Sunday, April 11, 2021

ECG Blog #213 (ECG MP-30) — Aberrant Conduction with AFib?

The lead II rhythm strip shown in Figure-1 was obtained from a hemodynamically stable patient.

  • WHAT is the rhythm? 


Figure-1: ECG obtained from a hemodynamically stable patient (See text).



MY Approach to the Rhythm in Figure-1:

There is a LOT going on in Figure-1. My approach to this rhythm was as follows:

  • I find it easiest whenever there are a number of things “going on” — to look first for an underlying rhythm. To do this — I return to the “Ps, Qs & 3R Approach” that I favor (Reviewed in ECG Blog #185).
  • The most commonly-occurring QRS complex in Figure-1 is an upright narrow R wave — so the beats I focused on to determine the underlying rhythm were beats #1, 2; 5; 10-thru-21; and 23. It is EASIEST to defer assessment of the different-looking beats (ie, beats #3,4 — 6,7,8,9 — and — #22) until after we determine the underlying rhythm!


Focusing my attention on beats #1, 2; 5; 10-thru-21; and 23:

  • As alluded to a moment ago — the QRS complex for this most-commonly occurring QRS morphology is narrow (ie, clearly not more than HALF a large box = ≤0.10 second in duration). 
  • NOTE #1: Ideally — we would have a complete 12-lead tracing to assist in assessing QRS duration and morphology. That said — given hemodynamic stability of the patient, and what appears to be a narrow and normal-looking QRS morphology in this lead II, I thought it safe to proceed for the moment on the assumption that the QRS complex of the underlying rhythm was narrow.
  • Normal sinus P waves are absent (ie, there is NO sign of upright P waves in this lead II monitoring lead).
  • There are therefore no sinus P waves that might be Related to neighboring QRS complexes.
  • The rhythm is Rapid — and clearly not Regular. The average R-R interval between the run of 12 consecutivebeats of this most commonly-occurring QRS morphology (that is, looking at beats #10-thru-21) — is no more than 2 large boxes in duration — or corresponding to a ventricular rate that is close to 150/minute.


Therefore — the underlying rhythm in Figure-1 appears to be AFib with a rapid ventricular response (ie, narrow QRS — no P waves — fast rate close to 150/minute — irregularly irregular for the run of consecutive beats from #10-thru-#21).

  • This leaves us with having to determine the etiology of beats #3, 4 and 22 — and — beats #6-thru-9.



NOTE #2: Some readers may prefer at this point to listen to the 7:40 minute ECG Audio PEARL before reading My Thoughts regarding other aspects of the rhythm in Figure-1. Feel free at any time to review my final thoughts on this rhythm (that appear below ECG MP-30).



Today’s ECG Media PEARL #30 (7:40 minutes Audio) — HOW to distinguish between Aberrant Conduction vs Ventricular Ectopy — when the underlying rhythm is AFib?

  • NOTE #3: For review of the Ashman Phenomenon which is mentioned in this Audio Pearl — Please See ECG Blog #70. Use of the Ashman phenomenon with AFib is reviewed in ECG Blog #71.


BACK to the Rhythm in Today’s Case:

We’ve established that the underlying rhythm is rapid AFib. This leaves us with other QRS morphologies that we need to assess (Figure-2).

  • Morphology #1 = the 3 wide QS (negative) complexes ( = beats #3, 4 and 22).
  • Morphology #2 = the 4 consecutive RS complexes ( = beats #6,7,8,9).

Figure-2: I’ve labeled Figure-1 to facilitate assessment (See text).

