Friday, January 26, 2024

ECG Blog #414 — What Kind of AV Block?

I was sent the ECG in Figure-1 — without the benefit of any history.

  • What kind of AV block is present in Figure-1

Figure-1: The initial ECG in today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Approach to Today’s Tracing:
As always — I favor beginning assessment with a quick look at the long lead rhythm strips at the bottom of the tracing. By the Ps, Qs, 3R Approach (which I review in ECG Blog #185):
  • P waves are present — and best seen in lead V1.
  • The QRS complex is narrow in all 12 leads.
  • The rhythm is not Regular. The ventricular Rate varies.
  • The 5th parameter of the Ps,Qs,3R Approach — is the 3rd R, which recalls Related” — or determining if P waves are (or are not) related to neighboring QRS complexes. This last parameter is best assessed by labeling P waves in the long lead V1 rhythm strip (which I have done in Figure-2).

Take a LOOK at Figure-2 — in which RED arrows highlight the P waves
  • How would YOU describe the regularity (or lack thereof) of P waves in today's rhythm?
  • Are all of the P waves originating from the SA node?
  • Are all P waves conducted to the ventricles?

Figure-2: I have labeled P waves with RED arrows in today's rhythm.

WHY are P waves so Irregular in Figure-2?
Now that we have labeled all P waves in Figure-2 with RED arrows — Isn't it much easier to appreciate the irregular irregularity of the atrial rhythm?
  • Most of the time — it is lead II that provides best visualization of sinus P waves. That said — this is not the case in today's rhythm, in that lead V1 provides more consistent visualization of atrial activity in Figure-2.

What We Know about the Rhythm ...
  • At least some of the P waves in Figure-2 appear to be sinus P waves that are conducted to the ventricles — namely the P waves before beats #3,7,8 and 9. I say this because each of these beats is preceded by a similar-looking P wave with a constant and normal PR interval.
  • Other P waves are clearly not conducted to the ventricles — because the PR interval in front of beats #1,2,5,6 and 11 appears to be too short to conduct. The fact that the R-R intervals that precede these beats are virtually the same (ie, ~5 large boxes in duration) — suggests that beats #1,2,5,6 and 11 are all junctional escape beats at an appropriate AV nodal escape rate of ~60/minute (ie, between the usual 40-60/minute range expected with junctional escape).

I am less certain about the origin of beats #4 and 10 — because these beats occur so much earlier-than-expected

  • The timing of beats #4 and 10 suggests that these beats may be PACs (Premature Atrial Contractions). Alternatively — it could be that each of the RED arrow P waves in Figure-2 is a sinus P wave that occurs in association with a very marked sinus arrhythmia
  • PACs typically manifest a different P wave morphology than sinus-conducted P waves, depending on how near or how far the site of the PAC is with respect to the SA node. For example, PACs arising from very close to the SA Node — may look very similar to sinus-conducted beats in most (if not in all) 12 leads.
  • Balanced with the above considerations regarding potential variation in P wave morphology — is the clinical reality that a certain amount of inherent variation in P wave morphology is common in sinus rhythms (See Pearl #6 in ECG Blog #413).

  • BOTTOM Line: I do not believe it possible to determine what the etiology of beats #4 and 10 is with any certainty from Figure-2. We simply cannot tell from this single tracing if the slight variability in P wave morphology that we see in lead II (not so much in lead V1) is "real" vs the effect of some normal variation in P wave morphology expected with sinus rhythms. 

Putting It All Together  in a Laddergram:
A picture "is worth 1,000 words" — so it's easier to illustrate the above description by means of a laddergram.
  • As I note above — it's not possible from this single tracing to determine with any certainty if beats #4 and 10 represent PACs — vs participants in a marked sinus arrhythmia. For the purpose of simplicity (ie, of not having to assume the additional element of there also being PACs in today's rhythm) — I assumed all RED arrow P waves were sinus impulses (ie, beats #3,7,8,9 — also being sinus-conducted beats).
  • Beats #1,2,5,6 and 11 represent junctional escape beats at an appropriate escape rate of ~60/minute (the P waves occurring just before these beats manifesting a PR interval too short to conduct).

  • NOTE: There is transient AV dissociation in today's rhythm — in the form of sinus P waves not related to neighboring QRS complexes (ie, the P waves before beats #1,2,5,6,11 manifesting a PR interval too short to conduct). As discussed in detail in ECG Blog #192 — there are 3 Causes of AV Dissociation: i) By "default"; ii) By "usurpation"; and, iii) As a result of AV block. Regardless of whether there is marked sinus arrhythmia vs short pauses following PACs — the cause of the transient AV dissociation seen in today's rhythm is a form of "default" — because the reason junctional escape beats #2,5,6 and 11 occur is a result of the slightly longer R-R intervals that precede each of these beats.
  • BUT — there is no evidence of any AV block in today's tracing — because we never see on-time sinus P waves that should conduct, yet fail to do so. 

  • P.S. (Beyond-the-Core): We were not provided with any history in today's case. Another possible explanation for today's rhythm might be a marked increase in vagal tone (as per the case presented in ECG Blog #61) — which could result in not only marked sinus arrhythmia — but also some variation in the PR interval of sinus-conducted beats (ie, It looks like the PR interval before conducted beats #4 and 10 may be slightly longer than the PR interval of other sinus-conducted beats).

Figure-3: Laddergram illustration for today's rhythm. The laddergram suggests there is marked sinus arrhythmia — that results in transient AV dissociation by "default" because junctional "escape" beats #1,2,5,6,11 occur before the delayed sinus impulses have an opportunity to conduct. The junctional escape rhythm is set at ~60/minute, such that these junctional escape beats are appropriate (and do not represent any form of AV block! ).


Acknowledgment: My appreciation to Danilo Franco (from Italy) for the case and this tracing.




Related ECG Blog Posts to Today’s Case:

  • ECG Blog #185 — Reviews my System for Rhythm Interpretation, using the Ps, Qs & 3R Approach.
  • ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 90 laddergram cases — many with step-by-step sequential illustration).

  • ECG Blog #192 — The Causes of AV Dissociation.
  • ECG Blog #191 — Reviews the difference between AV Dissociation vs Complete AV Block.

  • ECG Blog #389 — ECG Blog #373 — and ECG Blog #344 — for review of some cases that illustrate "AV block problem-solving".

  • ECG Blog #267 — Reviews with step-by-step laddergrams, the derivation of a case of Mobitz I with more than a single possible explanation.

  • ECG Blog #61Vagotonic block

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