Thursday, December 9, 2021

ECG Blog #267: Group Beating (a Changing QRS)

You are given the 2-lead tracing shown in Figure-1 without the benefit of any history.

  • What is the rhythm?
  • Is there complete AV block? 

Figure-1: A long-lead rhythm strip (showing simultaneously-recorded leads I and II) — but without any history. What is the rhythm?



There are 2 Parts to my Discussion of today's case:

  • PART-1: This is "the Quick Answer" — in which I'll review how in less than 30 seconds, I was able to determine the essentials for the rhythm diagnosis.
  • PART-2: My step-by-step Laddergram derivation — in which I illustrate the mechanism I favor for the etiology of today's rhythm (as well as a possible alternative explanation). I'll emphasize — Regardless of your comfort level with laddergrams, you will be able to follow my step-by-step laddergram illustrations!



NOTE #1: Some readers may prefer at this point to listen to the 6:10 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-75).


Today’s ECG Media PEARL #75 (6:10 minutes Audio) — Reviews how to tell IF a Junctional (AV Nodal) Rhythm is pathologic or appropriate?



PART-1: My "Quick Answer" for the Rhythm Diagnosis

  • Although this is a challenging tracing — application of the basic principles for arrhythmia interpretation should enable accurate and rapid assessment.
  • KEY Technical Point: As is common these days with smart phone photographs of ECGs sent via the internet (with the goal to expedite receiving a 2nd opinion) — there are technical shortcomings to this tracing (Figure-2). The tracing is unevenly angled (Note the non-vertical RED lines in Figure-2, which I drew parallel to the nearest ECG grid line). This angling of the smart phone photo leads to some distortion of certain measurements (Note that the size of the little RED boxes in Figure-2 is not the same!). This effect from angling is relevant to today's case — because the BEST clues for determining IF certain beats on a tracing are conducted and IF escape beats are present — are dependent on precise measurement and comparison of PR and R-R intervals. Despite these technical shortcomings of today's tracing — I feel this rhythm strip still is adequate for interpretation.

Figure-2: For clarity — I've numbered the beats. The slightly slanted RED lines and different sizes of the RED boxes highlight technical shortcomings of this tracing (See text).


MY Approach to Today's 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).

  • When applying the Ps, Qs, 3R System — You do not have to go in sequence. In today's case — I looked first at QRS width. Although QRS morphology looks similar for all beats in lead I — there is a definite difference in QRS morphology for every-other-beat in lead II. That said — the QRS does not look wide, which suggests that the rhythm is supraventricular.
  • The rhythm is not Regular. That said — there is a "pattern" to this rhythm that repeats, in the form of Group Beating (ie, alternating longer-with-shorter R-R intervals). Although slanting of the rhythm strip (especially near the beginning of the tracing) distorts measurements — the R-R intervals are shorter between beats #1-2; 3-4; 5-6; 7-8; 9-10; and 11-12.
  • Since the rhythm is not regular — the Rate of the ventricular rhythm will vary. The overall rate appears to be between 60-75/minute
  • P waves are present — and, are best seen in lead II. Although some of the P waves in Figure-2 are partially hidden within the initial part of the QRS complex — we can verify that the atrial rhythm is regular by identifying 2 P waves that we can clearly see in a row (ie, the 2 PINK arrows at the beginning of the tracing in Figure-3) — and then setting calipers to this P-P interval, which can then be "walked out" through the entire rhythm strip (RED arrows in Figure-3). 

Figure-3: To assess for regularity of the atrial rhythm — Look for 2 P waves you can clearly identify that occur in a row (PINK arrows). Set your calipers to this P-P interval — and note that you can walk out regular P waves throughout the entire rhythm strip (RED arrows).


The last parameter to assess in the Ps, Qs, 3R Approach — is whether at least some P waves are Related to neighboring QRS complexes?

  • Look in front of each QRS complex in Figure-3 to see IF the QRS is preceded by a P wave? IF so — Are there any PR intervals that repeat?


The RED arrows in Figure-4 show that although the PR interval is prolonged (to ~0.28 second) — this PR interval is the same before every-other-beat (ie, before beats #2, 4, 6, 8, 10 and 12). This tells us these even-numbered beats are all being conducted.

  • PEARL #1: The finding of group beating in a rhythm in which P waves are regular throughout and in which we clearly see that a number of these P waves are conducting — is most often the result of Wenckebach conduction. Awareness of this Pearl literally allowed me to suspect some form of AV Wenckebach within seconds of seeing today's tracing.


Figure-4: A constant PR interval is seen to precede every-other-beat (RED arrows). This confirms that beats #2, 4, 6, 8, 10 and 12 are all conducted (See text).

PEARL #2: When confronted with a challenging arrhythmia, in which there are findings that are EASY to interpret, as well as other findings that are not easy to interpret — Begin with those ECG findings about which you can be certainSAVE those ECG findings about which you are not yet certain for last.

