Monday, April 5, 2021

ECG Blog #211 (ECG MP-28) — Why does Aberrant Conduction Occur?

Interpret the 2 rhythm strips shown below in Figure-1, using the Ps, Qs & 3R Approach. Both patients are hemodynamically stable.

  • NOTE #1: Lead MCL-1 is a right-sided monitoring lead that provides a similar perspective as is seen in lead V1 on a 12-lead tracing. And, although ideally, we would have a complete 12-lead ECG available for rhythm assessment — sufficient information is evident in the single lead given to answer the Question below.


QUESTION: The different-looking beats in both Tracing A and Tracing B show "rabbit ears". WHICH of the following statements is true?

  • a) Aberrant conduction is present in both tracings.
  • b) Aberrant conduction accounts for the different-looking beats in A — but not in B.
  • c) Aberrant conduction accounts for the different-looking beat in B — but not for the different-looking beats in A.
  • d) Neither tracing shows aberrant conduction. The different-looking beats in both A and B are all PVCs.


Figure-1: Interpret Rhythm A and Rhythm B. Are the different-looking beat PVCs or PACs that conduct with aberration?




NOTE #2: Some readers may prefer at this point to watch to the 4:45 minute ECG Video PEARL before reading My Thoughts regarding the 2 rhythms shown in Figure-1. Feel free at any time to review to My Thoughts on these rhythms.


Today’s ECG Media PEARL #28 (4:45 minutes Video) — Reviews WHY some early beats and some SVT rhythms are conducted with Aberration (and why the most common form of aberrant conduction manifests RBBB morphology).

  • NOTE #3: I have excerpted a 6-page written summary regarding Aberrant Conduction from my ACLS-2013-ePub. This appears below in the Addendum (in Figures-4, -5, and -6).
  • CLICK HERE — to download a PDF of this 6-page file on Aberrant Conduction.



MY THOUGHTS on Rhythm A in Figure-1:

There are several things going on in Rhythm A. In such cases — I find it easiest to first look for an underlying rhythm:

  • The underlying rhythm in A is sinus — as determined by beats #1, 2  4, 5, 6  8, 9, 10 and 13. Although there is slight variation in QRS morphology (due to baseline artifact) — it should be apparent that for each of the above 9 beats, the QRS complex is narrow — the R-R interval that separates these beats is constant (and measures just over 3 large boxes in duration) — and a similar-morphology P wave with a constant and normal PR interval is seen (RED arrows in Figure-2). Therefore — there is an underlying sinus rhythm at ~90-95/minute.


Figure-2: I’ve added colored arrows to Rhythm A to highlight atrial activity (See text).


The QRS complex for 4 of the 13 beats shown in rhythm A looks different than the 9 sinus-conducted beats. There are a number of reasons why we can say with 100% certainty that beats #3, 7, 11 and 12 are all PACs that conduct with aberration.

  • Beats #3, 7 and 11 are all preceded by early-occurring P waves (light BLUE arrows). The most obvious of these premature P waves is the 1st BLUE arrow (that appears before beat #3) — which highlights an extremely tall and pointed P wave. The next 2 BLUE arrows (before beats #7 and 11) are more subtle — but the notching within the T waves is early and unmistakably different in morphology from the biphasic P waves of sinus beats.
  • QRS morphology for each of the different-looking QRS complexes in A is highly typical for RBBB conduction because: i) The initial QRS deflection for beats #3, 7, 11 and 12 is a small and thin r wave — that is similar to the small initial r wave in the sinus-conducted beats; andii) Each of these different-looking beats manifests an S wave that descends below the baseline — and finishes with a tall and slender R’ complex (ie, a taller right “rabbit ear”).
  • The reason beat #7 is a little bit narrower and lacks a tall R’ wave — is that this PAC conducts with a pattern of incomplete (rather than complete) RBBB aberration.
  • For MORE regarding the expected typical QRS morphology for RBBB aberration — See Sections 19.4, 19.5 and 19.6 in Figure-5 and Figure-6 in the Addendum below.
  • BOTTOM LINE: The rhythm in A (as shown in Figure-2) — is sinus with PACs that conduct with RBBB aberration.


MY THOUGHTS on Rhythm B in Figure-1:

There is only 1 different-looking beat in Rhythm B ( = beat #4).

  • Once again — I find the easiest way to assess this tracing is to look for an underlying rhythm (Figure-3):


Figure-3: I’ve added RED arrows to Rhythm B to highlight persistence of regular sinus P waves (See text).


