ECG Blog #538 — An Unusual Finding

The ECG in Figure-1 was obtained from an adult who complained of frequent “palpitations”.  Imagine no history is available.

Choose the BEST Answer regarding the rhythm in Figure-1:

• a) The probability of VT is ~50%.
• b) The probability of VT is ~75%.
• c) The probability of VT is virtually 100%.
• d) The rhythm is a reentry SVT (ie, AVNRT or AVRT) with aberrant conduction.
• e) The rhythm is AFlutter with 2:1 AV conduction. 

Figure-1: The initial ECG in today's case — obtained from an adult with frequent "palpitations". (To improve visualization — I've digitized the original ECG using PMcardio).

My Initial Thoughts:

The rhythm in Figure-1 is a regular WCT (Wide-Complex Tachycardia) at ~160/minute, without clear sign of sinus P waves.

• As always, the diagnosis of VT must be considered until you prove otherwise — whenever you encounter a regular WCT rhythm without clear sign of sinus P waves.

Additional factors in favor of VT include the following:

• The frontal plane axis during the WCT rhythm is indeterminate (predominantly, but not completely negative in both leads I and aVF). This degree of frontal axis deviation favors VT — because it is not consistent with either LAHB (Left Anterior HemiBlock) or LPHB (Left Posterior HemiBlock) conduction. That said, because the frontal plane axis is not “extreme” (ie, not completely negative in either lead I or aVF) — this degree of axis deviation is suggestive but not diagnostic of VT (See Rule #1 and Table-2 in ECG Blog #42-bis, among many other examples throughout my ECG Blog of "extreme" axis deviation as a sign of VT).
• QRS morphology in lead V1 is all positive, but amorphous (ie, completely lacking in the triphasic rsR’ morphology characteristic of RBBB conduction). Although this lead V1 appearance does not completely rule out RBBB conduction (from either preexisting bundle branch block or rate-related aberrant conduction) — this QRS morphology does somewhat favor VT (as per Figure-2 in ECG Blog #42-bis). That said, wide terminal S waves are seen in left-sided leads I and V6 — and there is enough of a positive r wave in lead V6 such that RBBB conduction is still possible (ie, QRS morphology in Figure-1 is suggestive but not diagnostic of VT).
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• BOTTOM Line: At this point in our assessment — We lack a definitive answer. Without knowledge of this patient’s age, awareness of his/her prior medical history (ie, Any history of underlying heart disease?) — and without the benefit of a prior ECG that might reveal similar QRS widening during sinus rhythm — We are left with the statistical reality that the clear majority of regular WCT rhythms with atypical morphologic features and without clear sign of sinus P waves (as is the case in Figure-1) — are likely to be VT (although some of these regular WCT rhythms will turn out to be supraventricular).
• Looking again at the answer choices provided at the beginning of today’s post — the BEST choice would seem to be b) The probability of VT is ~75%.

BUT — There is One More Important Clue!

• HINT #1: This KEY clue that I have not yet mentioned tells us that the answer is not choice "b" — but something else. This KEY clue is related to atrial activity.

Take another LOOK at Today's Tracing in Figure-2:

• HINT #2: Why have I numbered the beats in Figure-2?

Figure-2: I've numbered the beats in today's tracing.

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Answer: The Clue provided by Atrial Activity ...

It turns out that atrial activity is present in today's tracing — in the form retrograde P waves (highlighted by YELLOW arrows in Figure-3).

Figure-3: YELLOW arrows highlight retrograde P waves.

Retrograde P Waves:

Retrograde P waves typically appear as a negative notch or a negative deflection that occurs after the QRS in one or more of the inferior leads (ie, P wave negativity in the inferior leads being an indication that atrial depolarization is moving away instead of toward the AV Node).

• PEARL #1: The finding of AV dissociation during a regular WCT rhythm is an important clue that a regular WCT rhythm is the result of VT (as was shown in ECG Blog #133). But the finding of consistent 1:1 VA (retrograde) conduction is not AV "dissociation" — because in this case, each QRS complex is related to each neighboring QRS complex (ie, by a constant RP' interval). Both VT and reentry SVT rhythms may manifest consistent 1:1 VA conduction — such that the finding of 1:1 VA conduction during a regular WCT rhythm is of no assistance for distinguishing between VT vs an SVT rhythm (See ECG Blog #240 — for full discussion of the role that consistent 1:1 retrograde atrial activity may play in sustaining the reentry SVT rhythms of AVRT and AVNRT).
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• PEARL #2: In Figure-3, we do not see consistent 1:1 VA conduction — because retrograde P waves are not seen after each QRS complex (ie, There is no retrograde P wave seen after the QRS of beats #6, 12, 18 and 21 in Figure-3).

