local activation time.

Changing one can change the final map without necessarily changing either of the other two.

Figure 6. The Window of Interest Trap.
EP Master original educational illustration. Conceptual representation, not patient-specific data.

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The classic presentation is well known. A relatively young patient, often without structural heart disease, develops a ventricular tachycardia with right bundle branch block morphology and usually a superior or left axis. In the most common form, the arrhythmia is related to the left posterior fascicular system and is usually sensitive to verapamil.[1]

From a mapping perspective, however, the case can become more complex.

If we build an activation map using only the ventricular electrogram, the earliest ventricular activation is usually found in the inferoseptal region. That information is useful, but several electrophysiological studies have shown that activation of the Purkinje system may start earlier and extend through a much larger portion of the septum.[1–3]

This Deep Dive is not intended to show a perfect 3D map of fascicular VT. I do not have a clinical 3D case that allows me to reconstruct every part of this mechanism, and I also think that trying to create one ideal anatomical circuit could give more certainty than the available evidence allows.

The exact structure of the reentrant circuit is still being studied. Different groups have demonstrated different configurations involving P1, P2, ventricular myocardium, the posterior fascicle and, in some studies, more proximal parts of the left conduction system.[1,4,5]

For this reason, there will be fewer clinical images in this Deep Dive. I will use mainly simple conceptual schematics to organize the electrophysiology. When a specific circuit comes from a study, I will describe what the investigators found and refer to the original paper, where the recordings and anatomical models can be seen in their original context.

The purpose is more practical: to understand which signals may appear during the procedure, what they may represent, and what the mapper can look for when the tachycardia is present, when it becomes unstable, or when it is no longer inducible.

What you will find in this Deep Dive

• How classic reentrant fascicular VT is induced, and why atrial or ventricular stimulation can both access the circuit.

• How to recognize and interpret P1 and P2, including their timing, activation sequence and possible role in the reentrant mechanism.

• Why different studies proposed different circuit models, and why one anatomical drawing may not explain every patient.

• How catheter design, electrode spacing, high density mapping and annotation strategy can change what we are actually recording and mapping.

• How to approach the case when sustained VT is available, and what can still be mapped when the tachycardia becomes noninducible.

• Practical mapping concepts for the EP lab, including ventricular exit, Purkinje substrate during sinus rhythm, pacing maneuvers, possible ablation targets and procedural endpoints.

• Direct references to the original studies, with links to the papers so the reader can review the real electrograms, maps and authors’ proposed models.

Which fascicular VT are we talking about?

Fascicular ventricular arrhythmias are not one single entity.

The classic reentrant form can involve the left posterior fascicle, the left anterior fascicle or the upper septal region. Left posterior fascicular VT, or LPFVT, is by far the most frequently described form and is the main subject of this article.[1]

The usual ECG shows right bundle branch block morphology with left or superior axis deviation. The QRS can be relatively narrow for a ventricular tachycardia because ventricular activation reaches the Purkinje system early and uses part of the normal conduction network to activate the ventricle.

The mechanism is supported as reentry by several electrophysiological observations. The tachycardia can be induced and terminated by programmed stimulation, can be reset or entrained, and characteristic Purkinje related potentials can be recorded during the tachycardia.[1,2]

This article will therefore focus mainly on the classic verapamil sensitive reentrant LPFVT. Purkinje related PVCs, papillary muscle Purkinje arrhythmias, bundle branch reentry and other nonreentrant Purkinje VTs should not automatically be interpreted using the same model.

Why can a ventricular tachycardia be induced from the atrium?

Atrial induction has been part of the electrophysiological description of fascicular VT since the early reports of this arrhythmia.[6]

At first, it seems unusual. If the reentrant circuit is located in the ventricle, why can atrial pacing initiate it?

The explanation is related to how the premature impulse reaches the substrate.

An atrial stimulus can conduct normally through the AV node and His bundle and then enter the left ventricular conduction system. The atrium does not need to be part of the final reentrant circuit. It can simply provide a route for a premature wavefront to reach the Purkinje network under conditions that allow reentry to start.

Conceptually:

Figure 1. Conceptual pathway showing how an atrial stimulus may reach the left ventricular conduction system and initiate fascicular VT. EP Master original educational illustration. This is a simplified conceptual schematic created only to facilitate understanding and study. It does not represent exact cardiac anatomy, tissue structure, spatial relationships, conduction velocity, or a mechanism that must occur identically in every patient.

Once the tachycardia starts, the atrium may be completely dissociated from it.

Warren and colleagues provide a nice example. In their case, incremental atrial pacing during isoproterenol initiated a right bundle branch block, superior axis VT. During tachycardia there was VA dissociation, supporting that the atrium was not necessary to maintain the arrhythmia.[7]

Atrial pacing is only one way to induce the tachycardia. Ventricular stimulation is also frequently used.

Gopi and colleagues evaluated a systematic induction protocol in 68 patients with documented fascicular VT. Stimulation was performed from atrial and different ventricular sites, with ventricular extrastimuli and pharmacological provocation when necessary. Sustained FVT was induced in 64 patients, or 94%, and 18 of these required pharmacological provocation.[8]

For the mapper, this has a practical consequence. The clinical VT may appear easily at the beginning of the procedure, but it may become difficult to reproduce after catheter manipulation, pacing or the first RF applications. A mapping strategy based only on having sustained VT available can become limited very quickly.

Verapamil gave an early clue about the substrate

The relationship between this VT and verapamil became clear after the report from Belhassen and colleagues in 1981. They described a patient with recurrent sustained VT and no demonstrable structural heart disease in whom intravenous verapamil repeatedly terminated the tachycardia. During the EP study, verapamil also slowed the VT and prevented its reinitiation by pacing.[9]

Later electrophysiological studies helped connect this clinical observation with the Purkinje related substrate.

In the classical model, part of the circuit has decremental and verapamil sensitive conduction properties. This electrophysiological behavior became closely associated with the potential later called P1.[1,2]

The terminology needs some care. P1 should not be understood simply as “a slow potential”, and verapamil sensitivity does not mean that the complete Purkinje system depends on the same ionic mechanism.

What the studies show is that the specialized tissue associated with P1 behaves differently from normal fast Purkinje conduction. In the Nogami study, verapamil prolonged the tachycardia cycle length and intervals involving P1, while the relationship between P2 and ventricular activation was much less affected.[1,2]

That pharmacological behavior became another piece of evidence that P1 is related to a different component of the circuit.

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Coveney S, Cantwell C, Roney C. Atrial conduction velocity mapping: clinical tools, algorithms and approaches for understanding the arrhythmogenic substrate. Medical & Biological Engineering & Computing. 2022;60:2463–2478. doi:10.1007/s11517-022-02621-0.

About Paper to Practice

Paper to Practice is not a literature summary.

It is where I take one published paper and translate it into reasoning for the EP lab.

The idea is to understand the question behind the paper, the physiology behind the findings, what the mapper should actually look for and, especially, where we need to be careful before turning a result into a conclusion.

This edition is based on a review by Coveney, Cantwell and Roney about atrial conduction velocity mapping.

The paper is quite technical and goes deeply into different methods and algorithms used to estimate conduction velocity. But for me, the most interesting question is simpler:

When the system tells you that one region is slow, what exactly are you trusting?

In this Paper to Practice, you will find:

• What conduction velocity really represents

• Why the same tissue may show different conduction velocities

• How LAT, wavefront direction and geometry can change the result

• Where omnipolar mapping fits into this discussion

• What I would check before accepting a slow conduction zone as relevant substrate

Slow conduction sounds objective.

A wavefront moves between two regions, there is a distance between them and there is a difference in activation time. From there, we calculate a velocity.

At first sight, it looks simple.

Inside a real atrium, however, each part of that calculation can become more complicated.

We first need to know when local activation really happened at each point. We need a signal that actually represents local tissue. We need adequate contact. We need to understand whether the points belong to the same propagating wavefront and whether activation really traveled between those locations.

Direction also changes the interpretation. The wavefront may be traveling along the predominant myocardial fiber orientation or across it.

Geometry adds another layer. Two points can be close in three-dimensional space but separated by a ridge, scar or another structure that forces activation to take a longer pathway.

Even cycle length can change how quickly tissue conducts.

So the conduction velocity value that appears on the screen may look like a direct measurement of the tissue, but it is not exactly that.

It is an estimate built from several previous pieces of information.

The catheter records an electrogram, that signal is associated with a position. In many methods, a local activation time is assigned. The algorithm then uses spatial and temporal relationships between this point and other points to estimate how activation propagated through the region.

By the time we see the final number, several decisions have already happened.

That is where this paper becomes important for the mapper.

Figure 1. Conduction velocity may look like a simple relationship between distance and time, but the value displayed on a mapping system is a derived estimate built from electrical and spatial information. Depending on the method, signal acquisition, activation timing, geometry, wavefront behavior and algorithmic assumptions may all influence the final result.

EP Master original educational illustration. Conceptual representation created to simplify the explanation. It does not represent patient data and does not reproduce cardiac tissue, anatomy, or structures with anatomical or histological accuracy. Based on concepts discussed in Coveney, Cantwell and Roney, 2022.

What does conduction velocity actually represent?

Conduction velocity describes how fast and in which direction an electrical wavefront propagates through myocardium.

But atrial tissue does not conduct equally in every direction or under every condition.

The review discusses several structural and functional factors that can influence conduction, including fibrosis, changes in cell to cell coupling, alterations in sodium current and the direction of propagation relative to myocardial fiber orientation.

This last point deserves special attention.

Atrial myocardium is anisotropic. Electrical propagation depends on the direction in which the wavefront travels relative to the underlying tissue architecture.

Imagine mapping one atrial region during activation coming from one direction. Then we change the pacing site and activate the same tissue from another direction.

Nothing anatomical changed during those few minutes. The amount of fibrosis did not change and the tissue did not suddenly become healthier or more diseased.

But the measured conduction velocity may change because the direction of propagation changed.

This gives us an important concept:

The same tissue can show different conduction velocities when activated from different directions.

Once we understand this, saying that a region is simply “slow” becomes less straightforward.

We need to know how it was activated, from which direction, at which rate and under which mapping conditions.

Conduction velocity is therefore not only related to the substrate itself. It also reflects the interaction between that substrate and the wavefront traveling through it.

Why does this matter in the EP lab?

Slow conduction has an important relationship with reentry.

For a reentrant wavefront to continue circulating, previously activated tissue needs enough time to recover before the impulse reaches it again. Slow propagation can help create that time.

This is one reason why identifying areas of conduction slowing can be useful when studying arrhythmogenic substrate and potential reentrant pathways.

It also explains why conduction velocity mapping is attractive.

Voltage gives us information about the amplitude of the recorded signal and may help characterize abnormal substrate.

Activation mapping tells us when different regions activate.

Conduction velocity adds another type of information by trying to describe how the wavefront moves between those regions.

That can be very useful, but we need to remember that the catheter does not directly record conduction velocity.

It records electrical signals.

The velocity map appears only after those signals are processed and combined with timing, position, geometry and an algorithm.

This means that the CV map is already several steps away from the raw electrogram.

For the mapper, I do not think it is necessary to understand every mathematical equation behind these tools. But understanding the logic behind the calculation becomes important when the result starts influencing the interpretation of the substrate or the ablation strategy.

What was the question behind this paper?

Coveney and colleagues reviewed published methods used to calculate atrial conduction velocity and organized them into three broad categories:

Local methods

Global methods

Inverse methods

The authors then compared these approaches using some important concepts. They looked at whether the methods account for the physics of wavefront propagation, whether uncertainty can be incorporated into the calculation and whether the method respects the complex curved surface of the atrium.

At first, these sound like engineering questions.

But they translate quite naturally into the EP lab.

Does the result make physiological sense?

How confident should we be in the estimate?

And does the calculation respect the anatomy through which activation actually traveled?

This is why I think the review is useful even for a mapper who has no interest in programming or computational modeling.

The important part is not learning the equations.

It is understanding what assumptions can exist behind a number that sometimes looks much more certain than it really is.

One distinction is important before going further.

The categories used in this review describe published approaches for estimating conduction velocity. They should not be used as a direct classification of commercial mapping systems.

Clinical platforms may use proprietary algorithms, combine elements from different approaches or process the information in ways that are not fully described in public technical documentation.

So the objective here is not to say that one manufacturer is “local”, another is “global” and another is “inverse”.

The objective is to understand the different ways conduction velocity can be estimated and why the method used to reach the final number matters.

From this point on, the article enters the premium section. We will look at how LAT errors can become conduction velocity errors, why different ways of calculating CV can produce different interpretations, how anisotropy and wavefront collision affect the result, where omnipolar mapping helps and what I would check before accepting a slow conduction zone as meaningful.

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All of this sounds valuable. And I really do it often.

But there is an important question behind it:

Does a better map actually change the outcome?

That is what makes the study by Di Cori and colleagues interesting.

Not because it shows that high density mapping produces prettier maps.

We already know that higher spatial resolution can reveal electrophysiological details that a conventional mapping strategy may miss.

The more difficult question is whether that additional information changes what happens during ablation and, eventually, what happens to the patient.

I really like to read articles that test what we’ve been doing all day long. They try to validate (or not) that technology is helping us to get better results with less effort.

And this is one of those articles. So, let’s find out what they found.

What did the study actually ask?

Di Cori and colleagues retrospectively evaluated patients undergoing atypical atrial flutter ablation using two different mapping strategies.

One group was mapped using a conventional lower density approach with the ablation catheter.

The other was mapped using multipolar high density catheters.

Different electroanatomical systems were represented, including CARTO, EnSite Precision and Rhythmia HDx.

Here I have to say: the study was not comparing one proprietary mapping platform against another, and neither do I.

It was comparing two different ways of reconstructing an arrhythmia.

A lower density strategy and a high density strategy.

The authors included 87 patients treated over a six year period.

Most of the arrhythmias originated from the left atrium, and mitral dependent and roof dependent circuits represented a substantial proportion of the cases.

The question was:

Would the high density strategy change acute procedural success and long term arrhythmia recurrence?

What did they find?

Sinus rhythm was restored during ablation in approximately 79% of patients mapped with the high density strategy compared with 56% in the lower density group.

That difference occurred without a significant increase in total procedural time, mapping time or radiation exposure.

There was another finding that deserves attention.

Pacing maneuvers were performed in approximately 8% of the high density cases compared with 53% of the lower density cases.

And the long term results were even more provocative.

Overall recurrence of AF, atrial flutter or atrial tachycardia was:

14% versus 37% at one year

14% versus 48% at two years

14% versus 50% at three years

favoring the high density strategy.

In the multivariable analysis, high density mapping was associated with fewer arrhythmia recurrences during follow up.

Well, I think those were good numbers.

But this is where reading the result is not enough, we need to ask what those numbers actually mean.

Published observation

The study showed that patients treated with the high density mapping strategy had better acute outcomes and fewer documented atrial arrhythmia recurrences during follow up.

