Thursday, September 16, 2021

ECG Blog #251 (65) — HOW Does the Cycle End?

The ECG in Figure-1 was obtained from an 85-year old man who presented with "dizziness".



  • Can you explain what is happening with each beat on this tracing?


Figure-1: 12-lead ECG and long lead II rhythm strip from a 85-yo man with dizziness (See text).



My Sequential Approach to Today's Case: This is indeed a challenging tracing! I allowed myself a brief look at the 12-lead ECG before delving into details of the complex lead II rhythm strip — since I wanted to first ensure that the cause of the rhythm was not an acute event.

  • All of the beats on this tracing are narrow — therefore supraventricular! This means that the "usual" criteria for chamber enlargement and ST-T wave assessment can be applied.
  • Several criteria for LVH are clearly satisfied — based on very deep S waves in V3,V4 (Peguero Criteria) — and very tall (overlapping) R waves in V5,V6  (For more on LVH — See ECG Blog #245 — with attention to the Audio Pearl and Figures in the Addendum for specifics).
  • There is modest T wave inversion in a number of lateral leads (ie, leads I, aVL and V6) that is consistent with LV "strain" and/or some ischemia — but nothing that looks like an acute cardiac event! 


My Sequential Approach to the Rhythm:

As always — I began my systematic approach to the rhythm with assessment of the PsQs and 3Rs (as discussed in detail in ECG Blog #185).

  • P waves — A fairly (but not completely) regular atrial rhythm is seen (RED arrows highlighting sinus P waves in Figure-2).
  • As previously stated — the QRS is narrow — which confirms that the rhythm is supraventricular.
  • The rhythm is obviously not completely Regular — so the Rate varies. Regarding the 3rd R (ie, the "Relation" between P waves and neighboring QRS complexes) — it looks as if most of the sinus P waves (RED arrows) may be related to neighboring QRS complexes — but we need to look closer!



Figure-2: I've added RED arrows for the sinus P waves that were seen in Figure-1.


Additional QUESTIONS:

  • Take another look at Figure-2. In addition to the sinus P waves (RED arrows) — Are there other signs of atrial activity in this tracing?
  • Of all of the P waves that are seen in Figure-2  WHICH ones do you suspect are (and are not) being conducted?



The Case Continues:

In additon to the sinus P waves (RED arrows) — I've labeled the signs of other atrial activity with a YELLOW and a BLUE arrow (Figure-3). The challenge is to figure out the relationship (if any) to neighboring QRS complexes for each of the P waves in Figure-3.

  • PEARL #1: When faced with a complex rhythm that manifests a number of different features (such as the rhythm strip shown in Figure-3) — it is sometimes easiest to begin by looking to see IF there are any features that I can definitely explain. I then look to see if there is an "underlying rhythm" — and, I save for last the interpretation of those features that I am not yet certain about.
  • In Figure-3 — Although a sinus P wave does precede beat #1 — this PR interval is clearly too short to conduct. Since QRS morphology of beat #1 is narrow and virtually identical to the QRS morphology of all other beats on this tracing — beat #1 is not sinus-conducted. Note the long pause that precedes beat #1. This suggests that beat #1 is probably a junctional escape beat.
  • No QRS complex follows the negative P wave highlighted by the YELLOW arrow in Figure-3. This suggests that there is no "forward conduction" of this negative P wave (that looks very different in shape than the sinus P waves).
  • The P wave highlighted by BLUE arrow also looks very different than the sinus P waves (RED arrows) in this tracing. As a result — beat #6 is not sinus-conducted. Note the pause (ie, of 7.5 large boxes = 1.5 second) between beat #6 and the preceding QRS complex. Given that QRS morphology of beat #6 is narrow and virtually identical to the QRS morphology of all other beats on this tracing — this suggests that beat #6 is most probably either an ectopic atrial or junctional escape beat

PEARL #2: The most common form of 2nd-degree AV block is Mobitz Type I ( = AV Wenckebach— which is diagnosed by the finding of a progressively increasing PR interval until a beat is dropped. There follows a brief pause — after which the next Wenckebach cycle begins with a PR interval that is shorter than the longest PR interval that was seen right before the pause.

  • What is not commonly appreciated — is that there can be other ways that a Wenckebach cycle ends (See the Audio Pearl below).


Figure-3: I've labeled the additional signs of atrial activity with a YELLOW and a BLUE arrow.


  — The Case Continues BELOW today's Audio Pearl ... —



NOTE: Some readers may prefer at this point to listen to the 6:20-minute ECG Audio PEARL before reading My Thoughts regarding the Laddergram that I derive below.



Today's ECG Media PEARL #65 (6:20 minutes Audio) — Reviews the different ways that an AV Wenckebach cycle may end (as well as reviewing the "Footprints" of Wenckebach).




Deriving the LADDERGRAM:

The complex mechanism of today's case is best explained by step-by-step derivation of a Laddergram (See ECG Blog #188 for review on how to read and/or draw Laddergrams).

  • NOTE: Today's case provides an example in which I needed to construct a valid laddergram explanation in order to "solve" the arrhythmia.
  • Sequential legends over the next 7 Figures illustrate my thought process as I derived this laddergram.