  • Thus far, in today's tracing — we know that the QRS is most probably narrow (at least based on the 2 leads that we are provided with). We also know that there is group beating — that there are regular P waves throughout the tracing (Figure-3) — and, that the P waves highlighted by RED arrows in Figure-4 are all conducted, albeit with a prolonged PR interval.


Look now at the P waves highlighted by WHITE arrows in Figure-5. These P waves coincide with the initial portion of the QRS for beats #3, 5, 7, 9 and 11 — and manifest a PR interval that is clearly too short to conduct.

  • Does this mean that beats #1, 3, 5, 7, 9 and 11 must be junctional escape beats?
  • OR — Is there another possibility?

Figure-5: The P waves highlighted by WHITE arrows have a PR interval that is too short to conduct.

Each of the 2-beat groups in today's tracing are enclosed by 3 P waves. For clarity — I have chosen a different color for the repetitive pattern of these P waves (Figure-6):

  • Each of the P waves highlighted by RED arrows in Figure-6 are conducted with 1st-degree AV block.
  • P waves highlighted by WHITE arrows manifest a PR interval that is too short to conduct.


This leaves us with 2 QUESTIONS:

  • Question #1 — Are beats #1, 3, 5, 7, 9 and 11 junctional escape beats?
  • Question #2 — Could the P waves highlighted by the GREEN arrows possibly be conducting with a very long PR interval?


Figure-6: I have chosen 3 colors for the repetitive pattern of P waves that enclose each of the 2-beat groups.



PEARL #3: In my experience — the BEST clue for determining IF certain beats in a given rhythm strip are (or are not) conducted — is to see IF there are certain beats in the tracing that are preceded by a constant PR interval.

  • Note in Figure-7 that there are 2 consistent PR intervals that are repeated throughout the tracing. This suggests that RED arrow P waves are conducting with a PR interval of 0.28 second ( = 1st-degree AV block) — and GREEN arrow P waves are conducting with a markedly longer PR interval (of 0.72 second). 

Figure-7: I have added PR intervals to Figure-6 to indicate the distance from RED and GREEN arrow P waves until the next QRS. 

(NOTE: I believe the reason the PR interval from the 1st GREEN arrow until the onset of beat #3 is a little bit longer than 0.72 second — is the angulation that is most marked in the beginning of this tracing — which slightly distorts measurements. That said — the "theme" of today's tracing is that the same PR interval precedes each of the odd-numbered beats in this tracing).


PEARL #4: There are numerous variations of AV Wenckebach. Today's tracing illustrates one of these variations — in that the PR interval "increment" (ie, the increase in PR interval from 1 beat within a Wenckebach cycle to the next) is much greater than is usually seen with AV Wenckebach. It's important to appreciate that the presence of dual AV Nodal Pathways within the AV Junction, each with different conduction properties — could easily account for the marked PR interval increment seen in Figure-7 (For more on dual AV Nodal Pathway physiology — See this review by Mani BC and Pavri BB).

  • MY Conclusion: The 2-lead rhythm strip in Figure-7 manifests an unusual form of 2nd-Degree AV BlockMobitz Type I (AV Wenckebach) — in which there is 3:2 Aconduction (progressive increase in the PR interval from the RED to GREEN arrow P waves in each group — until the WHITE arrow P wave is blocked — followed by resumption of the cycle with the next RED arrow P wave).


KEY Clinical Point: I arrived at My Conclusion in less than 30 seconds! My goal for explaining my thought process in such elaborate detail is to hopefully assist others not only in arriving at a logical rhythm diagnosis — but to expedite the process for doing so! My suggestions:

  • Get good at recognizing group beating! The "groups" of beats will not always be as consistent as the alternating longer-shorter R-R intervals seen in today's tracing.
  • Use calipers. Doing so allowed me to establish that the atrial rhythm was regular in no more than 5-10 seconds.
  • It took no more than an additional few seconds to recognize the repetitive PR interval present before each of the RED arrow P waves in Figure-4.
  • Recognition of the findings in the above 3 bullets should suggest that the rhythm will be some form of AV Wenckebach until proven otherwise! Total time to arrive at this suspicion = less than 30 seconds!



Additional PEARLS associated with today's tracing include:

  • PEARL #5: I initially thought that beats #3, 5, 7, 9 and 11 might be junctional escape beats — because the PR interval of the WHITE arrow P waves that precede these beats is clearly too short to conduct. But the usual rate range of a junctional escape rhythm in adults is ~40-60/minuteand, IF junctional "escape" occurred following failure of GREEN arrow P waves in Figure-7 to conduct — the junctional rate would have to be much faster than this junctional "escape" rate range. As emphasized in today's Audio Pearlaccelerated junctional rhythms are distinctly uncommon in adults — and, one would have to postulate simultaneous existence of Mobitz I and an accelerated junctional pacemaker. As a result — I felt it far less likely that beats #3, 5, 7, 9 and 11 were junctional beats. (For an example of junctional escape beats in association with 2nd-degree AV Block, Mobitz Type I See ECG Blog #63).
  • PEARL #6: We have previously noted the definite difference in QRS morphology that is seen every-other-beat in lead II. While impossible to establish without a 12-lead ECG, whether QRS morphology of the slightly wider beats #2, 4, 6, 8, 10 and 12 is the result of a specific conduction defect — Common things are common. The most likely explanation for this change in QRS morphology with alternating beats is aberrant conduction either from: i) Improved conduction of beats #1, 3, 5, 7, 9 and 11 following recovery, because of a longer preceding R-R interval; or, ii) A manifestation of the Ashman phenomenon, in which aberrant conduction is seen in the beat after a relative pause (See ECG Blog #70 for detailed discussion of the Ashman Phenomenon)