It also showed that complex multiple loop circuits were identified more frequently in the high density group and that additional pacing maneuvers were used much less often.

Those are observations from the study.

Now comes the harder part.

Does this prove that high density mapping caused the better outcome?

No. It’s not that simple.

And this matters. The study was retrospective.

The procedures occurred between 2014 and 2019.

The two groups therefore represent not only different mapping strategies, but also different procedural eras, mapping technology and ablation catheters.

Contact force technology became more reliable. Software improved.

Workflow changed.

And the mappers themselves accumulated experience.

The distribution of mapping systems was also different between the groups. CARTO predominated in the conventional group, while Rhythmia was used only in the high density group.

So we cannot reduce the study to:

More points caused fewer recurrences.

What they show is that a more contemporary strategy incorporating high density mapping was associated with better outcomes in this cohort.

That is still important. And make us get back where we begin. Technology seems to be helping us to get better results with less effort.

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So where might the benefit actually come from?

This is where I think the paper becomes useful for the mapper.

Imagine a complex scar related atrial flutter.

The real challenge is reconstructing how activation moves through diseased tissue.

Where are the true boundaries?

Where does conduction slow?

Are there multiple loops?

Which corridor is necessary for circuit maintenance?

A higher density map gives us more information to answer those questions.

But the number of points itself is not the mechanism.

The possible mechanism is:

better resolution leading to —> better circuit reconstruction leading to —> better target selection leading to —> more effective ablation.

That chain is biologically plausible. Of course, in safety ways and correct ablation targets.

And that is the difference for the mapper: published data and what are you going to do with it on your next map.

The reduction in pacing maneuvers

The difference was large.

Approximately 53% of the lower density cases used pacing maneuvers compared with only 8% of the high density cases.

One interpretation is that a sufficiently complete activation map allowed the operators to understand more circuits without repeatedly using entrainment.

That makes sense. But we should be careful with the next step.

This study does not demonstrate that high density mapping eliminates the need for entrainment.

If the activation map clearly reconstructs the tachycardia, defines the boundaries, covers the cycle length and identifies a convincing critical region, additional pacing may not add useful information.

But when uncertainty remains, physiology still matters.

And here I get a point: If you have been reading my posts and thoughts on Substack or LinkedIn a while, you know that I always come with the idea that you should know how to question your map or hypothesis.

And that can be the negative side of the technology, I have to say. You get 15 cases that everything goes really fine, just one circuit, no passive loops. But suddenly, that difficult case appears. And you have to question your map, but you forget how entrainment works. Because you don’t use it a lot.

And that is just one thing, we have a lot of problems that may appear:

Automatic annotation can still be wrong.

Far field activity can still be accepted as local activity.

A low voltage corridor can still be a bystander.

The question should never be:

Do I use the map or do I use entrainment?

The better question is:

What is still uncertain after the map?

Then use the tool that answers that uncertainty.
And that is what I’m trying to do with EP Master. Make us better mappers.

This is where high density mapping can be misunderstood

The easiest way to use high density technology is to collect more points.

That is not necessarily the best way to use it. The value is not point density alone.

The value is what those additional points allow you to understand that you could not understand before.

Can you distinguish a broad region of abnormal tissue from a constrained conducting corridor?

Can you recognize real isochronal crowding?

Can you find surviving low amplitude tissue inside an apparent scar?

Can you identify where the activation map stops making physiological sense?

Can you choose a more specific lesion set because the mechanism is clearer?

If the answer is yes, the map changed the decision.

And that is probably where its real value begins.

Video. Atypical atrial flutter activation mapping.

Anonymized clinical data. For educational purposes only.

This video shows an atypical atrial flutter activation map from the author’s own clinical archive. No information that could identify the patient is displayed.

The map represents a specific moment during the procedure and is presented only to illustrate mapping concepts. It should not be interpreted as a complete case report, final diagnosis, or procedural endpoint.

Video: Author’s own archive.

But the map still does not create the lesion

There is another important separation.

Mapping and ablation solve different problems.

Mapping tells us where we believe the circuit can be interrupted.

Ablation determines whether we actually create a durable electrical effect there.

You can identify the correct isthmus and still create an incomplete lesion. Or the isthmus may be in a place where you cannot safely ablate.

You can terminate the tachycardia and still fail to achieve the appropriate conduction endpoint.

So even if better mapping improves target selection, long term outcome still depends on what happens after the target has been identified.

Lesion formation matters. Contact matters. Stability matters.

Tissue thickness matters.

And when a line is created, confirmation of durable bidirectional block matters.

This is one reason why we should be careful when connecting better visualization directly to better long term outcomes.

There are several physiological steps between the map and the recurrence curve.
Here I have to say again: The patient safety is the priority. As mappers, we cannot decide alone where the safest ablation site is. But with the right tools, we can help evaluate if the endpoint was achieved.

What should the mapper take from this study?

Not that high density mapping is automatically superior because it collects more points.

And not that a modern activation map makes conventional EP reasoning unnecessary.

The practical lesson is more useful than that.

If you have access to high density mapping, use its resolution to reduce uncertainty.

Do not simply increase density everywhere.

Interrogate the regions that determine the mechanism.

Review annotation where conduction appears slow.

Look at the raw electrograms when the map creates an unexpected channel.

Challenge apparent scar when the activation sequence suggests that conduction must cross it.

Look for competing loops when the cycle cannot be explained by the first circuit.

And if the map still leaves a mechanistic question unanswered, test it.

The purpose of technology is not to remove thinking from mapping.

It is to give us better information to think with.

EP Master interpretation

To me, the strongest message from this study is not just:

High density mapping improves outcomes.

The study is not designed to establish that causal statement.

Of course, high-density mapping can make things easier and faster. If you have, use it.

The question is no longer:

How many points did you collect?

It becomes:

What did the additional information allow you to understand that changed the procedure?

That is the question I would bring back to the lab.

A better map is valuable when it leads to a better decision.

Remember, technology is here to help, but as a mapper, you have to know the tools to question when it comes out of the pattern.

Further reading

Di Cori A, Mazzocchetti L, Parollo M, Giannotti M, Canu A, Barletta V, della Volpe S, De Lucia R, Viani S, Segreti L, Soldati E, Zucchelli G, Bongiorni MG.

Clinical impact of high density mapping on the acute and long term outcome of atypical atrial flutter ablations.

Journal of Interventional Cardiac Electrophysiology. 2024;67:43–51.

doi:10.1007/s10840-022-01121-3

The original paper is worth reading for the complete methodology, procedural details, statistical analysis and the authors’ own interpretation of the findings.

EP Master

The goal here is not to replace the paper.

It is to take published evidence and ask what it actually changes when we are standing in front of the mapping system.

This is the kind of practical electrophysiology reasoning I am building inside EP Master.

And soon, I hope to organize much more of it in one place.

EP Master Hub. More to come.

Subscribe now

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Leave a comment

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The more we understand mechanisms, the better we can interpret signals, choose maneuvers, and support better decisions during the case.

We followed the initial ventricular wavefront.

We asked whether activation appeared to begin on the right or on the left.

We used the inferior leads to understand whether that wavefront was moving superiorly or inferiorly.

And we explored how the degree of fusion changes what the ECG allows us to see.

But the ECG only takes us so far.

It may suggest where ventricular activation begins.

It does not show the complete accessory pathway.

This becomes important when we enter the heart and find something that does not fit the simple model.

The ECG suggests one region.

The earliest ventricular activation is recorded somewhere nearby, but not exactly where expected.

The earliest retrograde atrial activation appears at a different annular position.

The first RF application produces only transient block.

Or the map displays a convincing focal breakout, but several applications at that site fail to eliminate pathway conduction.

At that moment, the easiest conclusion is that the map is incomplete.

Sometimes it is.

But another possibility is that the anatomical model is wrong.

Most accessory pathway diagrams teach us to imagine a small bridge crossing the atrioventricular junction at one precise point.

One atrial insertion.

One ventricular insertion.

One location around the annulus.

One target.

That model is useful when we are learning the basic mechanism.

It is not always how the pathway exists inside the heart.

A conventional atrioventricular accessory pathway may cross the atrioventricular groove obliquely.

Its atrial and ventricular insertions may occupy different annular positions.

It may have a broad or branched insertion rather than behaving as one focal connection.

The accessible breakout may arise from deeper or epicardial tissue.

The connection may involve coronary sinus musculature or a cardiac vein.

And the surface accessible to the catheter may show only one part of that connection.

This is the central idea of this article:

The map does not show the accessory pathway as a complete structure. It shows where activation produced by that pathway reaches the surface being mapped.

Understanding that changes how we interpret the earliest point.

It changes how we compare antegrade and retrograde maps.

It changes what we do when apparently adequate RF does not eliminate conduction.

And it changes the question the mapper should ask when the ECG, intracardiac signals, and anatomy do not seem to agree.

Most of this article focuses specifically on conventional atrioventricular accessory pathways, meaning muscular connections linking atrial and ventricular myocardium across the atrioventricular junction. The three component model developed below applies to that anatomy. It should not be extended indiscriminately to every form of preexcitation. Fasciculoventricular connections originate within the His Purkinje system and connect to ventricular myocardium, while nodoventricular and nodofascicular connections originate from the AV nodal region and therefore do not have an atrial insertion in the same sense. [3] Atriofascicular pathways retain an atrial insertion but connect distally to the right ventricular conduction system and will be used later in this article as an extension of the broader three dimensional concept. [2]

In this article, we will explore:

• Why a conventional AV accessory pathway should be understood as a connection rather than a point

• The difference between atrial insertion, pathway course, and ventricular insertion

• Why antegrade and retrograde mapping may identify different locations

• How oblique, broad, branched, deeper, and epicardial connections can change the map

• Why coronary sinus musculature can become part of the connection

• What failed ablation can and cannot teach us about anatomy

• How the mapper can decide which part of the pathway is actually being mapped

The point model is useful until it becomes the problem

When we describe a pathway as left lateral, right free wall, inferior paraseptal, or anterosuperior paraseptal, we are simplifying a three dimensional structure into one regional label.

That simplification is necessary, it helps us communicate, organize the ECG, decide where to begin mapping.

But the label can gradually become a mental picture.

We start imagining the pathway as a vertical line crossing a flat annulus.

The atrium is directly above; ventricle is directly below.

The point of earliest atrial activation, the point of earliest ventricular activation, and the ablation target should all align.

When they do not, we often assume that the catheter is unstable, the reference is wrong, or the map lacks points.

Well, those are real possibilities.

But the anatomy it self may explain the disagreement.

The atrioventricular junction forms an insulating plane between atrial and ventricular myocardium. Conventional atrioventricular accessory pathways are muscular connections that cross this plane and restore electrical continuity between the two chambers. They can occur along the extensive right and left parietal junctions or within the much smaller true septal component. Some connections also involve myocardial tissue associated with coronary sinus abnormalities or coronary veins. [1]

The pathway is therefore not defined only by where it crosses an annular line.

It has a beginning, a course and it has an end.

FIGURE 1. FROM A DOT TO A CONNECTION

A conventional atrioventricular accessory pathway is commonly represented as a point around the annulus. In reality, it is a three dimensional muscular connection with an atrial insertion, a course across or around the atrioventricular junction, and a ventricular insertion.

A conventional atrioventricular pathway has at least three spatial components

To map a conventional atrioventricular accessory pathway correctly, we need to separate three concepts.

The atrial insertion

This is where pathway tissue connects with atrial myocardium.

During retrograde conduction, activation leaving the pathway enters the atrium through this region.

The earliest retrograde atrial electrogram may therefore help identify the atrial insertion or the first atrial breakout produced by it.

But the earliest atrial breakout is not automatically the ventricular insertion.

And it may not always represent the pathway body itself.

The pathway course

This is the connection between the atrial and ventricular insertions.

The pathway may be short and relatively perpendicular to the atrioventricular groove.

It may also travel obliquely along the groove before reaching the other chamber.

It may remain close to the annulus.

It may extend away from it.

And the tissue carrying pathway conduction may be closer to one mapping surface than another.

This course is usually not displayed directly by a conventional activation map.

The ventricular insertion

This is where activation leaves the pathway and enters ventricular myocardium.

During manifest preexcitation, this insertion strongly influences the first ventricular wavefront and therefore the delta wave.

During antegrade mapping, the earliest local ventricular activation may help identify this region.

But even that earliest ventricular breakout is not necessarily the only location from which pathway conduction can be interrupted.

The ECG, atrial activation map, and ventricular activation map may therefore describe three related but different parts of the same connection.

The ECG suggests the ventricular breakout.
Retrograde mapping may emphasize the atrial insertion.
The successful lesion may interrupt the connection from an accessible part of its course.

The ECG sees the breakout, not the complete pathway

This was the central limit established in the previous article.

The delta wave reflects the direction of the earliest ventricular activation produced after the impulse reaches the ventricular insertion.

It does not show where the pathway begins in the atrium and it does not show how long the pathway is.

And it does not show whether the pathway reaches the ventricle directly beneath the atrial insertion or after traveling along the atrioventricular groove.

This explains why an ECG may correctly predict a left posterior ventricular insertion while the earliest retrograde atrial activation is recorded at a more lateral or more paraseptal position.

Both observations may be correct.

They may be showing opposite ends of an oblique pathway.

The mistake would be forcing them to represent the same point.

Oblique pathways are not unusual exceptions

The idea of an oblique accessory pathway is sometimes discussed as though it were a rare anatomical curiosity.

Electrophysiological studies suggest that at least some degree of obliquity can be demonstrated frequently when the pathway is approached from opposite wavefront directions.

Otomo and colleagues studied 114 patients with a single AV accessory pathway. Reversing the direction of ventricular pacing increased the local VA interval by at least 15 ms in 91% of evaluable patients, while reversing the atrial wavefront increased the local AV interval by at least 15 ms in 73% of the subgroup in whom antegrade evaluation was performed. The authors interpreted these changes as evidence that an oblique course was present in most pathways studied with this protocol. Mapping during the wavefront direction that produced the longer local interval also exposed an accessory pathway potential in 102 of 114 patients. [4]

Why would the pacing direction matter?

Imagine an oblique pathway running along the annulus.

If the paced wavefront approaches the ventricular insertion from one side, ventricular activation may arrive at the local recording site before the pathway mediated atrial signal becomes clearly separated.

The local ventricular and atrial components overlap.

The pathway potential may disappear inside the continuous electrogram.

Reverse the pacing direction, and the timing relationship changes.

The ventricular wavefront reaches the insertion from the opposite side.

The interval between local ventricular activation and pathway activation becomes longer.

A discrete pathway potential may now become visible.

The pathway did not move.

The direction from which activation reached it changed what the catheter was able to record.

This has a direct practical consequence:

An apparently continuous atrioventricular electrogram does not always mean that the catheter is positioned directly over a perpendicular pathway. It may reflect wavefront direction and signal overlap along an oblique connection.

