Saturday, January 30, 2021

ECG Blog #189 (ECG MP-6) — Is this Some Type of Wenckebach? (SVT - PACs - Flutter - ATach - Laddergram)

I thought today’s case would be an excellent follow-up to my last post, which was ECG Blog #188 — in which I reviewed a step-by-step approach for drawing laddergrams, with links to more than 20 clinical examples of detailed laddergram analysis.

  

BForewarned! — Laddergram analysis of the arrhythmia that appears in Figure-1 is complex. I fully acknowledge that it took me considerable time to figure out the mechanism of this arrhythmia. That said:

  • As interesting as the laddergram of the ECG mechanism is — the laddergram is not the “Take-Home” Point in today's case. So, while true that only more advanced interpreters may figure out the complete answer to this case — what is much MORE important, is the problem-solving approach to this arrhythmia.
  • Regardless of your ECG experience level — I cover many PEARLS in arrhythmia interpretation in this post that are relevant to any clinician charged with interpreting ECG rhythms.
  • I walk you through a step-by-step approach to how I derived my laddergram for this tracing in the 2-part ECG Video that is found just below Figure-1 (ECG Media Pearl #6). Please spend a few minutes to WATCH this ECG Media PEARL before you check out my detailed laddergram in Figure-2 at the bottom of this post.
  • Your COMMENTS on this case are WELCOME!

 

Today's CASE: Interpret the ECG shown below in Figure-1:

  • Does this rhythm represent some form of Wenckebach?


Figure-1: How would YOU interpret this 2-lead rhythm strip? Are you up for the challenge of drawing a laddergram? NOTE: No history is available on the patient — but the history is really not essential for assessing this arrhythmia (See text).


The ANSWER:
Detailed review of my approach to this arrhythmia is covered in the 2-part video of my ECG Media PEARL #6: 

NOTE: Because of video size limitations — I’ve had to break this video into 2 parts:

  • Part 1 (6:50 minutes) — Initial approach to the rhythm.
  • Part 2 (5:00 minutes) — Deriving the laddergram.
  •   — NOTE: I've reproduced the final laddergram in today's case in Figure-2, which is shown below the videos.



Part 1 Video (6:50 minutes) — Initial approach to the rhythm.




Part 2 Video (5:00 minutes) — Deriving the laddergram.



Review of the Final Laddergram:

The final laddergram in today's case that was derived in Part-2 of the Video is shown below in Figure-2:


Figure-2: Review of the final laddergram.


Laddergram Review: The underlying rhythm in Figure-2 is ATach (Atrial Tachycardia), as shown by the RED arrows that represent regular-occurring P waves.

  • The first finding to note is group beating — in that a repetitive pattern of alternating long-short intervals is seen. That this is not due to chance should be obvious from the fact that shorter R-R intervals are all almost the same duration — and — longer R-R intervals are also almost all of the same duration.
  • Many P waves are not conducted. The marked difference in duration between longer and shorter R-R intervals is what suggested to me that there might be dual-level block within the AV Nodal Tier. (Part-2 of the video showed why the more common single level block was unlikely in today's case.)
  • Many P waves in Figure-2 are conducted! We know this to be true — because there are 2 sets of PR intervals that continually repeat. That is — the relatively long PR interval that is seen before beat #1 is the same as the PR interval before beats #3, 5, 7 and 9.
  • And, the same very short PR interval that we see before beat #2 — is seen to repeat before beats #4, 6 and 8. However, this PR interval looks too short to conduct. 
  • KEY Point for Constructing Laddergrams: Much of the time with faster atrial rhythms — the P waves that conduct to the ventricles are not those P waves that are closest to the next QRS complex. This is a result of "concealed" conduction that delays forward conduction of selected atrial impulses.
  • As a result of the above observations — I contemplated a number of different possibilities for the mechanism of conduction within the 2 levels of the AV Nodal Tier. The one that "fits" best is shown in Figure-2 — in which I postulate 4:3 AV Wenckebach conduction within the upper AV Nodal Tier — and — 3:2 AV Wenckebach conduction within the lower AV Nodal Tier.