MORPHOLOGY #1: I strongly suspect that beats #3, 4 and 22 are PVCs (Premature Ventricular Contractions):

  • The QRS complex for these 3 negative beats looks extremely wide (especially for beats #3 and 22)  and, QRS morphology of these QS complexes is very different compared to the all-positive (R wave) morphology of QRS complexes during the underlying AFib rhythm.
  • PEARL #1: Since the majority of ventricular beats originate from a site in the ventricles that lies outside of the conduction system — these ventricular beats not only manifest a different QRS morphology — but also often manifest a slowing in the slope of the initial QRS deflection. This is because initial conduction arising from ventricular myocardium is slower than conduction of supraventricular impulses, that when the ventricular conduction system is intact, are accelerated after exiting the AV node. We especially see this reduced slope for the initial QRS downstroke deflection of beats #3 and 22 (RED arrows in Figure-2).
  • PEARL #2: As a result of retrograde conduction from ventricular beats — PVCs are often followed by a brief post-ectopic pause, even when the underlying rhythm is AFib. This differs from the usual situation seen with aberrant conduction in AFib — in which the irregular irregularity of the AFib rhythm continues throughout, completely uninfluenced by whether or not there is aberrant conduction. Although no post-ectopic pause is seen after beat #4 in Figure-2 — a post-ectopic pause is clearly seen after beat #22 (PURPLE lines in Figure-2). Note that this brief pause seen between beats #22-23 is the longest pause in this tracing — and that it clearly changes the pattern of the underlying AFib rhythm.

MORPHOLOGY #2: In contrast to beats #3,4,22 — I strongly suspect that beats #67, 8 and 9 represent a run of supraventricular beats with aberrant conduction.

  • The QRS complex for these 4 consecutive beats (with biphasic RS morphology) does not look overly wide (if widened at all, in this single monitoring lead).
  • The underlying irregular irregularity of the AFib rhythm in Figure-2 is not altered at all by the change in morphology of these 4 beats. 
  • There is no post-ectopic pause after beat #9.
  • The initial direction and slope of the R wave upstroke for beats #6-thru-9 is virtually the same as the rapidly rising R wave upstroke for the 16 normally conducted beats in the underlying AFib rhythm (both light and dark BLUE arrows in Figure-2 showing a rapidly-rising R wave of similar slope).
  • There is a reason to explain WHY beats #6-thru-9 might conduct with aberrancy — namely, that the shortestcoupling interval in this tracing is seen before the first different-looking beat (ie, the shortest coupling interval is seen between beats #5-6).

BOTTOM LINE: The underlying rhythm in today’s case is rapid AFib. Three different QRS morphologies are seen in this lead II rhythm strip. Even without additional monitoring leads — this single lead II rhythm strip suggests that beats # 3,4,22 are PVCs — and beats #6,7,8,9 are AFib impulses conducted with aberration

  • Because AFib is the underlying rhythm in this case — this means we have lost the diagnostic feature of identifying a premature P wave (there are no P waves with AFib). As a result — I can not be 100% certain from this single lead rhythm strip of the etiology of the different-looking beats.
  • Use of additional monitoring leads (or obtaining a 12-lead ECG during tachycardia) — could greatly assist in further assessment of this rhythm.
  • Clinically — it probably does not matter if beats #6-thru-9 represent a run of NSVT (Non-Sustained Ventricular Tachycardia) or AFib with aberrant conduction. Regardless, the “theme” of the rhythm in Figure-2 is rapid AFib, with at least some ventricular ectopy. As long as the patient remains stable — management with goal of controlling the rapid rate of this AFib plus correcting potential precipitating factors, may be all that is needed to improve the clinical situation.



Relevant ECG Blog Posts to Today’s Case:

  • ECG Blog #185 — Systematic Approach to Rhythm Interpretation
  • ECG Blog #70 — Reviews the basics of the Ashman phenomenon
  • ECG Blog #71 — Is the Ashman phenomenon accurate in AFib?
  • ECG Blog #211 — Reviews in detail WHY aberrant conduction occurs (and why RBBB aberration is the most common form).
  • ECG Blog #212 — Shows application of the Ashman phenomenon — and of assessment of QRS widening in the setting of underlying fast AFib.

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