Looking Closer at Today's Retrograde Activity ...

To better visualize the nature of retrograde atrial activity in today's tracing — I've magnified in Figure-4 an excerpt of of beats #5-thru-15 from Figure-3.

• Can you appreciate what's happening to the RP' interval following beats #7-thru-12?

Figure-4: I've magnified beats #5-thru-15 from Figure-3. What's happening to the RP' interval following beats #7-thru-12?

Laddergram Illustration:

My laddergram in Figure-5 schematically shows that the RP' interval is progressively increasing as we move from beat #7 to beat #11 (dotted RED lines after these beats) — until we see that there is no retrograde P wave after beat #12.

• Retrograde conduction resumes after the brief pause between beats #12-13 — albeit once again with a shorter retrograde RP' interval after beat #13.
• PEARL #3: With this laddergram in Figure-5 — the YELLOW arrows represent retrograde P waves. Although subtle, the RP' interval is increasing. This is most easily appreciated by looking at the RP' interval before the retrograde Wenckebach conduction is blocked (darker BLUE double arrows seen in the rhythm strip after beat #11). Note subtle increase in the AV Nodal Tier in the angle of retrograde P wave conduction, that becomes maximal just before the P wave is dropped (dotted BLUE line in the AV Nodal Tier). And then, following the brief pause between beats #12-13 — the Wenckebach cycle begins again with the RP' interval shortening after beat #13. 

Figure-5: Laddergram showing retrograde Wenckebach conduction for magnified beats #5-thru-15 from Figure-4.

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Significance of Intermittent Retrograde Conduction

In Figure-6 — I've drawn theoretical laddergrams to illustrate why the presence of intermittent retrograde conduction that does not disturb the regularity of a WCT rhythm is virtually diagnostic of VT (Roig et al — Circulation 153(15):1171-1173, 2026 — and — Pilecky et al — Eur Heart J 7:1-2, 2023).

• Panel A (Top laddergram in Figure-6) — A PAC (beat #3) is seen after 2 normal sinus beats. If the timing is "just right" — this PAC may initiate a reentry SVT rhythm (usually either AVNRT or AVRT). But because reentry SVT rhythms are dependent on continued retrograde conduction (dotted lines during the SVT run from beat #3-thru-11) — the reentry SVT will abruptly end if for any reason retrograde conduction fails (as it does here in this theoretical laddergram after beat #11).
• Panel B (Bottom laddergram in Figure-6) — Following 2 sinus beats, a run of VT begins with beat #3. I've drawn in some different possibilities for different VA conduction ratios. It should be apparent in Panel B that regardless if 1:1 VA conduction persists (as it does from beats #4-to-9) — or is intermittent with a 2:1 VA conduction ratio (as it is from beats #9-to-12) — or manifests retrograde Wenckebach conduction with progressive RP' prolongation until retrograde conduction fails (as occurs from beats #13-to-17) — the regularity of the VT rhythm is unaffected! This proves that ventricular activation is independent of atrial activity — thereby essentially confirming VT by eliminating the possibility of a reentry SVT that is dependent on persistence of retrograde conduction with a reentry circuit.

Figure-6: Theoretical laddergrams illustrating the expected effect of intermittent retrograde conduction on a reentry SVT vs the effect on VT.

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Final Thoughts on Today’s CASE:

Intermittent block of retrograde conduction during a regular WCT rhythm (be this by retrograde Wenckebach or other intermittent VA conduction phenomenon) — is not a common occurrence. This is an advanced concept that you will not often see. But it does occur (as in today's case) — and you can detect it if looked for.

• Going back to Figure-1 (and to Figure-3) — the fact that the WCT rhythm in today's case maintains a regular ventricular rate despite the failure to conduct retrograde with every beat essentially proves that this WCT rhythm is sustained VT (with the only rare exception being the possibility of a wide junctional tachycardia that conducts retrograde).

Clinically — this subtle informative clue of intermittent retrograde conduction during a regular WCT rhythm will usually not be needed to determine optimal initial management. 

• This is because IF your patient in a regular WCT rhythm without sinus P waves is hemodynamically unstable — then electrical cardioversion will be needed regardless of whether the rhythm is VT or an SVT.
• IF on the other hand, your patient in a regular WCT is stable — then a trial of medical therapy is reasonable. And, if you do recognize intermittent retrograde conduction despite maintenance of the regular WCT rhythm — then I'd skip a trial of Adenosine, because we would then know that the WCT rhythm is VT.

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Acknowledgment: My appreciation to Khaled Elashiq, Hasan Al-Qassim and Mahmoud Al-Rahmoun (from Syria) for contributing this case.

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