Figure-4: It is usually easiest to begin a laddergram by marking the path of sinus P waves through the Atrial Tier (RED lines drawn directly below the onset of each of the sinus P waves — as shown by the large GREEN arrows). Note that these RED lines in the Atrial Tier are nearly vertical — since conduction of sinus P waves through the atria is rapid. Note also that the P-P interval between successive sinus P waves (vertical RED lines) is equal!


Figure-5: The most challenging part of most laddergrams is construction of the AV Nodal Tier — so I generally save that for last. Therefore, after drawing sinus P waves into the Atrial Tier — I prefer to next add indication of all narrow QRS complexes into the Ventricular Tier. The large GREEN arrows show that my landmark for QRS complexes in the laddergram is the onset of the QRS. Note that the RED lines in the Ventricular Tier are also nearly vertical — since conduction of these narrow QRS complexes through the ventricles is rapid.


Figure-6: It's time to begin "solving" what we can in the laddergram. I do this by connecting those P waves in the Atrial Tier that might logically be conducting to narrow QRS complexes in the Ventricular Tier (slanted BLUE lines within the AV Nodal Tier)Doesn't it appear as if the angle of slant for these BLUE lines in the AV Nodal Tier is increasing?


Figure-7: Caliper measurement of the PR intervals for beats #2-thru-5 confirms progressive increase in the PR interval (from 340 to 440 msec.). The PR interval shortens before beat #7 (to 340 msec.) — and then lengthens again, to 410 msec. before beat #8. This is Wenckebach conduction! But unlike a typical Mobitz I block — the Wenckebach cycle from beats #2-thru-5 does not end with an on-time non-conducted sinus P wave. 


Figure-8: It's time to assess the "missing parts" of our laddergram. As we previously suggested — the PR interval of the sinus P wave preceding beat #1 is clearly too short to conduct. This strongly suggests that beat #1 is a junctional escape beat. We schematically represent this with a BLUE circle within the AV Nodal Tier that conducts retrograde (dotted BLUE line), thereby preventing forward conduction of this first sinus P wave. Since beat #6 is preceded by a dfferent-looking (negative) P wave with a short PR interval — this suggests beat #6 is a low atrial escape beat (BLUE circle within the Atrial Tier).


Figure-9: The remaining "missing part" of our laddergram relates to the small-amplitude negative P wave deflection highlighted by the YELLOW arrow. I introduced the concept of Echo beats in ECG Blog #239. The dotted BLUE lines propose retrograde conduction originating from the path of the sinus P wave that precedes beat #5 (as it passes through the AV Nodal Tier) as the source of this negative P wave. This is a logical proposal — because Echo beats are most likely to originate following PR interval prolongation (as discussed in detail in ECG Blog #239).


Figure-10: For clarity — I've labeled the unusual elements in this tracing. The rhythm strip begins with a junctional escape beat (JE), which is beat #1. There follows a Wenckebach cycle for beats #2-thru-5, with progressively increasing PR intervals. Instead of ending with a non-conducted on-time P wave — this short run of Mobitz I, 2nd-Degree AV Block is terminated by an Echo beat (YELLOW arrow). Presumably — this Echo beat resets the SA Node — so that instead of a sinus P wave, the 1.5 second pause between beats #5-and-6 ends with a low atrial escape beat (AE). This finally allows enough time for the SA Node to recover (RED arrow preceding beat #7) — as the next Wenckebach cycle begins (ie, with beats #7 and 8).


Conclusion of Today's Case:

In addition to Mobitz I 2nd-Degree AV Block — this 85-year old man with dizziness had significant bradycardia with pauses on additional monitoring. A permanent pacemaker was placed.




Acknowledgment: My appreciation to 林柏志 (from Taiwan) for the case and this tracing.




Related ECG Blog Posts to Today’s Case: 

  • ECG Blog #185 — Reviews the Ps, Qs & 3R Approach to Systematic Rhythm Interpretation.
  • ECG Blog #164 — Reviews a case of Mobitz I 2nd-Degree AV Block, with detailed discussion of the "Footprints" of Wenckebach.
  • ECG Blog #236 — Reviews in our 15-minute Video Pearl #52 how to recognize the 2nd-Degree AV Blocks (including "high-grade" AV block)
  • ECG Blog #186 — Reviews when to suspect 2nd-Degree, Mobitz Type I.
  • ECG Blog #239 — Reviews the concept of Echo Beats, and its clinical applications (showing another case of Wenckebach terminated by Echo beats).
  • ECG Blog #188 — Reviews how to understand (and how to drawLaddergrams!


  1. Thanks Professor for your detailed interesting interpretation. I just have a query, that is the probability that negative P wave after beat# 5 is nonconducted PAC. How can we rule out this probability? And if there is echo beat after beat#5, then why there are NOT echo beats after beats# 2,3,4?.

    1. Excellent Question! I discuss Echo beats in detail in Blog #239 ( ) — and this is the Subect of the Audio Pearl in this Blog 239. So if you look at my explanation for Figure-9 (above, in today Blog #251) — Echo beats are most likely to originate following PR interval prolongation for the simple reason that the longer preceding PR interval allow MORE OPPORTUNITY for this retrograde conduction to occur. So while impossible to obtain “absolute proof” that the YELLOW arrow is an “Echo beat” (and not a PAC) — the fact that we do not see a similar negative P wave anywhere else on this tracing EXCEPT following the longest PR interval strongly suggests that my laddergram IS the most likely etiology for this rhythm — :)