FIGURE 2. ONE PATHWAY, TWO DIFFERENT MAPS

An oblique conventional AV pathway can produce different earliest activation sites during antegrade and retrograde conduction. The two maps may identify opposite ends of the same connection rather than contradictory locations.

Earliest atrial activation does not necessarily identify the ventricular insertion

This distinction becomes especially important during mapping of concealed pathways.

A concealed pathway does not conduct anterogradely.

There is no delta wave and no antegrade ventricular insertion map during sinus rhythm.

The mapper often relies on retrograde conduction during ventricular pacing or orthodromic AVRT.

The earliest atrial activation identifies where the retrograde wavefront enters the atrium.

But an oblique pathway may connect to the ventricle at one location, travel along the groove, and reach the atrium somewhere else.

Yamabe and colleagues studied 39 patients with concealed left sided accessory pathways using ventricular pace mapping along the mitral annulus. The ventricular insertion, defined by the shortest stimulus to atrial interval, did not coincide with the site of earliest retrograde atrial activation in 19 of 39 patients. The authors interpreted that separation as evidence of an oblique course. [5]

For the mapper, the practical question becomes:

Am I mapping the place where the atrium becomes activated, or the place where the pathway connects to the ventricle?

Both can be meaningful targets.

But they are not necessarily the same target.

One connection can create several apparent focal points

Activation maps are designed to display breakout.

Where did activation first reach the surface?

But breakout is not always equivalent to the entire anatomical insertion.

Kipp and colleagues reported three concealed pathways in which electrophysiological findings supported a single ventricular insertion with two discrete atrial insertion sites. [6] Li and colleagues subsequently described 10 patients with an unusual right free wall pathway phenotype characterized by multiple separate atrial breakout sites away from the tricuspid annulus. These were selected case series demonstrating that this anatomy can occur, not studies establishing how frequently it occurs across accessory pathways in general. [7]

In the Li series, ablation of the first earliest atrial breakout did not eliminate pathway conduction. Stepwise remapping exposed additional insertions, and conduction was eliminated only after the relevant sites were addressed. [7]

The published observation is that anatomically complex pathways can generate more than one discrete atrial breakout.

The anatomical interpretation depends on the individual case.

And the EP Master practical synthesis is:

A focal activation map can be produced by a structure that is not anatomically focal.

That statement should not be treated as a universal property of focal maps.

It is a reason to question the anatomical model when the behavior of the pathway no longer fits a single point.

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Not a map in the same sense as an electroanatomical mapping system.

The ECG will not reconstruct the atrioventricular junction. It will not draw the pathway, separate its atrial and ventricular insertions, or identify the best site for ablation.

But before the first catheter enters the heart, the ECG may already suggest where ventricular activation begins and in which direction the first wavefront travels.

Today’s subject is dense.

There is no honest way to explain vectors and ventricular preexcitation without asking the reader to think in three dimensions. The difficulty is not memorizing that a delta wave is positive or negative. It is imagining activation moving through the heart and understanding how that movement becomes a deflection on the body surface.

But dense does not need to mean confusing.

We will reduce the problem to a few physiological questions and use the figures to carry part of the explanation. This is one of those subjects that becomes much easier when it is seen, not only described.

I have heard this many times:

“ECG localization is the physician’s job. The mapper only needs to map.”

I disagree.

Not because the mapper should replace the electrophysiologist or decide the procedural strategy alone.

But because mapping is not a neutral process.

The first region we investigate, the rhythm we choose to map, the pacing condition we use, the reference we select, the electrogram we accept as relevant, and the way we react when the map does not fit the expected anatomy all depend on an initial hypothesis.

Without that hypothesis, the mapper may still collect points.

But collecting points is not the same as building a meaningful map.

This understanding is not limited to accessory pathways.

Learning how electrical activation becomes an ECG pattern changes the way a mapper approaches ventricular ectopy, ventricular tachycardia, atrial tachycardia, flutter, preexcitation, and any arrhythmia in which the direction of activation matters.

The ECG should not tell us where to ablate.

It should help us decide where to begin, what to expect, and whether the map is telling a physiologically coherent story.

ECG suggests. Mapping confirms.

In this article, we will explore:

• What the delta wave actually represents

• How to read the direction of initial ventricular activation

• How an ECG pattern becomes an anatomical hypothesis

• Why the same pathway may produce different morphologies

• How ECG interpretation changes the mapper’s first decisions

Why should a mapper understand the ECG?

Understanding activation direction is not defined by a job title.

It is a technical skill.

A mapper who understands the ECG is not simply trying to predict an ablation point before entering the heart. The purpose is broader.

The ECG gives the mapper an independent source of information against which the map can be tested.

A probable left sided accessory pathway may change the access that needs to be prepared.

A probable paraseptal pathway immediately raises the importance of the His bundle, AV node, coronary sinus ostium, and ablation risk.

A posterior pattern may increase the relevance of the coronary sinus and cardiac venous system.

A minimally preexcited ECG may suggest that sinus rhythm is not providing enough pathway mediated ventricular activation for reliable antegrade mapping.

The same reasoning appears in other procedures.

When mapping a premature ventricular complex, the ECG provides a spatial hypothesis before the catheter reaches the ventricle.

When mapping ventricular tachycardia, the QRS morphology may suggest the exit and the direction of ventricular activation, even when the critical substrate is located somewhere else.

When mapping an atrial tachycardia, the P wave can support or challenge the chamber and region suggested by the activation map.

During flutter, the surface morphology does not define the circuit, but it remains another piece of evidence that should be compatible with the atrial activation sequence.

Understanding the ECG pushes the mapper outside the comfort zone of the mapping system.

The map is no longer interpreted only through colors, timing values, or the earliest displayed point. The mapper begins to compare different expressions of the same activation process:

Where did activation begin?

Where did it move?

What did the body surface record?

What did the intracardiac catheter record?

Does the anatomy support both?

Imagine that the surface ECG strongly favors a left posterior ventricular insertion, but the left endocardial map shows only modestly early ventricular electrograms.

The correct response is not automatically to collect more points.

The mapper should ask:

Is the ventricle sufficiently preexcited?

Am I comparing the local ventricular electrogram with the true onset of the delta wave?

Am I mapping the appropriate surface?

Could the atrial and ventricular insertions be separated by an oblique pathway?

Could the connection involve coronary sinus musculature or another epicardial structure?

Could more than one pathway be present?

The ECG becomes an external test of the map’s coherence.

It does not prove the target.

It tells us whether the explanation we are building is plausible.

A mapper who understands the ECG does not simply search for an earlier point. That mapper builds maps with more direction, coherence, and purpose.

FIGURE 1. THE ECG BEFORE THE MAP

What is an ECG lead actually recording?

An ECG lead does not directly see an anatomical wall.

It records how the electrical activity generated by the heart projects onto a specific axis.

When activation moves toward the positive pole of a lead, the deflection tends to become positive.

When activation moves away from the positive pole, the deflection tends to become negative.

When activation moves approximately perpendicular to that axis, the deflection may be small, biphasic, or nearly isoelectric.

Imagine an activation wavefront moving directly toward an observer.

That observer records a positive deflection.

Now rotate the wavefront so that it moves away.

The same observer records a negative deflection.

The observer did not change.

The direction of activation did.

This is how we should begin thinking about the delta wave.

A negative delta wave does not mean that the pathway is located beneath that lead.

It means that the earliest ventricular wavefront is moving away from the positive pole of that lead.

A positive delta wave means that the wavefront has a component moving toward it.

The ECG does not show the source directly.

It shows the direction of activation produced by that source.

FIGURE 2. TOWARD, AWAY, AND PERPENDICULAR

The polarity of the delta wave reflects the direction of the first ventricular wavefront relative to each lead. The lead records movement, not anatomy itself.

The heart cannot be read as a flat diagram

Accessory pathways are commonly represented around two flat circles.

One represents the tricuspid annulus.

The other represents the mitral annulus.

These diagrams are useful for communication, but the heart does not sit inside the chest like a map opened on a table.

The atrioventricular junctions are oblique, three dimensional structures. The mitral and tricuspid junctions do not occupy one flat plane, and regions that appear widely separated on an opened annular diagram may be closely related when approached from different surfaces.

The inferior paraseptal region is surrounded by the right and left atrioventricular junctions, the coronary sinus ostium, the proximal coronary sinus, the middle cardiac vein, and the central fibrous structures.

The true septal component of the atrioventricular junction is limited. The traditional nomenclature was partly created from surgically distorted views that placed the valvar rings and the triangle of Koch into a single plane.

This three dimensional orientation explains why one lead cannot provide a complete location.

The precordial leads help us examine activation across the horizontal plane.

The limb leads help us examine activation in the frontal plane.

Only after combining those projections can we begin to build an anatomical hypothesis.

We do not need to calculate the electrical axis in exact degrees every time.

We need to understand direction.

Did activation probably begin on the right or on the left?

Is the first wavefront moving superiorly or inferiorly?

Does the pattern look strongly lateralized or closer to a paraseptal region?

These questions are more useful than memorizing a long list of morphologies.

FIGURE 3. THE HEART IS NOT FLAT

Flat annular diagrams simplify communication, but accessory pathways exist in three dimensional anatomy. The ECG provides the first directional hypothesis. The intracardiac map must test that hypothesis on the surfaces that can actually be reached.

The delta wave is the beginning of the story

The delta wave is often described as slow conduction through the accessory pathway.

That explanation is incomplete.

A conventional atrioventricular accessory pathway may conduct rapidly and with little decrement compared with the AV node.

The initial slurring of the QRS appears after the impulse leaves the pathway and begins spreading through working ventricular myocardium.

This local myocardial activation is slower and less efficiently distributed than activation through the Purkinje system.

The sequence begins when the atrial impulse reaches the accessory pathway.

The pathway crosses the atrioventricular junction and activates ventricular myocardium near its ventricular insertion.

That wavefront begins spreading through local myocardium.

Meanwhile, another impulse travels through the AV node, His bundle, bundle branches, and Purkinje system.

The two ventricular wavefronts eventually meet.

The complete preexcited QRS is therefore not generated by the accessory pathway alone.

It is a fusion between early myocardial activation through the pathway and normal ventricular activation through the His Purkinje system. The morphology depends both on pathway location and on the degree of fusion between these two routes.

This is why the first milliseconds are so important.

The beginning of the QRS is more closely related to the ventricular insertion of the pathway.

Later portions increasingly reflect the interaction between both activation fronts.

A lead may show a small negative delta wave followed by a large positive R wave.

The first ventricular wavefront moved away from that lead.

Later activation moved toward it.

Reading only the global polarity of the QRS could hide the information contained in its beginning.

The delta wave does not show the entire pathway. It shows how ventricular activation begins after the impulse exits the pathway.

FIGURE 4. THE DELTA WAVE AND FUSION

The preexcited QRS is a dynamic fusion complex. The initial deflection reflects early myocardial activation through the accessory pathway. Later portions increasingly reflect normal His Purkinje activation.

Before trying to localize the pathway, ask one question

Is the ECG actually showing enough of it?

A delta wave may be visible, but that does not mean that the entire QRS carries the same amount of pathway specific information.

When preexcitation is minimal, only a small amount of ventricular myocardium has been activated through the pathway before the normal conduction system arrives.

The delta wave may be subtle.

The remaining QRS may be dominated by normal ventricular activation.

When preexcitation increases, the pathway activates a larger portion of the ventricle before the His Purkinje wavefront arrives.

Its spatial signature becomes easier to recognize.

This changes how much confidence we should place in the ECG.

Before attempting precise localization, confirm that the beginning of ventricular activation can be identified, that preexcitation is sufficient, that the morphology is stable, and that lead placement and baseline conduction are not distorting the pattern.

The question is not simply:

Where does this ECG say the pathway is?

The better question is:

How much of this ECG truly belongs to activation through the pathway?

Once that is clear, we can begin translating the surface pattern into anatomy.

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“Perimitral.”

“Roof-dependent.”

“Scar-related.”

Those names may describe the broad path of the re-entrant wavefront.

But they do not necessarily tell you where the circuit is most vulnerable.

And that distinction is the real reason this paper matters.

In the era of high-density mapping, the goal is no longer simply to recognize that activation travels around the mitral annulus, the pulmonary veins, a surgical scar, or a previous ablation line.

The harder question is:

Where is the tissue without which this circuit cannot continue?

That is the critical isthmus.

And it may not be located where the name of the flutter makes you expect.

About Paper to Practice Series

Paper to Practice is not a literature summary.

It is where I take one published study and translate it into EP lab reasoning: what question the authors were trying to answer, what the results really mean, what should change in map interpretation, and what the mapper should do differently during the next procedure.

This edition is based on Atypical Atrial Flutter Catheter Ablation in the Era of High-Density Mapping, by Raymond-Paquin and colleagues, published in the Journal of Interventional Cardiac Electrophysiology in 2023.

The study evaluated a contemporary strategy for atypical atrial flutter ablation based primarily on high-density activation mapping, with entrainment reserved for cases in which the circuit or critical isthmus remained uncertain.

In this Paper to Practice, you’ll find:

• Why an anatomical flutter label does not automatically identify the best ablation target

• Why mapping the entire circuit matters even when the flutter appears conventional

• What distinguished conventional from non-conventional atypical flutter circuits in this study

• Why shorter critical isthmuses were more frequently associated with slow conduction

• How to identify a likely critical isthmus using activation, isochrones, electrograms, voltage, and anatomical boundaries

• When high-density mapping may be enough — and when entrainment should still be used

• What the study demonstrated, what it only suggested, and what should not be overinterpreted

• A practical framework for complex atypical flutter in the EP lab

The clinical problem

Atypical atrial flutter is a macroreentrant tachycardia that is independent of the cavotricuspid isthmus.

It frequently occurs in atria modified by previous AF ablation, cardiac surgery, atriotomy, fibrosis, or linear lesions.

Those interventions create anatomical and functional barriers.

But they may also leave gaps, channels, and slow-conduction corridors capable of sustaining re-entry.

The resulting chamber may contain:

• previous lines of block;

• incomplete lesion sets;

• areas of dense scar;

• viable low-amplitude tissue;

• multiple possible channels;

• anatomical obstacles;

• functional block;

• and more than one visually plausible circuit.

That is why these cases are difficult.

Not because the chamber lacks activation.

But because the chamber may offer too many possible explanations.

Historically, the response was to combine activation mapping with multiple entrainment maneuvers.

But entrainment has limitations.

Pacing may terminate the flutter, induce a different tachycardia, degenerate the rhythm into AF, produce rate-related conduction slowing, or fail to capture scarred tissue.

Modern high-density mapping offers a different possibility:

map the entire circuit with enough resolution to identify the mechanism and critical isthmus before repeatedly perturbing the tachycardia.

The question is whether that strategy is reliable enough to guide ablation while using entrainment only when it adds decisive information.

If you want to understand more about typical / atypical flutter, mechanisms and a deep dive about reentrant tachycardia, EP Master has a mini series about it.

Start here:

The question behind the paper

The authors were essentially asking two questions.