To clarify the mechanism I favor for the etiology of today’s rhythm — I've derived a laddergram with step-by-step annotations. I begin with Figure-8.

  • NOTE: For review of the Basics for HOW to read (and drawladdergrams  See ECG Blog #188).   

Figure-8: The 1st step in laddergram construction — is to represent each of the P waves in the Atrial Tier. Since conduction through the atria is fast — vertical lines are used, drawn from each of the ARROWS that overlie P waves in this tracing.

Figure-9: Since all QRS complexes are narrow — each of the 12 beats in this tracing is supraventricular. Knowing this allows me to draw in conduction within the Ventricular Tier — which I represent with slightly inclined forward-directed lines with an arrow to indicate the downward direction of conduction. NOTE: I find Power Point optimal for drawing laddergrams — as it allows ready duplication of laddergram elements and precisely vertical displacement to ensure laddergram elements appear exactly below P waves and QRS complexes in the original rhythm strip.


Figure-10: For clarity (and to assist for drawing in conduction through the AV Nodal Tier) — I've colored P waves using the same representation I chose earlier in Figure-6.


Figure-11: It's now time to "solve" the laddergram. I began with those P waves I was most certain about = the RED arrow P waves, which are clearly conducting and manifest  a constant PR interval of ~0.28 second (BLUE lines within the AV Nodal Tier).


Figure-12: I next chose to represent the WHITE arrow P waves — which clearly were not conducting given how short the PR interval preceding beats #3, 5, 7, 9 and 11 is (BLUE butt ends drawn within the AV Nodal Tier). This left only the GREEN arrow P waves unaccounted for ...


Figure-13: I connected the remaining atral impulses (highlighted by the GREEN arrows) — with the remaining (unconnected) QRS complexes in the Ventricular Tier (BLUE lines within the AV Nodal Tier).


Figure-14: My completed laddergram illustrating the mechanism I favor for the etiology of today's rhythm. This laddergram appearance is consistent with 2nd-Degree AV Block, Mobitz Type I (AV Wenckebach) — in which there is 3:2 AV conduction.


Figure-15: Complex arrhythmias may sometimes have more than a single plausible explanation. In Panel A — I reproduce from Figure-14 the mechanism I favor for today's tracing. In Panel B — I consider an alternative possibility, in which there is 3:1 block with odd-numbered beats being junctional in origin. While not impossible — this alternative theory would require the simultaneous coincidental presence of an accelerated junctional pacemaker, which seems unlikely. BOTTOM Line: While it's educational to consider alternative possibilities — I think the mechanism illustrated in Panel A is the most logical explanation for the rhythm in today's case.



Acknowledgment: My appreciation to Drs. Arshad Ali and Kanwal Naz (from Karachi, Pakistan) for the case and these tracings.



Relevant ECG Posts to Today's Case:

  • ECG Blog #185 — Use of a Systematic Approach to Rhythm Interpretation.

  • ECG Blog #63 Mobitz I, 2nd-Degree AV Block with Junctional Escape Beats.

  • ECG Blog #70 — Reviews in detail the Ashman Phenomenon as a cause of aberrant conduction.
  • Mani BC and Pavri BB: Dual AV Nodal Pathway Physiology (Indian Pacing Electrophysiol J 14[1]:12-25, 2014).
  • ECG Blog #188 — How to Read (and DrawLaddergrams.


  1. Thank you so much for this great explanation. In my humble opinion, I believe the correction of aberrant QRS 2, 4,6, 8 10 and 12 into 3,5,7,9,11 and 13 is more from "gap phenomenon" rather than "Ashman"

  2. @ NP — Thank you for your comment! From my understanding of the "gap" phenomenon — it would be difficult (impossible) to prove from the surface ECG whether your theory or mine is the correct explanation for the change in QRS morphology — :)

  3. Pardon the ignorance, it the simultaneous coexistence of WPW and abnormal AV-conduction possible?..

    1. Excellent question. It IS possible for a patient with WPW to manifest AV block! The interesting thing that happens (for example, if there is 1st degree AV block) — is that you will then see delta waves and QRS widening — but with a normal, if not prolonged PR interval! Similarly — you can on occasion, see 2nd- or 3rd-degree AV block in association with WPW.