BOTTOM
LINE:
 The important point to recognize, is that the combination of group beating — dropped beats but regular-occurring P waves — and — the identical PR interval preceding the QRS complex that ends each of the pauses in this tracing — overwhelmingly suggest that some type of Wenckebach conduction is present.
  • The rhythm is not 3rd-degree (complete) AV Block — because identical PR intervals prior to multiple QRS complexes (ie, prior to beats #1, 3, 5, 7 and 9) in this regular pattern prove that there is conduction of at least some beats.
  • The rhythm is not Mobitz II 2nd-degree AV Block — because the PR interval never stays the same for consecutively conducted beats.
  • This patient may not necessarily need a pacemaker — because the mechanism of the conduction defect is AV Wenckebach (albeit at 2 levels within the AV Node) — and the overall ventricular response is not excessively slow.
  • Additional clinical details would be needed to better appreciate the overall longterm outcome.

  • P.S. — I intentionally did not comment regarding QRS duration on this tracing because I thought it impossible to tell from the 2 leads given (especially in view of the suboptimal resolution) — if the QRS was wide or narrow. The terminal S wave in left-sided lead V6 suggests that there may be some right-sided delay — but whether this reflects incomplete vs complete RBBB is impossible to tell without better resolution and a 12-lead tracing. That said — regardless of QRS width, my interpretation of the rhythm would be the same.

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Acknowledgment: My appreciation to Abo Ali Mohamed (from Libya) for the case and this tracing.

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4 comments:

  1. Very interesring tracing. Grateful for your time and effort explaining it. My question is why isn't it possible that the second blocked P wave is due to pause dependent block (caused by the first blocked P) in the INFRANODAL tissue? If this is the case, wouldn't this patient need a pacemaker?
    Thanks again for sharing

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    1. @ Ahmed — Excellent thought! Phase 4 block (sometimes called “pause-dependent” block) is a mechanism of paroxysmal AV block in patients with diseased His-Purkinje systems. It is typically initiated in such patients by premature beats (ie, PACs or PVCs) — however, in this nice review by Uhm et al ( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6003538/ ) — they indicate that “various [other] beats or rhythms can cause phase 4 block”. So, as is OFTEN the case — complex arrhythmias (especially when there is associated AV block) often have more than a single potentially correct explanation — and perhaps the initial laddergram I drew (which I show in Part-2 of my above ECG video) could be correct after all — IF this particular patient had severe underlying conduction system disease and was manifesting Phase 4 block …

      I still favor my laddergram that I show in Figure-2, because it seems to work so well … but without additional monitoring strips and more history — I can NOT rule out the possibility of Phase 4 (pause-dependent) block as you suggest.

      THANKS again for your comment! — :)

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  2. great teaching. Wonder why it is atrial instead of sinus tachycardia?

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    Replies
    1. @ JJ — Thanks for your EXCELLENT question. I decided not to go into detail about this — because of the problems with poor resolution (virtually impossible to be certain of the ECG grid, therefore the rate) — and because of lack of other leads with P waves of sufficient amplitude to be able to distinguish P wave morphology from sinus vs ectopic atrial …

      As stated — your question is EXCELLENT — and it raises the point that sometimes it is very difficult (if not impossible) from a single tracing to tell if we are dealing with “sinus tach” vs ectopic ATach. So, IF you see clear difference in P wave morphology between what you KNOW is a sinus P wave — and especially IF you see other clear features of ectopic ATach (ie, gradual, progressive increase in speed = “warm-up” from an ectopic atrial pacemaker — followed by “cool-down” as the rate gradually slows) — then you KNOW. But in a case like the one we have — I did NOT know for certain.

      So, I decided to suggest that the underlying atrial rhythm is ectopic atrial tach. ATach does not have to be overly “fast” (ie, could be close to 100-110/minute in a symptomatic patient) — and ATach more than sinus tach in the absence of acute inferior MI would be more likely to produce Wenckebach conduction … but I fully acknowledge that I do NOT know for certain that this is ATach and not sinus tachycardia (which is why I indicated in my “Bottom Line” that additional clinical details would be needed to know better how to approach this patient.

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