First:

Can atypical flutter be mapped and ablated successfully using a contemporary high-density strategy while minimizing entrainment?

Second:

Do non-conventional flutter circuits have electrophysiological characteristics that distinguish them from familiar mitral-isthmus and roof-dependent circuits?

These questions are connected.

Once high-density mapping allows us to move beyond standard anatomical labels, we begin to see that many circuits do not follow the routes we historically expected.

And when the circuit is different, the critical isthmus may also be different.

What the study actually did

The study included 62 consecutive patients undergoing atypical flutter ablation.

Eighty-five percent had previously undergone AF or flutter ablation, and 44% had prior cardiac surgery.

A total of 95 macroreentrant atypical flutter circuits were mapped.

The authors classified the circuits as:

Conventional circuits

• Clockwise mitral-isthmus dependent

• Counterclockwise mitral-isthmus dependent

• Left atrial roof-dependent

Non-conventional circuits

• Left atrial anterior wall

• Posterior wall

• Septal wall

• Left atrial floor

• Left atrial ridge or appendage

• Right atrial scar-related circuits

• Coronary sinus ostium

• Superior vena cava

• Biatrial circuits

• Other right atrial loops

This classification was based mainly on the location of the critical isthmus.

The mapping workflow included:

• analysis of coronary sinus activation to help decide which atrium should be mapped first;

• high-density LAT acquisition;

• a window of interest covering 95% of the TCL;

• local activation annotation using maximum dV/dt;

• isochronal and propagation maps;

• analysis of anatomical and functional barriers;

• voltage mapping;

• and an effort to reconstruct the complete macroreentrant circuit.

Slow-conduction zones were defined by a combination of:

• isochronal crowding;

• low-amplitude bipolar electrograms;

• fractionation;

• and prolonged electrogram duration.

The voltage threshold could be lowered to 0.05 mV when necessary to identify viable tissue inside regions initially displayed as scar.

Entrainment was not prohibited.

It was reserved for cases in which high-density mapping could not clearly define the circuit or the critical isthmus.

Ablation targeted the narrowest and/or slowest portion of the circuit.

When a linear lesion set was required, lesions were created between nonexcitable barriers, and bidirectional block was subsequently confirmed.

That last detail is critical.

The authors did not simply identify a loop and draw a conventional anatomical line.

They tried to identify the most vulnerable part of the actual circuit.

Figure 1. The name of the flutter describes the broad circuit, but it does not necessarily identify its most vulnerable point. High-density mapping should help define the narrowest or slowest tissue required for circuit maintenance.

Source: EP Master original illustration, based on the concepts reported by Raymond-Paquin et al.

What the study found

Of the 95 mapped flutters:

• 44, or 46%, were classified as conventional;

• 51, or 54%, were classified as non-conventional.

More than half of the circuits therefore did not fit the familiar mitral-isthmus or roof-dependent categories used in the study.

Acute ablation success was 97%.

Success was similar between conventional and non-conventional circuits:

• 98% for conventional circuits;

• 96% for non-conventional circuits.

But the critical isthmuses were not electrophysiologically identical.

Compared with conventional circuits, non-conventional circuits had:

• shorter critical isthmuses: 10.8 ± 6.3 mm versus 19.0 ± 9.0 mm;

• more frequent slow conduction at the critical isthmus: 86% versus 59%;

• fractionated activity spanning a larger proportion of the TCL: 55% versus 42%;

• and less RF time to flutter termination: 51 ± 66 seconds versus 117 ± 119 seconds.

Entrainment was attempted in only 19 of the 95 flutters.

Its use declined over time from 33% in the first half of the cohort to 6% in the second half, while acute procedural success remained high.

At follow-up, freedom from recurrent atypical flutter was 84%, while freedom from any atrial arrhythmia was 65% over a mean follow-up of approximately 14 months.

These numbers matter.

But the most useful lesson is not simply that acute success was high.

It is that many non-conventional circuits contained critical isthmuses that were shorter, slower, and less anatomically predictable.

Figure 2. Conventional and non-conventional atypical flutter circuits had similar acute ablation success, but their critical isthmuses showed different electrophysiological characteristics. In this cohort, non-conventional circuits had shorter critical isthmuses, a higher prevalence of slow conduction, fractionated activity spanning a greater proportion of the TCL, and less RF time to termination.
Source: EP Master original table based on data from Raymond-Paquin A, Pillai A, Myadam R, et al. Atypical atrial flutter catheter ablation in the era of high-density mapping. J Interv Card Electrophysiol. 2023;66:1807–1815.

From this point on, the article enters the premium section: the mechanism behind the findings, how to identify the true critical isthmus, what the mapper should look for in activation, voltage, isochrones, and electrograms, when high-density mapping may be enough, when entrainment still adds decisive information, the main pitfalls, and a practical step-by-step framework for complex atypical flutter.

A quick note for free readers: If the paid upgrade option does not appear properly in your Substack app, try opening this publication in your browser and upgrading there instead. Depending on your region, platform, or app behavior, the upgrade flow may appear differently. The browser remains a useful workaround when needed.

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The key idea was that AVNRT is not just “short VA tachycardia.”

It is a nodal/perinodal reentry that exists because of AV nodal physiology: decremental conduction, dual AV nodal inputs, refractoriness, and heterogeneous recovery.

In the second article, we explored AVRT.

The key idea was that AVRT is not just “SVT with an accessory pathway.”

It is an atrioventricular reentrant circuit in which the accessory pathway and the normal conduction system become opposite limbs of the loop.

Now we arrive at the next step.

How do we prove the mechanism?

Because in the EP lab, naming the tachycardia is not enough.

A regular narrow QRS tachycardia can be AVNRT.

It can be orthodromic AVRT.

It can be atrial tachycardia with 1:1 AV conduction.

Sometimes the ECG helps.

Sometimes the VA interval helps.

Sometimes the activation sequence helps.

But none of these should be interpreted alone.

The real work begins when we ask:

what does each maneuver prove?

That is the purpose of this article.

Not to memorize tricks.

Not to repeat criteria automatically.

But to understand the physiology behind the maneuvers used to differentiate AVNRT, AVRT, and atrial tachycardia.

In this article, we are going to explore:

• Why SVT diagnosis is a mechanism problem, not a label problem.

• Why baseline observations come before maneuvers.

• What the VA interval can and cannot prove.

• Why atrial activation sequence helps, but can mislead.

• What spontaneous AV block during tachycardia can teach.

• Why His-refractory PVC is so powerful in suspected AVRT.

• How ventricular overdrive pacing helps separate AV node-dependent reentry from atrial tachycardia.

• Why V-A-V and V-A-A-V responses matter.

• What PPI-TCL and SA-VA are actually trying to measure.

• Why termination pattern matters.

• How adenosine can help, but also mislead.

• Why no maneuver should be interpreted out of context.

• How to build a practical framework for the mapper.

Image 1.
Column 1: AVNRT — nodal/perinodal circuit.
Column 2: AVRT — AV node + ventricle + accessory pathway circuit.
Column 3: Atrial tachycardia — atrium owns the rhythm, AV node only conducts.
SVT maneuvers are not isolated tricks. They are tests of circuit participation.

Before going into the maneuvers, start with the mechanisms.

This article is the third part of the SVT series.

If you have not read the previous two articles yet, I recommend starting there before continuing.

The AVNRT article explains the nodal/perinodal circuit, dual AV nodal physiology, decremental conduction, refractory periods, and why AV nodal dependence matters.

The AVRT article explains accessory pathway participation, manifest and concealed pathways, orthodromic versus antidromic AVRT, retrograde atrial activation, and why ventricular maneuvers become so important.

The maneuvers discussed in this article are not isolated tricks.

They are tests built on those mechanisms.

Start here:

Then continue here:

The first mistake: starting with the maneuver

Many people learn SVT maneuvers as isolated facts.

His-refractory PVC advances the atrium: AVRT.

V-A-V response: AV node-dependent reentry.

V-A-A-V response: atrial tachycardia.

Long PPI-TCL: not AVRT.

Short SA-VA: AVRT.

These statements are useful.

But if they are memorized without mechanism, they become forgettable.

Because every maneuver has conditions.

Timing matters.

Capture matters.

His refractoriness matters.

Fusion matters.

The pacing site matters.

The tachycardia cycle length matters.

The atrial activation sequence matters.

The response after pacing matters.

A maneuver is only useful if we know what question it is answering.

So before asking:

“Which maneuver should I do?”

The better question is:

“What am I trying to prove?”

Are we trying to prove that the ventricle is part of the circuit?

Are we trying to prove that an accessory pathway connects ventricle to atrium?

Are we trying to prove that the atrium is driving independently?

Are we trying to prove that the AV node is part of the mechanism and not just conducting the rhythm?

These are different questions.

And different questions require different tests.

IMAGE 2 — MANEUVERS ARE QUESTIONS
A maneuver should not be performed as a ritual. It should answer a specific mechanism question.

Before maneuvers: read the tachycardia

Before any pacing maneuver, the mapper should read the tachycardia itself.

The first map is not a 3D map.

The first map is the signal map in your head.

Ask:

Is the QRS narrow or wide?

Is the tachycardia regular?

Is the AV relationship 1:1?

Is the VA interval short or long?

Where is the earliest atrial activation?

Is the atrial activation sequence concentric or eccentric?

Does the atrial activation sequence stay stable?

Is there spontaneous VA or AV block?

Does the tachycardia start with A, V, or both?

Does it terminate with A or V?

Are there changes in cycle length before termination?

Are there signs of pre-excitation in baseline rhythm?

These observations do not prove everything.

But they build the hypothesis.

A good maneuver is not performed in a vacuum.

It tests a hypothesis that was already built from the ECG and intracardiac signals.

IMAGE 3 — BASELINE SIGNAL CHECKLIST
Before pacing, read the tachycardia. The maneuver should test the hypothesis built from the signals.

The VA interval: useful, but not enough

The VA interval is one of the first things we look at during SVT.

A very short VA interval supports typical AVNRT.

A longer VA interval raises the possibility of AVRT, atypical AVNRT, or atrial tachycardia.

But the VA interval alone should not close the case.

Typical AVNRT often has near-simultaneous atrial and ventricular activation.

The atrial electrogram may be buried in the QRS or appear just after it.

Orthodromic AVRT often has a longer VA interval because the impulse must travel to the ventricle and return to the atrium through the accessory pathway.

Atrial tachycardia may also show a long RP or a variable VA relationship depending on AV conduction.

Atypical AVNRT can also produce a longer VA interval.

Septal accessory pathways can make AVRT look more like AVNRT.

So VA helps.

But it does not prove.

It should guide the next question.

If VA is short:

is this typical AVNRT, or could it be septal AVRT?

If VA is long:

is this AVRT, atypical AVNRT, or atrial tachycardia?

The VA interval should not be treated as the answer.

It should be treated as the first fork in the reasoning.

IMAGE 4 — VA INTERVAL INTERPRETATION
Short VA: typical AVNRT more likely, but septal AVRT possible.
Long VA: AVRT, atypical AVNRT, or AT.
The VA interval narrows the differential. It does not prove the mechanism.

Atrial activation sequence: helpful, but not absolute

Retrograde atrial activation sequence is another powerful clue.

If atrial activation is eccentric, especially earliest in the distal coronary sinus, an accessory pathway becomes more likely.

If atrial activation is concentric, AVNRT becomes more likely.

But this is not absolute.

A septal or a right accessory pathway can produce concentric retrograde activation.

Atypical AVNRT can produce delayed atrial activation.

Atrial tachycardia can produce atrial activation patterns that mimic reentrant SVT, depending on the focus or circuit.

Catheter positioning can also mislead.

If the coronary sinus catheter is not well positioned, if the right atrial annulus is poorly covered, or if the earliest atrial activation is simply not being recorded, the apparent sequence may be incomplete.

So the activation sequence should be respected.

The mapper should ask:

Does the sequence make anatomical sense?

Does it remain stable?

Does it change with pacing?

Does it match the suspected pathway location?

Does it match the response to maneuvers?

Activation sequence suggests.

Mechanism requires proof.

IMAGE 5 — ATRIAL ACTIVATION SEQUENCE PITFALLS
Atrial activation sequence is a clue. It becomes stronger when it matches the rest of the physiology.

Spontaneous AV block during tachycardia

Sometimes the tachycardia teaches before we pace.

If AV block occurs during tachycardia and the atrial tachycardia continues, that strongly suggests that the atrium is driving independently.

This supports atrial tachycardia.

Why?

Because if the tachycardia requires the AV node or ventricle as part of the circuit, AV block should usually interrupt the loop.

In AVNRT, the AV node/perinodal region is central to the circuit.

In AVRT, the circuit usually requires conduction from atrium to ventricle through the AV node-His-Purkinje system in orthodromic AVRT.

If the ventricle is no longer activated and the atrial rhythm continues unchanged, we should think about an atrial mechanism.

But again, context matters.

Not every episode of block is easy to interpret.

We must be sure what blocked.

AV block?

VA block?

Atrial signal continuing independently?

Tachycardia continuing or changing?

The key teaching point is simple:

if the atrium continues driving the rhythm despite interruption of AV conduction, atrial tachycardia rises in the differential.

IMAGE 6 — AV BLOCK DURING TACHYCARDIA
Spontaneous AV block can reveal whether the atrium owns the rhythm or whether the AV node/ventricle is required. But remember, there is exceptions, like AVNRT slow-slow, sometimes it cans produce

His-refractory PVC: testing for an accessory pathway

Now we can talk about one of the most elegant maneuvers in SVT diagnosis.

The His-refractory PVC.

The idea is simple.

If a premature ventricular beat is delivered when the His bundle is refractory, that beat should not be able to travel retrogradely through the His-Purkinje system and AV node to reach the atrium.

So if the atrium is advanced by that PVC, how did the impulse get there?

The most likely answer is:

through an accessory pathway.

That is why a His-refractory PVC that advances atrial activation supports AVRT.

It suggests that the ventricle can reach the atrium through a connection outside the normal His-AV nodal axis.

This is especially useful when the differential is AVNRT versus septal AVRT.

Because both can show concentric atrial activation.

Both can look similar.

But if a His-refractory PVC advances the atrium, the maneuver is telling us that the ventricle has access to the atrium through a pathway independent of the His bundle.

That is a powerful statement.

But only if the maneuver is correct.

The PVC must capture the ventricle.

It must be delivered when the His is truly refractory.

The atrial electrogram must truly advance.

The atrial activation sequence should be analyzed.

The tachycardia should continue or respond in a way that makes physiological sense.

If these conditions are not met, the maneuver can mislead.

So the phrase should not be:

“PVC advanced A, therefore AVRT.”

The better phrase is:

“A correctly timed His-refractory PVC that captures the ventricle and advances atrial activation supports an accessory pathway participating in the circuit.”

Precision matters.

IMAGE 7a — HIS-REFRACTORY PVC
A His-refractory PVC tests whether the ventricle can reach the atrium without using the His-AV nodal axis.

