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).

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 (5:30 minutes) — Initial approach to the rhythm.
  • Part 2 (4:00 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 (5:30 minutes) — Initial approach to the rhythm.

Part 2 Video (4:00 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.

 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.


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




  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

    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 ( ) — 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! — :)

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

    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.