There are a number of reasons why despite the presence of a taller right “rabbit ear” for beat #4 — this beat is not aberrantly conducted. Instead — we can say with virtual certainty that beat #4 in B is a PVC.

  • FIRST — The presence of “rabbit ears” is only helpful when seen in a right-sided lead (such as lead V1 on a standard 12-lead tracing — or lead MCL-1 of a monitoring lead). There is no diagnostic benefit from the rR’ complex we see for beat #4 in B.
  • The underlying rhythm in B is sinus with a long PR interval (RED arrows in Figure-3). The QRS complex is wide (about 0.12 second) — so some type of underlying bundle branch block is present. Nevertheless — the presence of regularly-occurring upright P waves in lead II with a constant (albeit prolonged) PR interval confirms that the underlying rhythm is sinus.
  • KEY POINT: Instead of the P wave preceding beat #4 being early — this P wave ( = the 4th RED arrow in Figure-3) occurs precisely on time! Note that the PR interval preceding beat #4 is unmistakably shorter than the PR interval preceding each of the sinus-conducted beats in this tracing. This means that “something else” (ie, something arising from below the AV node) must have occurred before the P wave preceding beat #4 had a chance to conduct to the ventricles, which by itself proves that beat #4 must be a PVC.
  • Unlike the situation in rhythm A — the pause that contains the different-looking beat (ie, the R-R interval between beats #3-to-5) is “compensatory” (ie, equal to exactly twice the R-R interval of normal sinus beats). The R-R interval that contains isolated PVCs often (albeit not always) is perfectly “compensatory” — reflecting the fact that many PVCs do not conduct back to the atria to reset the SA node. This is different than what occurs with PACs — that routinely reset the SA node because of their occurrence in the atria.
  • There is no reason for beat #4 in B to conduct with aberration. This is because this premature beat occurs late in the cycle (ie, well after the end of the T wave of the preceding beat). Aberrantly-conducted beats generally occur much earlier in the R-R interval, during which time a portion of the conduction system is far more likely to still be refractory (as was the case for the aberrantly conducted PACs seen in rhythm A).
  • Finally — QRS morphology of beat #4 in rhythm B is completely different than the QRS morphology of normal sinus beats. Note in particular, how the initial deflection of beat #4 (which is a small-but-wide and rounded initial positive deflection) is nothing like the thinner, more-rapidly-rising and taller initial R wave seen in each of the sinus-conducted beats.
  • BOTTOM LINE: Taken together, the above-noted findings overwhelmingly suggest that the rhythm in B (as shown in Figure-3) — is sinus with 1st-degree AV block and a PVC ( = beat #4).




ADDENDUM (4/5/2021) In the following 3 Figures — I post written summary from my ACLS-2013-ePub regarding the basics of Aberrant Conduction.

  • CLICK HERE — for a PDF of this 6-page file on the basics of Aberrant Conduction that appears in Figures-4, -5, -6.


Figure-4: Aberrant Conduction — Refractory periods/Coupling intervals (from my ACLS-2013-ePub).


Figure-5: Aberrant Conduction (Continued) — QRS morphology/Rabbit Ears.


Figure-6: Aberrant Conduction (Continued) — Example/Summary.



Relevant ECG Blog Posts to Today’s Case:

  • ECG Blog #140 — Example of alternating Bifascicular Block Aberration.
  • ECG Blog #14 — Example of Blocked PACs.
  • ECG Blog #15 — Example of a WCT due to Aberrant Conduction.
  • ECG Blog #33 — Example of PACs with varying degrees of Aberrant Conduction.


  1. Nice post. I have some questions. Why do PAC occur? What is your etiology? Why QRS is wide in Rhythm B? Thanks a lot, my professor.

    1. There are many possible causes of PACs — which include stimulants (ie, caffeine, alcohol, cocaine or other stimulant medications) — dehydration — hypoxemia — heart failure — fluid and electrolyte or acid-base problems — “sick patient” — psychological stress — inadequate sleep — etc., etc.).

      And MUITO OBRIGADO O Poder — for your 2nd question. Somehow, I forgot to make any mention of the baseline QRS widening in Rhythm B. The reason for this is preexisting bundle branch block. We KNOW that the rhythm is sinus — because there is an upright P wave with constant (albeit prolonged) PR interval in this lead II rhythm strip. THANKS to YOU — I have just now ADDED a bullet to the text, so as to emphasize the reason for the baseline QRS widening in Rhythm B! — :)