IMAGE 7b. His-refractory ventricular premature beat with atrial advancement.
A ventricular premature beat is delivered after the His potential, at a timing in which the His bundle should be refractory. Despite this, the next atrial activation is advanced. If ventricular capture and true His refractoriness are confirmed, this response supports retrograde conduction to the atrium through an accessory pathway, favoring AVRT over AVNRT in the differential diagnosis.

From this point on, the article enters the premium section: practical interpretation of ventricular maneuvers, termination patterns, adenosine response, VA linking, entrainment pitfalls, and how to build a stepwise differential diagnosis between AVNRT, AVRT, and atrial tachycardia.

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The key idea was simple:

in AVNRT, the ventricle is activated by the circuit, but it is not a required part of the circuit.

Now the reasoning changes.

In atrioventricular reentrant tachycardia, AVRT, the ventricle is no longer just an activated chamber.

It becomes part of the circuit.

That distinction changes everything.

It changes how we interpret the ECG.

It changes how we interpret the intracardiac recording.

It changes why ventricular maneuvers become so powerful.

It changes how we think about accessory pathways.

And, most importantly, it changes the question we should ask during the EP study.

The question is no longer only:

“Is the VA interval short or long?”

The better question is:

is the ventricle part of the circuit?

That is the central idea of this article.

In this article, we are going to explore:

• Why AVRT requires an accessory pathway.

• What an accessory pathway actually does.

• Why AVRT is a macroreentrant circuit.

• Why orthodromic AVRT usually has a narrow QRS.

• Why antidromic AVRT produces a wide QRS.

• What manifest, concealed, and bidirectional accessory pathways mean.

• Why WPW pattern is not the same thing as AVRT.

• How accessory pathway location changes atrial activation.

• Why septal pathways can mimic AVNRT.

• Why ventricular maneuvers become important in AVRT, without going deep into them yet.

• Why the ECG suggests accessory pathway location, but the EP study proves it.

• How this prepares the next step: maneuvers to differentiate AVNRT, AVRT, and atrial tachycardia.

Video 1 — In AVNRT, the ventricle is activated by the circuit. In AVRT, the ventricle is part of the circuit.”

The mistake starts when we define AVRT too quickly

The common definition sounds simple:

“AVRT is a tachycardia using an accessory pathway.”

That is true.

But it is not enough.

Because if the mapper stops there, AVRT becomes only a label.

The real question is:

what does the accessory pathway change?

A normal heart has one physiological electrical bridge between atrium and ventricle:

the AV node and His-Purkinje system.

This connection is specialized.

It delays.

It filters.

It protects.

It regulates how atrial impulses reach the ventricle.

An accessory pathway changes this architecture.

It creates an additional atrioventricular connection.

A second bridge.

A bypass.

A pathway that connects atrial myocardium and ventricular myocardium outside the normal AV nodal-His-Purkinje axis.

That second connection is the substrate.

But substrate alone is not tachycardia.

To have AVRT, the impulse must find a way to travel down one pathway and return through the other.

That is the mechanism.

One limb goes from atrium to ventricle.

The other limb returns from ventricle to atrium.

The circuit keeps rotating.

That is why AVRT is not simply “a patient with an accessory pathway.”

AVRT is what happens when the accessory pathway and the normal conduction system become opposite limbs of the same reentrant loop.

IMAGE 1 — An accessory pathway creates a second atrioventricular connection. That second connection can become one limb of a reentrant circuit.

AVRT is a circuit diagnosis, not just an ECG diagnosis

This point matters.

A patient may have ventricular pre-excitation on the ECG.

A patient may have a delta wave.

A patient may have a WPW pattern.

But this does not mean the patient is in AVRT at that moment.

WPW pattern means there is manifest anterograde conduction over an accessory pathway during sinus rhythm.

AVRT means a reentrant tachycardia is using that pathway as part of a circuit.

These are related, but they are not the same.

One is evidence of pathway conduction.

The other is a tachycardia mechanism.

That distinction is important for the mapper.

Because during the EP study, we are not only asking:

“Is there an accessory pathway?”

We are asking:

“Is this pathway part of the tachycardia circuit?”

That is a very different question.

A pathway can be present.

A pathway can conduct.

A pathway can be manifest.

A pathway can be concealed.

A pathway can be a bystander in some situations.

But in AVRT, the accessory pathway is not a bystander.

It is part of the loop.

IMAGE 2 - Delta wave shows pathway conduction. AVRT proves the pathway is participating in a reentrant circuit.

What does “orthodromic” AVRT mean?

Orthodromic AVRT is the most common form of AVRT.

The word sounds complicated, but the concept is simple.

In orthodromic AVRT, the impulse travels anterogradely through the normal conduction system:

atrium,

AV node,

His-Purkinje system,

ventricle.

Then it returns retrogradely to the atrium through the accessory pathway.

So the circuit is:

atrium → AV node → His-Purkinje → ventricle → accessory pathway → atrium.

Because the ventricle is activated through the His-Purkinje system, the QRS is usually narrow.

This is one of the most important points.

Orthodromic AVRT can look like a regular narrow QRS tachycardia.

That means it can enter the same diagnostic territory as AVNRT and atrial tachycardia with 1:1 AV conduction.

If the mapper sees only “narrow QRS regular tachycardia,” the diagnosis remains open.

The mechanism is not proven yet.

The key is that in orthodromic AVRT, the ventricle is required.

The impulse must reach the ventricle, then return to the atrium through the accessory pathway.

The ventricle is not just being activated.

It is part of the loop.

IMAGE 3 - In orthodromic AVRT, the ventricle is activated through the His-Purkinje system, which is why the QRS is usually narrow.

Why does orthodromic AVRT usually have a narrow QRS?

This is where the mechanism explains the ECG.

If the impulse goes from atrium to ventricle through the AV node and His-Purkinje system, ventricular activation uses the normal rapid conduction network.

That produces a narrow QRS, unless there is baseline bundle branch block, rate-related aberrancy, or another ventricular conduction abnormality.

The accessory pathway is still critical.

But in orthodromic AVRT, it is usually not the anterograde route to the ventricle.

It is the retrograde route back to the atrium.

That is why the QRS can be narrow even though an accessory pathway is involved.

This is a common mental trap.

Some people associate accessory pathways only with wide QRS or delta waves.

But concealed pathways may conduct only retrogradely.

And during orthodromic AVRT, even a manifest pathway may participate retrogradely while the ventricle is activated normally.

So the absence of pre-excitation during tachycardia does not exclude AVRT.

The ECG is a clue.

The circuit is the answer.

IMAGE 4 — The QRS reflects how the ventricle is activated, not simply whether an accessory pathway exists.

What does “antidromic” AVRT mean?

In antidromic AVRT, the direction is reversed.

The impulse travels anterogradely through the accessory pathway to the ventricle.

Then it returns retrogradely through the AV node/His-Purkinje system or, in some cases, through another accessory pathway.

Now the ventricle is not activated through the normal His-Purkinje system first.

It is activated through ventricular myocardium from the insertion of the accessory pathway.

That produces a wide QRS tachycardia.

The circuit is still atrioventricular reentry.

But the ventricular activation pattern is completely different.

This is why antidromic AVRT can be confused with ventricular tachycardia or other wide complex tachycardias.

A wide QRS does not automatically mean VT.

But in clinical practice, wide complex tachycardia must be handled carefully.

For the mapper, the important point is mechanism:

orthodromic AVRT goes down the AV node and back through the pathway.

antidromic AVRT goes down the pathway and back through the normal system or another pathway.

Same components.

Different direction.

Different QRS.

Different diagnostic challenge.

IMAGE 5 — Direction determines ventricular activation. Ventricular activation determines QRS width.

Manifest, concealed, and bidirectional pathways

Not all accessory pathways behave the same way.

Some conduct anterogradely.

Some conduct retrogradely.

Some conduct in both directions.

This is why the terms manifest, concealed, and bidirectional matter.

A manifest pathway conducts from atrium to ventricle during sinus rhythm.

This creates ventricular pre-excitation.

On the ECG, we may see a short PR interval, delta wave, and widened QRS depending on the degree of pre-excitation.

A concealed pathway does not conduct anterogradely during sinus rhythm.

So there is no delta wave.

There may be no obvious clue on the baseline ECG.

But the pathway can conduct retrogradely.

That means it can still participate in orthodromic AVRT.

This is critical.

A patient can have a completely normal resting ECG and still have AVRT mediated by a concealed accessory pathway.

A bidirectional pathway can conduct both anterogradely and retrogradely.

It may produce pre-excitation in sinus rhythm and also participate in AVRT.

For the mapper, the baseline ECG helps.

But it does not close the case.

The EP study must define the pathway’s behavior.

Can it conduct anterogradely?

Can it conduct retrogradely?

Is the conduction decremental or nondecremental?

Where is the earliest atrial activation during retrograde conduction?

Does the pathway participate in the tachycardia?

Those are the questions that matter.

IMAGE 6 — The resting ECG may show the pathway (delta wave), or it may hide it. Concealed pathways can still sustain orthodromic AVRT. To prove a concealed pathway, stim on ventricles.

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The circuit: why the ventricle matters

Now we can return to the most important idea.

In AVNRT, the circuit is nodal/perinodal.

The ventricle is activated because the impulse exits the AV node/His-Purkinje system, but the ventricle is not required to maintain the reentry.

In AVRT, the circuit needs the ventricle.

This is not just semantic.

It changes the logic of the EP study.

If the ventricle is part of the circuit, then ventricular events can interact directly with the circuit.

This is why ventricular maneuvers become so important in AVRT.

A ventricular premature beat, a ventricular pacing train, the post-pacing response, VA linking, and interval analysis can all be used to test whether the ventricle and the accessory pathway are participating in the tachycardia.

But we should be careful here.

The purpose of this article is not to teach the maneuvers yet.

The purpose is to understand why those maneuvers matter.

They matter because AVRT is built differently.

When the ventricle is part of the loop, ventricular pacing is not just pacing.

It becomes a way to interrogate the circuit.

That is why AVRT is the bridge to the next article.

To understand SVT maneuvers, we first need to understand what they are testing.

They are not magic tricks.

They are circuit participation tests.

In this article, we build the architecture.

In the next one, we will test it.

From this point on, the article enters the premium section: orthodromic vs antidromic AVRT, concealed pathways, pathway localization, intracardiac activation patterns, septal pathway pitfalls, mapping strategy, ablation target, and how AVRT prepares the logic for the next article on SVT maneuvers.

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• Why recognizing AVNRT is not the same as understanding AVNRT.

• What decremental conduction in the AV node really means.

• Why the fast pathway conducts faster but usually has a longer refractory period.

• Why the slow pathway conducts slowly but allows reentry to occur.

• Whether AVNRT should be understood as microreentry, macroreentry, or compact nodal/perinodal reentry.

• Where the slow pathway is located and why the anatomy of the triangle of Koch matters.

• How to think about zones 3, 2, and 1 during RF ablation.

• What to look for on the ablation catheter before RF delivery.

• How to interpret junctional rhythm during RF.

• Why the endpoint should be non-inducibility with preservation of normal AV conduction.

• How this understanding prepares the next step: differentiating AVNRT, AVRT, and atrial tachycardia.

Everyone recognizes typical AVNRT.

Narrow QRS.

Regular tachycardia.

Short VA interval.

Concentric retrograde atrial activation.

Sometimes a pseudo-r’ in V1.

Sometimes pseudo-S waves in the inferior leads.

During the EP study, we see the classic AH jump, a nodal echo, and then tachycardia.

The diagnosis looks easy.

But here is where many got trapped:

recognizing AVNRT is not the same as understanding AVNRT.

Many mappers can say, “this is AVNRT.”

Fewer can explain, in real depth, why this tachycardia exists, what type of tissue allows it to happen, why the AV node conducts decrementally, why one pathway conducts faster and the other slowly, why we ablate the slow pathway region, why we avoid RF near the fast pathway, and why the endpoint is not simply “eliminating the AH jump.”

AVNRT is often treated as a basic arrhythmia.

But in reality, it is a compact lesson in electrophysiology.

It teaches reentry, unidirectional block, decremental conduction, refractoriness, functional anatomy, and ablation risk.

And above all, it teaches something every mapper needs to understand:

the mechanism matters more than just know the name of the arrhythmia.

Image 1.
Typical AVNRT: easy to recognize, much harder to understand deeply.

Source: personal archive

The problem starts when we teach AVNRT as an algorithm

Imagine the case.

We enter the procedure.

The patient is in sinus rhythm.

Even before the catheters are positioned, the preparation has already started.

We have reviewed the clinical scenario, discussed the diagnostic hypothesis, aligned with the team, and entered the lab with an initial reasoning plan.

Now the catheters are positioned.

Everyone is ready.

If you do not remember — or have not yet read — why this preparation matters before the first pacing maneuver, it is worth going back to the previous articles on procedural preparation. This is where we start building the questions that the EP study will answer (link bellow).

We start the study.

With the first pacing maneuver, we induce a regular narrow QRS tachycardia.

The VA interval is short.

We look at it and think:

AVNRT.

It was fast.

It was intuitive.

And in most cases, it will probably be correct.

This is exactly the type of situation we see every day in the EP lab.

The problem is not recognizing AVNRT.

The problem is when recognition becomes a shortcut and replaces reasoning.

Over time, the mapper learns to associate certain findings with certain diagnoses.

A short VA interval suggests AVNRT.

An AH jump suggests dual AV nodal physiology.

A nodal echo reinforces the hypothesis.

The response to adenosine helps build the scenario.

All of this is useful.

But none of these findings, in isolation, explains the mechanism of the arrhythmia.

And when the mapper learns only through algorithms, they start depending on shortcuts.

Shortcuts work well in easy cases.

But they fail exactly when the case requires reasoning.

A short VA interval suggests typical AVNRT, but it does not explain the circuit.

An AH jump suggests dual AV nodal physiology, but it does not confirm the diagnosis by itself.

A nodal echo shows the presence of a conduction pathway capable of supporting reentry, but it must be interpreted in context.

Adenosine may terminate AV node-dependent tachycardias, but it does not differentiate everything by itself.

The real diagnosis does not come from a single finding.

It comes from convergence:

mechanism, signals and response to maneuvers.

That is the point.

Do not just look at the criterion.

Ask what that criterion actually proves.

From this point on, the article enters the premium section: AV node physiology, decremental conduction, dual AV nodal physiology, reentrant mechanism, slow pathway anatomy, RF zones, ablation catheter signals, and safety during RF delivery.

A quick note for free users: If the paid upgrade does not appear properly in your Substack app, try opening this publication in your browser and upgrading there instead. Depending on region, platform, or app behavior, the upgrade flow may appear differently. The browser remains a useful workaround when needed.

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About Paper to Practice
Paper to Practice is not a literature summary.
It is where I take one paper and translate it into EP lab reasoning: what it changes in mechanism interpretation, what it changes in ablation strategy, and what it should change in the behavior of the mapper.

