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.
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| 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). |
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, among many other examples throughout this 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). 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).
- 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?
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| Figure-2: I've numbered the beats in today's tracing. |
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| Figure-3: YELLOW arrows highlight retrograde P waves. |
- 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).
- 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).
- Can you appreciate what's happening to the RP' interval following beats #7-thru-12?
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| Figure-4: I've magnified beats #5-thru-15 from Figure-3. What's happening to the RP' interval following beats #7-thru-12? |
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| Figure-5: XXXX |
With this laddergram — YELLOW arrows are retrograde P waves. Although subtle, the RP' interval is increasing (most easily appreciation by looking at the RP' interval before the retrograde Wenckebach conduction is blocked (darker BLUE double arrows). Note subtle increase in the angle of retrograde p waves that are maximal just before the P wave is dropped (BLUE lines in AV Nodal Tier) — and then the RP' interval shorts again beginning with beat #13.
I must cite the following as PROOF that retrograde Wenckebach confirms VT (with exception of Junctional tach with wide QRS)
— Roig et al — Circulation 153:1171-1173, 2026
https://www.ahajournals.org/doi/epub/10.1161/CIRCULATIONAHA.126.080138
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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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Case from Khaled Ash from Damascus Syria
— "An unusual finding ..." —
— NOTE — This was a very complex case, but I will only give a brief 1-line history — saying this is a young adult complaining of frequent palpitations
— The Dx is Fascicular VT (NOT RBBB aberrancy given atypical features)
— Me to ask, "How certain are you of the diagnosis? HINT: Is there atrial activity? If so — Is there something unusual about atrial activity?"
— Quote article — Retrograde Wenckebach in the setting of a persistently regular WCT is very strongly suggestive of VT (even more so than AV dissociation) — since if the rhythm was supraventricular, it would change the R-R interval!
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Hard to believe this patient was markedly hypothyroid!
That said — I want to make an ECG Blog of this case — and I've spent a few hours fixing up the tracing, since the original was rotated and missing a complex in lead V3. I'll add a laddergram.
It is RARE to see VT with retrograde Wenckebach like this — and as per the attached article, seeing this is diagnostic for VT (with the very rare exception that you could theoretically see retrograde Wenckebach in junctional tach with a wide QRS from BBB).
I'm happy to acknowledge both you and your colleague from who this case comes. That said — I don't feel obligation to necessarily acknowledge your colleage — since I have totally redone the tracing (attached) using PMcardio, and since I will NOT be using clinical details of the case (I'll only give a VERY simple 1-line history). I am HAPPY to acknowledge you if you like (Khaled Ash — from Damascus, Syria) — or I could make the case anonymous, as you prefer.
I've done lots of Fascicular VT cases — so the main reason for doing an ECG Blog is the rare example of retrograde Wenckebach that essentially PROVES this is VT (I suspect many readers won't know what to do with the concept of retrograde Wenckebach.
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Today's CASE:
Today's initial ECG makes for a worthy addition to the above list of problematic tracings — as I found myself wavering back-and-forth between VT vs an SVT with either rate-related aberrant conduction or preexisting bundle branch block. I ended up opting for "the default" (ie, When in doubt — assume VT until proven otherwise) — but I was in noway certain.
- And then I saw the one clue that all-but-confirmed the diagnosis.
Take another LOOK in Figure-1 below, at today's initial ECG. As per Dr. Smith's above discussion — ECG #1 manifests the following:
- A regular WCT (Wide-Complex Tachycardia) at a rate of ~175/minute, without clear sign of sinus P waves (ie, There is no clearly upright P wave with fixed PR interval in lead II).
- Looking for P waves in other leads is problematic because of the fast rate that results in what looks like the terminal upright portion of the T wave mimicking what otherwise might be thought to represent P waves in other leads (ie, a sinus P wave in lead I should not be taller than the sinus P wave in lead II — and if the rhythm is sinus, we should not see larger P waves in leads V5,V6 than in lead II — as highlighted by BLUE arrows in Figure-1).
- The QRS appears to be very wide (ie, 0.16 second — as suggested by the parallel RED lines in Figure-1) — which favors VT.
- But QRS morphology is potentially consisent with RBBB (Right Bundle Branch Block) + LAHB (Left Anterior HemiBlock) — which together with the marked fragmentation (ie, notching) of the QRS in multiple leads could represent as yet undiagnosed underlying heart disease in today's 38-year old patient.
- BOTTOM Line: I found myself unable to decide with any degree of certainty between VT vs some form of SVT. I knew statistics always favor VT as the most common cause of a regular WCT rhythm without clear sign of sinus P waves (with need to treat the patient accordingly until we can prove otherwise). But — I couldn't rule out aberrant conduction or a previously present bifascicular block with a supraventricular etiology of today's tachycardia.
- If we could find a previous ECG for comparison, we might be able too arrive at a more definitive diagnosis — but no prior tracing was available.
- Are we suspicious enough of VT in Figure-1 to apply synchronized cardioversion as our initial intervention?
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| Figure-1: I've reproduced today's initial ECG. |
— And then I saw the definitive clue ...
- HINT: Looking closely at Figure-1 — although clear indication of sinus P waves is not seen, perhaps there is otherevidence of atrial activity on this tracing? Do YOU see it?
= = =
Answer:
I believe there are retrograde P waves in today's initial ECG (YELLOW arrows in Figure-2). But unlike the 1:1 VA conduction that is most commonly seen when retrograde P waves are present — we only see retrograde P waves every-other-beat!
- Retrograde P waves are recognized by the presence of a consistent small negative deflection appearing after the QRS in one or more of the inferior leads. In Figure-2 — this is best seen in lead III, but it is also seen in leads IIand aVF as a consistent deepening of the terminal part of every-other QRS complex (YELLOW arrows in lead aVF and the long lead II).
- Panel A (Top laddergram in Figure-3) — 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-3) — 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.
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| Figure-2: XXXXXX |
Significance of Intermittent Retrograde Conduction
In Figure-3 — 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), 2026 — and — Pilecky et al — Eur Heart J 7:1-2, 2023).
- Panel A (Top laddergram in Figure-3) — 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-3) — 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.
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| Figure-3: Theoretical laddergrams illustrating the expected effect of intermittent retrograde conduction on a reentry SVT vs the effect on VT. |
Final Thoughts on Today’s CASE:
Intermittent block of retrograde conduction (be this by retrograde Wenckebach or other intermittent VA conduction phenomenon) is not a common occurrence. But it does occur (as in today's case) — and you can detect it if looked for.
- As seen in Figure-2 — the fact that the WCT rhythm maintains a regular ventricular rate despite a failure to conduct retrograde with every beat proves that the WCT rhythm is sustained VT (with the rare exception of a wide junctional tachycardia that conducts retrograde).
- Clinically, in today's case — this subtle informative clue was not needed to determine an optimal management plan because the patient's unstable hemodynamic status dictated the need for electrical cardioversion regardless of whether today's initial rhythm was VT or an SVT.
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