This edition is based on From High-Density Mapping to Low-Density Mapping: Outlining the Active Circuit in Complex Atrial Re-Entrant Tachycardias, by Rodríguez Font and colleagues, published in JACC: Clinical Electrophysiology in 2020. The paper proposes a 2-step approach in which a high-density activation map is combined with entrainment maneuvers to build a low-density map that displays only the active part of the re-entrant circuit.

Before we start, a quick note for free readers:
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In this Paper to Practice, you’ll find:

• Why this is not a paper against high-density mapping, but against overinterpreting it

• Why entrainment exposes the active circuit when complex flutter/AT becomes difficult to read

• How the paper transforms a high-density map into a low-density representation of the true active circuit

• Why not every colorful activation path deserves mechanistic trust

• What the elite mapper should do when the case becomes visually rich but mechanistically uncertain

• Why proving your hypothesis is more important than defending it

• Why this is one of those papers worth reading in full

In complex atrial re-entry, the problem is not always that the map shows too little.

Sometimes the problem is that it shows too much.

That is exactly why this paper matters.

Because high-density mapping can make a chamber look extraordinarily detailed while still leaving one essential question unanswered:

What part of this activation is truly participating in the circuit — and what part is only being passively activated?

That is the real issue.

And this is where the paper by Rodríguez Font and colleagues becomes so useful.

Not because it tries to reduce the value of high-density mapping.

But because it reminds us of something more important:

high density shows more.
entrainment proves what matters.

That is the central lesson here.

Read more

The problem is the moment before that.

The moment when atypical flutter still has not closed.
When the first map has not answered enough.
When the chamber looks promising, but not convincing.
When the initial hypothesis starts to weaken, but RF still should not begin.

That is the point where a complex case stops being theory and becomes decision-making.

Because in real EP lab practice, the greatest challenge in atypical flutter is not always “understanding the mechanism” in the abstract. The real challenge is knowing how to move forward when the case is still not clear.

Before we continue, a quick note: if you want to upgrade your subscription and the Substack app does not show the paid upgrade option, please open EP Master in your browser to complete your subscription. After that, the full article can be read normally in the app.

This is the point where many cases begin to get lost.

Not because of lack of technology.
Not because of lack of points.
But because of lack of practical direction.

Because complex cases do not just require more mapping.
They require better questions, better tests, and better decisions.

This article is about that.

Not atypical flutter as an abstract concept.
But atypical flutter as it appears in the lab: incomplete, ambiguous, uncomfortable, and demanding that the mapper knows what to do before the first RF lesion.

The real problem in a complex case

Read more

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There is a mistake in the EP lab that often happens before the first RF application.

Sometimes before the first pacing maneuver. Sometimes before the first real diagnostic thought.

The mistake is naming the tachycardia too early.

Once an organized atrial tachycardia “looks like flutter,” the lab becomes vulnerable to a dangerous shortcut: familiar appearance starts replacing proven mechanism. And from that point on, everything downstream becomes easier to distort — the ECG, the meaning of TCL coverage, the activation pattern, the role of entrainment, and ultimately the ablation strategy itself.

Not every organized atrial tachycardia is typical flutter.

And many procedural mistakes begin when visual familiarity is mistaken for proven mechanism.

This is where the mapper has to become more demanding.

Because in atrial tachycardia, “organized” is not a diagnosis. “Flutter-like” is not a mechanism. And a case that looks familiar may still be asking a very different electrophysiological question.

In this article, the goal is not just to classify rhythms. The goal is to sharpen discrimination. To understand when a tachycardia is likely to be typical flutter, when it may be focal AT, when it may be a non-CTI macroreentry, and when the lab is simply becoming too confident too early.

Read more

Typical flutter is one of the arrhythmias most familiar to every mapper.

And precisely because of that, it is also one of the most dangerous to oversimplify.

When a case looks obvious, the risk is stopping your thinking too early. The ECG looks familiar. The activation seems consistent. The CTI line appears straightforward. The tachycardia terminates. And without noticing it, the mapper replaces mechanism with habit.

But typical flutter is not just “a line case.”

Typical flutter is a macroreentrant atrial tachycardia, dependent on the cavotricuspid isthmus (CTI) as a critical segment of the circuit. In practical terms, this means a circuit that usually travels up the atrial septum, down the lateral wall of the right atrium, and crosses the CTI between the tricuspid annulus and the inferior vena cava. The ablation target is not the CTI by tradition. It is the CTI because of mechanism.

And that completely changes the logic of the case.

Because in typical flutter, the goal is not simply to terminate the arrhythmia. The goal is to demonstrate that the isthmus has truly stopped conducting in both directions. The endpoint remains bidirectional CTI block.

In this article, the goal is not to review typical flutter as a “classic” topic. The goal is to put the case back where it belongs: mechanism, maneuver, criteria, and confirmation.

Here is what you will find:

• how to suspect typical flutter from the ECG without becoming overconfident

• what defines typical flutter as macroreentry

• how the mapper recognizes the circuit on intracardiac recordings

• how to use entrainment to confirm CTI dependence

• where the RF line is usually delivered and where the anatomical trap lies

• how to truly confirm bidirectional block

• and how to search for gaps when the line looks complete but still is not blocked

Figure 1
Comparisons between typical flutter, clockwise typical flutter, atypical flutter, and 1:1 flutter.
Source: Bonakdar HR. Atrial Flutter: Diagnosis and Management Strategies. In: Cardiac Arrhythmias. IntechOpen; 2018. DOI: 10.5772/intechopen.74850.

When the ECG makes you think of typical flutter

The ECG can raise a strong suspicion of typical flutter. But it does not establish the mechanism by itself.

In counterclockwise typical flutter, flutter waves are usually most visible in V1 and the inferior leads, with a sawtooth pattern, constant morphology, constant polarity, and no true isoelectric baseline between one wave and the next. In the classic counterclockwise form, septal activation makes the flutter waves predominantly negative in the inferior leads. In clockwise typical flutter, polarity may reverse, and V1 becomes even more important for interpretation. The chapter itself emphasizes that flutter-wave morphology can change in the presence of atrial disease, prior surgery, or previous ablation, which makes the ECG useful, but not definitive, for determining flutter type.

That is an important point for the mapper: the ECG suggests. It does not exempt you from testing.

In practical terms, I strongly suspect typical flutter when I find a regular atrial tachycardia, with a stable cycle length, continuous atrial activity, no true isoelectric interval, and repetitive morphology that is coherent with an organized right atrial reentry. But I only begin the case with that hypothesis. I do not finish the case there.

Figure 2
Caption: The classic ECG pattern helps a lot, but it does not replace mechanism.
Source: Bonakdar HR. Atrial Flutter: Diagnosis and Management Strategies. In: Cardiac Arrhythmias. IntechOpen; 2018. DOI: 10.5772/intechopen.74850.

Figure 3
Caption: Typical flutter tends to occupy nearly the entire cycle. Focal AT usually does not.
Source: Bonakdar HR. Atrial Flutter: Diagnosis and Management Strategies. In: Cardiac Arrhythmias. IntechOpen; 2018. DOI: 10.5772/intechopen.74850.

Typical flutter is not a focus. It is a circuit.

This may be the most important conceptual shift in the entire article.

Typical flutter is a macroreentry. It is not a focal tachycardia arising from one point and spreading outward. It is a large, organized circuit, sustained by continuous propagation around anatomical and functional barriers, using the CTI as a critical conduction zone.

For the mapper, that changes everything.

If you think like a focal tachycardia, you search for the origin.
If you think like a macroreentry, you search for circuit coherence, activation direction, TCL coverage, and the critical conduction segment.

In other words:

In typical flutter, you are not chasing the earliest point.
You are dismantling a circuit.

What the intracardiac recordings need to show

In typical flutter, intracardiac electrograms show a constant cycle length, constant morphology, constant polarity, and an activation sequence that is consistent with macroreentry. In addition, endocardial recordings from the interatrial septum, upper anterolateral right atrium, and the CTI tend to cover 100% of the tachycardia cycle length. By contrast, in focal AT, even when looking only at the right atrium, activation usually covers less than 50% of the cycle.

This gives you an excellent discrimination point between a reentrant and a focal mechanism:

Does what I am see cover the whole cycle, as a large circuit should?
Or am I dealing with a more limited activation pattern, more consistent with a focal source?

In typical flutter, cycle-length stability is usually very high, with minimal variation. That also reinforces the impression of stable macroreentry rather than a more erratic or poorly defined mechanism.

Induction is not bureaucracy. It is hypothesis testing.

If the question of the case is whether the flutter is truly CTI-dependent, inducing the tachycardia is not just “making the arrhythmia appear.” It is creating the opportunity to test mechanism.

Distinguishing isthmus-dependent flutter from non-isthmus-dependent flutter is crucial before a curative ablation. Induction is often performed with programmed stimulation, frequently from the proximal CS, which tends to induce counterclockwise typical flutter. In some cases, isoproterenol may be needed to facilitate induction.

For the mapper, this means something very simple:

induction is there to confirm whether your circuit hypothesis truly holds up in practice.

Entrainment is not ornament. It is circuit confirmation.

The chapter is explicit: entrainment is the essential maneuver to confirm the diagnosis and determine whether the flutter is CTI-dependent. Overdrive pacing is performed at a cycle length 10–20 ms shorter than the tachycardia cycle length. During the maneuver, all atrial activity accelerates to the pacing rate; if pacing is progressively faster, progressive fusion may appear; and when pacing is stopped, the same tachycardia returns with the same cycle length and the same activation sequence.

In addition, check the morphological pattern on the diagnostic catheters during the tachycardia and compare it with what you see during entrainment pacing. If the site you are pacing changes the electrogram morphology, that already suggests the pacing site is probably not part of the circuit. If the morphology remains the same, that is an excellent sign that you are pacing inside the tachycardia circuit.

The key value we obtain from this is the post-pacing interval (PPI).

If the pacing site is in the circuit, or very close to it, the PPI will be very close to the tachycardia cycle length. When the catheter is at the CTI and the PPI is equal to, or within 20 ms of, the TCL, that supports the idea that the CTI is part of the circuit and confirms isthmus dependence.

If you suspect the CTI is part of the circuit, it is not enough to believe that based on how the case looks. Pace the CTI. Measure the PPI. See whether the response is truly that of a site belonging to the circuit.

Figure 4
Caption: Entrainment. Return cycle of 286 ms minus tachycardia cycle length of 282 ms = 4 ms. Result: inside the circuit.
Source: Personal archive.

A practical entrainment framework in typical flutter

First, confirm a real and stable cycle length.

Then, with the mapping catheter at the CTI, perform overdrive pacing 10–20 ms faster than the tachycardia cycle.

Observe whether the tachycardia is being entrained coherently.

When pacing stops, measure the PPI at the same site.

If the PPI minus TCL is very short, especially up to 20 ms, that strongly favors the isthmus being part of the circuit.

The key sentence here is:

Entrainment is not only there to say, “this is flutter.”
It is there to test whether the point you are studying truly belongs to the circuit you are trying to interrupt.

That is why understanding entrainment is so important, not only in typical flutter, but in any reentrant tachycardia. The principle of entrainment is a principle of circuit localization.

Where the RF line is usually delivered

The classic line is delivered between the tricuspid annulus and the inferior vena cava, crossing the CTI.

The article describes a classic setup using three catheters: the ablation catheter, a CS catheter, and a duodecapolar catheter positioned around the tricuspid annulus. The duodecapolar catheter is placed so that its distal poles record the middle and lateral portions of the CTI. The ablation catheter is adjusted in LAO at approximately 6–7 o’clock. The first application is usually delivered near the tricuspid annulus, with a small A/V ratio. After each application lasting 30–60 seconds — depending on the catheter type, irrigated or 8 mm — the catheter is dragged back roughly 4 mm until a new sharp atrial electrogram is found, not far-field signal, and this is repeated until the border of the inferior vena cava is reached.

What the mapper needs to watch during the line

Adequate contact.

A true local electrogram.

An A/V ratio that helps orient position.

Coherent progression of the line.

And most importantly, the difference between an anatomically drawn line and a functionally blocked line.

Because those two things are not the same.

Where the anatomical difficulty lies

Not every CTI is the same.

The article describes variable anatomy: a wider lateral portion, a thinner central portion, the presence of concavity and pouches in a relevant proportion of patients, and the Eustachian ridge forming the posterior border of the isthmus. This anatomy can make line continuity much more difficult.

There is one particularly important detail for the article: the Eustachian ridge may prevent a simple drag-ablation technique from adequately reaching the most posterior part of the CTI. In these cases, it may be necessary to curl the catheter in order to access the floor of the pouch and complete the line.

In addition, the chapter reminds us that the septal/medial portion of the CTI lies close to the posterior extension of the AV node, which explains the higher risk of AV block when ablation is performed too medially, around 5 o’clock. The practical message is clear: the farther you stay from the septum, the lower the risk of that complication.

Figure 5
Caption: Sometimes the problem is not “lack of RF.” It is anatomy hiding the gap.
Source: Bonakdar HR. Atrial Flutter: Diagnosis and Management Strategies. In: Cardiac Arrhythmias. IntechOpen; 2018. DOI: 10.5772/intechopen.74850.

Terminating the tachycardia is not the endpoint

This point needs to be stated plainly.

Seeing flutter terminate during RF is important.

But it is not the endpoint.

The endpoint remains confirmation of bidirectional block, and the chapter itself emphasizes that this block should be checked at the end of the procedure and reassessed after 20–30 minutes, because conduction can recover.

That is the difference between a case that is executed and a case that is truly understood.

How the mapper confirms bidirectional block

The chapter organizes this well, and it can be turned into a field framework.

1. Activation sequence while pacing from one side of the line

When you pace from the medial side of the line, for example from the proximal CS, and a medial-to-lateral block is present, the activation wave must travel up the septum, around the right atrium, and only then reach the distal poles of the duodecapolar catheter. That is why the distal duodecapolar poles become the last ones to record activation.

2. Activation sequence in the opposite direction

To confirm lateral-to-medial block, you pace laterally to the line, for example from the distal poles of the duodecapolar catheter or with the ablation catheter at approximately 8 o’clock. If true block is present, activation travels up the lateral wall, around the superior right atrium, and then down the septum until it reaches the proximal CS.

This may be the simplest way to teach the logic to the mapper:

If block is present, the impulse is forced to go around.
If it still crosses directly, the line is not ready.

3. Trans-CTI conduction time

The chapter also gives a very useful number: a greater than 50% prolongation in conduction time across the CTI, or an absolute value of 150 ms or more, strongly favors isthmus block.

That is useful because it transforms the feeling of block into an objective check.

4. Differential pacing

This part is absolutely worth including because it gives real sophistication to the article.

To assess, for example, lateral-to-medial block, you first pace close to the line, such as from duodecapolar 1–2, measure the time to the proximal CS, and then pace farther from the line, such as from duodecapolar 3–4. If block is present, that interval becomes shorter as you move away from the line. If the CTI is still conducting, the opposite happens: the interval becomes longer, because the wavefront is still crossing the isthmus directly.

Figure 6
Caption: Bidirectional block is not a visual impression. It is activation behavior tested in both directions.
Source: Bonakdar HR. Atrial Flutter: Diagnosis and Management Strategies. In: Cardiac Arrhythmias. IntechOpen; 2018. DOI: 10.5772/intechopen.74850.

When the line looks complete, but a gap is still there

This is one of the most valuable parts of the chapter.

The text reminds us that residual gaps are common because of isthmus anatomy and pouches, and that finding them is mandatory to achieve complete bidirectional block and reduce recurrence. These gaps can be detected through fractionated electrograms and through the behavior of double potentials along the line. When you move the catheter toward the gap, the interval between the two components of the double potential becomes progressively shorter, until, at the gap itself, the signals fuse. When the line is truly blocked, the interval between the components becomes longer again because the wavefront now needs to travel all the way around the right atrium to reach the other side of the line.

This deserves to become a signature sentence:

A gap is tracked by signal physiology.

Figure 7
Caption: The gap is not where the line looks bad. It is where physiology shortens the interval and fuses the signals.

The mapper’s framework for calling true block

If I had to summarize all of this into one practical field logic, it would be this:

After the CTI line, I only call true block when I find a coherent set of signs.

The tachycardia terminated, but I do not stop the case there.

I test activation from one side of the line and see whether the impulse is now forced to travel around the atrium.

I test the opposite direction and look for the same logic.

I compare trans-CTI time with baseline and want to see important prolongation, ideally more than 50% or at least around 150 ms.

I use differential pacing when I need to distinguish true block from residual conduction.

I walk the line searching for gaps whenever something still does not fit.

That is how the mapper stops operating by impression and starts operating by electrophysiological evidence.

What typical flutter teaches about the mapper

Perhaps the greatest educational value of typical flutter is not that it is common.

Perhaps it is that it exposes, very clearly, the difference between execution and understanding.

The mature mapper thinks:

“Did I demonstrate true bidirectional block?”

That is the right question.

In the end, typical flutter is not simple because the circuit is simple. It looks simple because we have seen it so many times. But in electrophysiology, repetition without interpretation creates too much confidence and not enough precision.

The goal is not simply to cross the CTI with RF.

The goal is to demonstrate, with criteria, that the circuit has lost its critical conduction pathway.

And that difference completely changes the level of the case.

References

Bonakdar HR. Atrial Flutter: Diagnosis and Management Strategies. IntechOpen. 2018. DOI: 10.5772/intechopen.74850.

Saoudi N, Cosío F, Waldo A, et al. A classification of atrial flutter and regular atrial tachycardia according to electrophysiological mechanism and anatomical bases.

Chen J, De Chillou C, Basiouny T, et al. Cavotricuspid isthmus mapping to assess bidirectional block during common atrial flutter radiofrequency ablation.

Oral H, Sticherling C, Tada H, et al. Role of transisthmus intervals in predicting bidirectional block after ablation of typical atrial flutter.

Shah D, Haïssaguerre M, Takahashi A, et al. Differential pacing for distinguishing block from persistent conduction through an ablation line.

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In this Paper to Practice, you’ll find:

• Why this is not just an outcome paper, but a strategy paper

• Why eliminating the index AT is not the same as eliminating the atrial substrate

• How ultra-high-density mapping supports a more tailored and less empirical ablation strategy

• What an elite mapper should actually do when facing complex post-ablation AT

• A practical step-by-step for reading the chamber before reacting to the map

• Why recurrence after a good first procedure does not automatically mean the first interpretation was wrong

In atrial tachycardia ablation, success depends heavily on what question you think you are answering.

If the goal is to eliminate the active tachycardia, ultra-high-density mapping supports a very strong strategy.

If the goal is to eliminate the arrhythmogenic potential of a heavily scarred atrium, that is a different challenge entirely.

That distinction matters more than it seems.

Because one of the easiest ways to misread a paper like this is to walk away thinking that better mapping simply produces better outcomes.

But that is not what this study really shows.

What it shows is something more useful for the lab:

ultra-high-density mapping becomes valuable when it allows a more tailored ablation strategy against the active circuit — reducing empirical lesion delivery, sharpening target selection, and improving mechanistic confidence.

But it also shows the limit of that success.

Eliminating the index atrial tachycardia is not the same as eliminating the atrial substrate that allowed it to exist in the first place.

That is the real tension of this paper.

Jungen and colleagues studied 250 consecutive patients with atrial tachycardias after prior catheter ablation or cardiac surgery; 98% had prior ablation, and the cohort was made of previously modified, electrically complex atria rather than simple, clean AT substrates.

And the mechanisms were not all the same.

This matters.

Because one of the traps in post-ablation atrial tachycardia is to think too early that everything is just another scar-related macroreentry. But in the index procedures, the 354 mapped ATs included macroreentry (64%), localized reentry (27%), and focal AT (9%).

That alone should already change the mapper’s posture.

Because when mechanism diversity is real, interpretation has to become more disciplined.

Not faster.
Not louder.
More disciplined.

Figure 1. Post-ablation atrial tachycardia is not a single mechanistic problem. Different mechanisms and different atrial locations require a more tailored reading of the map.
Source: Adapted from Jungen C, Akbulak R, Kahle A-K, et al. Outcome after tailored catheter ablation of atrial tachycardia using ultra-high-density mapping. J Cardiovasc Electrophysiol. 2020;31:2645–2652. Figure 1, page 5.

The procedural logic in this study is deceptively simple, but very important.

The chamber of interest was mapped during the tachycardia. The operators analyzed wavefront propagation, activation patterns, areas of slow conduction, anatomical and functional barriers, and lines of block. If the mapped chamber did not explain enough of the tachycardia cycle length, mapping was extended to the remaining atrium and the coronary sinus.

That sounds obvious on paper.

But in real life, this is exactly where many cases are lost.

Because the average operator sees an abnormal chamber and wants to ablate.

The elite mapper sees an abnormal chamber and asks:

What part of this chamber is actually necessary for the arrhythmia I am seeing right now?

That is the difference.

And this paper helps define what that mindset should look like.

What the elite mapper should do in cases like this

Read more

If you haven’t read it, I strongly recommend starting there.

Just click bellow:

In this second one, the question is more useful for the lab:

if the power was the same, why wasn’t the application?

This is one of the central questions in modern electrophysiology. Because in the real world, two 30 W applications may look identical on the screen and still produce completely different results. One may generate an effective, coherent, durable lesion. Another may be superficial, inconsistent, or dangerously efficient.

The reason is simple, but it requires maturity to read it correctly:

power does not define the lesion.
Power defines only the generator’s intention.

What defines the lesion is the interaction between current, contact, stability, impedance, tissue geometry, convective cooling, and local response.

That is why the average technician watches parameters.
But the elite technician watches the relationship between parameters, tissue, and response.

What this article delivers in practice

By the end of this article, the goal is that you will be able to:

• better understand why two applications with the same wattage may produce very different lesions

• use impedance with more judgment and less automatism

• recognize when steam pop is a real risk rather than an unpredictable complication

• read AI, LSI, FTI, and LI as useful tools without turning them into dogma

• adapt your reading to the platform used in your lab

• and elevate your RF analysis from “following parameters” to “reading tissue, energy, and response”

1. The most common mistake: treating power as if it were effect

Power is a delivery variable. Lesion is a biological event.

That difference may sound theoretical, but it explains much of the inconsistency in the lab.

Two applications with the same power may generate very different lesions if the following change:

• circuit impedance

• electrode-tissue coverage

• catheter orientation

• application stability

• local heat dissipation

In practice, the same wattage may lead to:

• a small lesion

• an effective lesion

• or overheating

So the useful question is never “how many watts did I use?”
The useful question is:

how much of that energy actually entered the tissue, with what footprint, with what stability, and with what thermal dissipation?

Image 1
Image adapted from Patil S, Deshmukh AJ. Relationship between power, current, resistance, and current density in radiofrequency lesion formation. Indian Pacing Electrophysiol J. 2025.

2. Impedance drop: the most useful signal and, at the same time, one of the most misread

Impedance remains one of the best real-time feedback signals during RF, as long as it is interpreted in the right context.

The problem is that many people only look at the final result:

“did it drop or not?”

But useful impedance is not simply “impedance dropped.”
It is impedance dropping in a coherent, progressive way, proportional to tissue and application time.

In practice, two mistakes are very common:

The first is using impedance drop as a trophy.
The second is ignoring how it happened.

A progressive and coherent drop is often a good sign.
An aggressive and very early drop should trigger suspicion, not enthusiasm.

Some applications look excellent in the first seconds precisely because they are being too efficient where they should not be.

That is where thermal risk begins.

Image 2
Image adapted from Patil S, Deshmukh AJ. Very rapid and excessive impedance drops may accompany intratissue overheating and steam pop during RF. Indian Pacing Electrophysiol J. 2025.

So far, we have covered two central ideas:
power alone does not define lesion quality, and impedance drop only makes sense when interpreted in context.

From this point on, we move into the most practical and valuable part of the article:

• current

• AI, LSI, FTI, and LI

• what to watch in each ecosystem

• contact force, stability, and steam pop

• and the EP Master framework for reading RF in real time

From this point on, the article enters the premium section.

A quick note for free users: If the paid upgrade does not appear properly in your Substack app, try opening this publication in your browser and upgrading there instead. Depending on region, platform, or app behavior, the upgrade flow may appear differently. The browser remains a useful workaround when needed.

If EP Master content is helping you think more clearly in the EP lab, please consider sharing this article so we can reach and help more people.

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Radiofrequency is often treated as something intuitive.

You step on the pedal.
You deliver power.
You watch impedance.
You look for an endpoint.

But radiofrequency is not simply “energy delivered at one point.”

It is a complete electrical circuit, and a lesion is only created when current, resistance, contact, and heat dissipation interact in the expected way. RF energy is delivered in a unipolar fashion, from the catheter tip to the dispersive electrode on the skin, and effective heating occurs because current crosses resistive tissue — not because the generator directly “burns” the myocardium.

So before discussing technique, indices, or strategy, it is worth returning to the most basic question:

what actually happens between the pedal and the myocardium?

1. Radiofrequency is not “energy at the tip.” It is a circuit.

Radiofrequency used in cardiac ablation is a high-frequency alternating current, typically around 500 kHz, delivered in a unipolar fashion from the catheter tip to a dispersive electrode applied to the skin. In other words, lesion formation does not depend on the catheter alone; it depends on the entire circuit.

Current leaves the generator, passes through the catheter, enters the myocardium, travels through the body, and returns via the dispersive patch. The point of highest current density is the catheter-tissue interface because the surface area of the tip electrode is much smaller than the surface area of the dispersive electrode. That difference in area is exactly why useful heating occurs in the heart and not in the skin.

Figure 1 — Unipolar radiofrequency circuit.
Electrical current travels through a complete circuit involving the generator, catheter, myocardium, and dispersive electrode. Lesion formation does not depend only on the catheter tip, but on the behavior of the entire circuit.
Adapted from Patil S, Deshmukh AJ. Biophysics of radiofrequency Ablation: An evolving paradigm. Indian Pacing Electrophysiol J. 2025.

2. How heat is actually produced

When current crosses tissue, it generates local electromagnetic heating — so-called resistive heating. This occurs in the small rim of tissue directly in contact with the electrode. From there, heat propagates into deeper layers through passive thermal conduction, creating a larger zone of conductive heating.

This point is central.

Radiofrequency does not create a homogeneous lesion from the start. First, it directly heats a small superficial rim. Then that energy spreads into deeper tissue. The final lesion depends precisely on the balance between these two phases.

There is also a concept many underestimates: thermal latency. Even after RF delivery stops, tissue temperature may continue to accumulate in deeper layers while the surface interface is already beginning to cool. This helps explain why the final lesion does not always correspond exactly to what seemed to be happening at the surface during the application.

Figure 2 — Resistive and conductive heating.
Radiofrequency produces direct heating in a small superficial zone at the catheter-tissue interface. The lesion then expands in depth through passive thermal conduction.
Adapted from Patil S, Deshmukh AJ. Biophysics of radiofrequency Ablation: An evolving paradigm. Indian Pacing Electrophysiol J. 2025.

3. When tissue becomes non-excitable

Irreversible injury does not occur just because tissue “gets a little warm.”

It occurs when tissue temperature crosses a biological threshold sufficient to produce permanent cellular damage. The article summarizes this thermal continuum well: below 45 °C, excitability is relatively maintained; around 48 °C, reversible loss of excitability may occur; above 50 °C, irreversible tissue injury begins; and at approximately 50.3 °C and 53.6 °C, transient and permanent conduction block appear, respectively.

This matters for two reasons.

First, because acute block is not always equivalent to permanent lesion formation.
Second, because effective RF does not require “boiling” tissue — in fact, once you get too close to that, the problem shifts from inefficacy to complication.

Table 1 — Thermal effects of radiofrequency on tissue.
Excitability, conduction block, and irreversibility of lesion formation depend directly on the temperature reached within the myocardium.
Adapted from Patil S, Deshmukh AJ. Biophysics of radiofrequency Ablation: An evolving paradigm. Indian Pacing Electrophysiol J. 2025.

4. Power is not the lesion

One of the most common conceptual mistakes in the lab is treating power as if it were synonymous with effect.

It is not.

Power is simply the rate of energy delivery per unit time. In the RF circuit, it relates to voltage, current, and resistance. The article summarizes this with the classic relationships: P = V × I and P = I²R, reinforcing that the amount of current actually crossing tissue depends on what the circuit allows.

This changes everything.

Because two applications with the same power may produce different lesions if circuit impedance, tip geometry, or current density differ. The paper provides an elegant example: applications of 20 W at 80 Ω, 30 W at 120 Ω, 40 W at 160 Ω, and 50 W at 200 Ω may result in the same current flow and therefore a similar degree of heating.

So the correct question is not simply “how many watts did I use?” but:

how much current actually entered the tissue, with what density, and under what mechanical and thermal conditions?

Figure 4 — Power, current, voltage, resistance, and current density.
Lesion formation does not depend only on the power value selected, but on the interaction between power, impedance, and the spatial distribution of current at the catheter-tissue interface.
Adapted from Patil S, Deshmukh AJ. Biophysics of radiofrequency Ablation: An evolving paradigm. Indian Pacing Electrophysiol J. 2025.

5. Why the same power does not create the same lesion

Once the circuit is understood, it becomes easier to see why the same power does not always produce the same lesion.

Lesion size increases with power because more current is delivered to tissue. But that relationship is not absolute. The article emphasizes that larger lesions may occur with higher power, but the risk of overheating at the electrode-tissue interface also rises, and the relationship between power and temperature depends on circuit impedance. This means that applications using the same power can generate different degrees of heating and therefore different lesion dimensions.

Time also changes lesion geometry.

In conventional ablation, most lesion growth occurs within the first seconds, and maximum size is reached as the system approaches thermal equilibrium. The article summarizes this clearly: at constant power, lesion growth occurs mainly in the first 10 seconds and reaches maximal size by about 30 seconds; prolonging beyond that adds little. It also highlights that high-power short-duration tends to produce wider and shallower lesions, whereas lower to moderate power for longer duration favors deeper lesions that depend more on conductive heating.

Figure 5 — Relationship between power and lesion depth.
Higher power tends to increase lesion depth and volume, but this effect depends on impedance, time, contact, and heat dissipation.
Adapted from Patil S, Deshmukh AJ. Biophysics of radiofrequency Ablation: An evolving paradigm. Indian Pacing Electrophysiol J. 2025.

6. The dispersive patch is not a detail

Because RF works through a circuit, the dispersive patch is not a logistical accessory. It is a functional part of energy delivery.

The article makes this clear in two ways. First, by explaining that little to no heating occurs at the patch when it has a large surface area and low current density. Second, by warning that poor skin contact, trapped air, sweat, or reduced effective patch area can create focal areas of high current density and skin burns.

This is an excellent practical teaching point, because many people treat the patch as a setup step rather than as a determinant of circuit safety.

In the real world, poor patch adhesion can mean:

• higher impedance

• less effective current delivery

• greater variability in RF application

• and, in extreme cases, skin injury

The reasoning is simple: if the small catheter tip acts as a source of high current density, any meaningful reduction in the effective area of the patch moves the system in the wrong direction from a safety perspective.

7. Tip size, current density, and the false intuition of the “larger catheter”

Another concept worth revisiting is the effect of electrode size.

In non-irrigated systems, larger electrodes tolerate higher total power delivery because they dissipate more heat into the blood pool. But that does not mean they will always produce a larger lesion at the same wattage. In fact, the article shows that, at fixed power, an 8 mm tip may produce a smaller lesion than a 4 mm tip because it reduces current density and increases convective heat loss.

In other words:

a larger electrode does not automatically mean a larger lesion.

The effect depends on how much current is actually concentrated at the useful catheter-tissue interface.

Figure 8 — Effect of tip size on lesion depth.
At the same power, a smaller tip may generate greater current density and a deeper lesion than a larger tip.
Adapted from Patil S, Deshmukh AJ. Biophysics of radiofrequency Ablation: An evolving paradigm. Indian Pacing Electrophysiol J. 2025.

8. Blood flow, irrigation, and heat sink

If heating depended only on power, the problem would be simpler.

But heat does not stay in the tissue.

It is continuously dissipated.

The article highlights two extremely relevant mechanisms of heat loss: convective cooling by circulating blood and the heat sink effect of epicardial vessels. Both reduce effective tissue heating and may limit final lesion size.

This helps explain why certain anatomical regions are more resistant to durable lesion formation even when the application appears adequate.

Irrigation enters exactly at this point. It allows greater power delivery by preventing overheating at the electrode-tissue interface, reducing char formation and enabling larger lesions. In open-irrigated systems, saline cools the tip internally and externally, reducing the risk of coagulum and steam pop. The article also notes that irrigation flow and irrigant composition may alter RF delivery efficiency.

Figure 9 — Convective heat loss and heat sink effect.
Circulating blood and epicardial vessels may dissipate heat and reduce effective lesion depth even when the application appears adequate.
Adapted from Patil S, Deshmukh AJ. Biophysics of radiofrequency Ablation: An evolving paradigm. Indian Pacing Electrophysiol J. 2025.

EP Master Framework

Before stepping on the pedal, think CIRCUIT

Understanding RF biophysics is essential.
But in the lab, knowledge only becomes valuable when it turns into practical reading.

So before each application, it is worth using a simple, repeatable, technically mature framework:

CIRCUIT

Not as a clever word.
But as a way of thinking about RF correctly.

C — Contact

Is the catheter truly supported on tissue, or merely brushing the surface?
Is contact continuous or intermittent?
Does catheter orientation favor real energy delivery, or only the appearance of stability?

Without true contact, there is no efficient resistive heating.

I — Impedance

What is the baseline impedance of this circuit?
Does it make sense for the anatomy, the system, and the intended application?
During RF delivery, is impedance dropping progressively and coherently — or erratically and without meaning?

Impedance is not just a screen detail.
It is an indirect readout of energy transfer.

R — Region

What tissue am I ablating?
Thin atrium, septum, isthmus, ventricular wall, structure near a vessel, region with heat sink effect?

The same power does not mean the same lesion in different regions.

C — Complete circuit

Is the dispersive patch well adhered?
Is irrigation adequate?
Are the generator, cables, and entire circuit functioning as expected?

Radiofrequency does not happen only at the tip.
It depends on the whole circuit.

U — Unit of heat

What type of lesion am I trying to build here?
More superficial? Wider? Deeper?

Power and time only make sense when subordinated to the desired thermal effect.

I — Instability

Is the catheter truly stable or moving on the tissue with respiration or drift?
Is there enough mechanical variation to turn a single application into several incoherent micro-applications?

Instability reduces consistency even when power appears correct.

T — Time

Is application time consistent with the goal, the tissue, and the selected power?
Am I stopping too early, or prolonging beyond the point where lesion gain has already plateaued?

Time is not just duration.
It is depth, thermal conduction, and accumulated risk.

O — Objective

What is the true endpoint of this application?
Block? EGM reduction? Line formation? Substrate modification? Focal elimination?

An application without a clear objective becomes ritual.
And ritual does not create better lesions.

Practical EP Master Box

What elite technicians look at before RF

Elite technicians do not look only at watts.

They mentally check whether:

• contact is real

• baseline impedance makes sense

• the anatomical region supports that strategy

• the dispersive patch is correctly positioned and well adhered

• catheter stability allows a consistent application

• time and power match the lesion objective

In other words:

they do not just observe energy.
They read circuit, tissue, and response.

Conclusion

If I had to summarize everything in one sentence, it would be this:

a radiofrequency lesion is not created by the generator. It is created by the behavior of the circuit inside the tissue.

RF works because current crosses a small interface, generates local resistive heating, propagates heat by conduction, and simultaneously loses heat to blood flow, irrigation, and neighboring structures. The dispersive patch, impedance, tip geometry, contact, and heat dissipation are not details. They are part of the lesion.

So understanding RF is not just understanding how many watts to use.

It is understanding:

• how current travels

• where heat is generated

• where heat is lost

• and which variables truly change what happens in the myocardium

That is the foundation.

But foundation alone does not solve the lab.

Because in the real world, the next question is unavoidable:

if I understand the physics, why does the same power still produce such different applications?

That is exactly where the next stage of the series begins.

In the next article, we will move into the most practical and decisive side of radiofrequency:

• how to read impedance drop in a useful way

• when an application is good and when it only looks good

• why contact and stability change everything

• how to better interpret overheating risk

• and what truly improves lesion quality in daily practice

This will be the first premium article in the series.

Because biophysics explains the lesion.
But correct interpretation of the application is what separates routine from consistency.

EP Master

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The problem is that, in complex atrial arrhythmias, conventional mapping often fails not only because of insufficient density.
It fails because it oversimplifies the mechanism.

And when that happens, the error is not merely visual.
It becomes diagnostic.

The article Ultra-High-Definition Mapping of Atrial Arrhythmias, by Bun, Lațcu, Delassi, El Jamili, Al Amoura, and Saoudi, sits exactly at that boundary: how ultra-high definition changes the way we understand mechanism, substrate, and ablation targets in the atrium, while also improving acquisition speed and reducing dependence on manual correction. The authors report an experience with 182 maps, an average of 13,724 EGMs per map, and a mean mapping time of 16.9 ± 14.5 min per map, in scenarios including typical flutter, persistent atrial fibrillation, post-AF ablation atrial tachycardia, and focal atrial tachycardia.

In this article, we will break down what this paper truly delivers to the mapper.

What you will find in this Paper to Practice

• Why, in the atrium, low resolution not only loses detail, but can actually change mechanism classification

• What UHD truly adds — and why this goes beyond simply “more points”

• How to read focal AT, microreentry, and macroreentry without falling into the trap of the “beautiful map”

• Why scar is not just a color, and how threshold and confidence mask can hide the functional isthmus

• How UHD changes the identification of critical isthmuses, gaps, and complete lines of block

• What this article can be translated into as practical workflow in the EP lab

1. The core problem: why the atrial map lies more often than we admit

In atrial tachycardia, the most common error is not the absence of a map.
It is accepting the first interpretation of the map too early.

This paper shows, both implicitly and practically, that conventional mapping often oversimplifies mechanism. The final map appears coherent, but that coherence may be artificial. Particularly in the atrium, low resolution, excessive interpolation, wider electrode spacing, and inadequate threshold settings tend to force the electrical phenomenon into simpler categories than it actually belongs to.

The result is familiar to any experienced mapper:

• what looks like a focal source may be microreentry

• what looks like block may actually be a gap

• what looks like dense scar may be viable tissue below the current threshold

• what looks like the “end of the problem” on the map may simply be the limit of what the system was able to show

The central thesis of the article is powerful precisely because it shifts the discussion:

UHD does not make the arrhythmia more complex.
It reduces the mechanistic fiction imposed by low resolution.

Read more

• The Prospective Window Illusion: The real engineering reason why modern systems limit your mapping window.

• The Technological Paradox: Why premium algorithms (Near Field, Peak Frequency) fail if your time configuration is wrong.

• The ZEALC Paradigm: The crucial difference between the classic Deceleration Zone and true isochronal compression (85% accuracy).

• Multi-System EP Master Playbook: How to force the machine to reveal hidden potentials on CARTO 3, EnSite X, and Rhythmia.

The Cognitive Bridge: What the Atrium Can Teach the Ventricle

If you map complex arrhythmias, this scenario is already familiar: in Atypical Flutter, the signal is continuous and occupies 100% of the cycle length. As an advanced mapper, you know you cannot simply look at the screen, accept the arbitrary boundary where red meets purple (Early Meets Late), and begin ablation.

Why? Because this meeting line is often just a visual artifact that shifts depending on how you configure your reference and your Window of Interest (WOI). What you are truly searching for is the critical isthmus: the region of compressed isochrones, which denotes the true physiological slowing of conduction.

Your clinical reasoning perfectly grasps the need to manipulate the window and seek the real mechanism in the atrium. But why, when we move down to the ventricle to map the substrate of a Ventricular Tachycardia (VT), do we often set this logic aside and start overly relying on the machine’s default settings?

In this exclusive Paper to Practice, we will dissect the article “Retrospective Window of Interest Annotation Provides New Insights Into Functional Channels in Ventricular Tachycardia Substrate” (Hawson et al., JACC, 2023). We will analyze the main biases in current mapping, break down the software limitations set by manufacturers, and deliver the exact workflow on how to apply the ZEALC concept in practice.

Read more

If you have not read the first two — focused on Mechanism and Mapping Interpretation — I strongly recommend going back and reading them before continuing.

In the first article, we established why AF happens. In the second, we discussed what the map truly shows (and what it hides).

Now, we cross the line from observation to intervention.

Facing a complex mechanism and an interpreted map, the unavoidable question arises:

How should we ablate?

The map, no matter how sophisticated, does not cure AF. It is the diagnosis. The lesion is the treatment.

In this article, we will explore:

• Why PVI treats the trigger, but not the atrial myopathy

• The golden rule of proportional ablation

• How PFA shifted the thermal safety paradigm

• The cognitive trap of the posterior wall

Because in the EP lab, strategy must dictate the use of energy, not the other way around.

1. Pulmonary Vein Isolation (PVI) as the Foundation

PVI is the indisputable cornerstone of AF treatment.

As we saw in Article 1, the pulmonary veins are the cradle of the ectopic triggers that initiate the arrhythmia. By isolating them, we eliminate the initiating factor in the vast majority of paroxysmal patients.

But PVI has a clear functional limit: It treats the trigger, not the atrial myopathy.

When the patient has a structurally healthy atrium (preserved voltage in sinus rhythm), PVI is enough.

The strategy is proportional to the disease. Isolated trigger. Problem solved.

But what happens when the disease has already taken over the atrium?

2. The Golden Rule: Proportional Strategy

The biggest cognitive error in the EP lab is not missing a transseptal puncture. It is treating a sick atrium with the same strategy as a healthy atrium.

Ablation cannot be a rigid protocol. It must be proportional to the substrate.

If the sinus rhythm voltage map (our best marker for true substrate, as seen in Article 2) reveals extensive scars, diffuse low voltage, and severe remodeling, the focal PVI strategy will fail.

In this scenario, the arrhythmia no longer relies solely on the trigger. It has found a home in the diseased tissue.

A proportional strategy requires: Healthy atria = Pulmonary Vein Isolation (PVI). Sick atria = PVI + Substrate Modification (e.g., posterior wall isolation, additional lines).

Under-ablation (ignoring fibrotic tissue) leads to recurrence. Over-ablation (burning healthy tissue “just in case”) leads to loss of atrial function and procedural complications.

The substrate must dictate the target.

3. The Energy Revolution: RF vs. PFA

Historically, extensive substrate modification hit a physical and anatomical wall: thermal safety.

Ablating the left atrial posterior wall with Radiofrequency (RF) is an exercise in extreme precision. The esophagus is right there. The risk of an atrio-esophageal fistula, although rare, is catastrophic.

Thermal energy limited the strategy.

It was exactly within this anatomical and strategic vacuum that Pulsed Field Ablation (PFA) gained immense traction.

PFA is not a mapping evolution. It is an energy delivery revolution.

By being largely non-thermal and tissue-selective, PFA drastically reduces the risk to collateral structures like the esophagus and the phrenic nerve. It delivers transmural lesions incredibly fast and with a highly favorable safety profile.

But here lies the greatest cognitive trap in modern electrophysiology.

4. The Posterior Wall Trap

With PFA, isolating the posterior wall became fast, easy, and significantly safer from a thermal standpoint.

The question every mapper and electrophysiologist must ask themselves now is: “Does the fact that we can isolate the posterior wall with a safer energy profile mean we should do it in every patient?”

The answer is categorical: No.

The ease of the tool does not justify the absence of clinical indication. If the patient has paroxysmal AF and a perfectly healthy posterior wall in sinus rhythm, isolating it with PFA is treating the operator’s fear, not the patient’s disease.

A wrong lesion, delivered quickly and with safer energy, is still a wrong lesion.

Technology shifted the safety paradigm. But it will never solve the indication problem.

5. Conclusion: The Tool vs. The Thinker

The evolution of electrophysiology has given us ultra-high-density 3D systems and revolutionary selective energies.

Today, we can see the atrium with unprecedented clarity. We can destroy tissue with impeccable safety.

But the system only displays the colors. The catheter only delivers the energy.

The one who decides the exact geographical coordinates of that lesion is the human brain that interpreted the physiology, read the voltage, and understood the proportion of the disease.

Energy destroys tissue. Reasoning destroys arrhythmias.

Mapping does not replace reasoning. It exposes it.

EP Master

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