Wednesday, February 2, 2011

ECG Interpretation Review #15 (Run of WCT - VT vs Aberrant Conduction)

QUESTION: Interpret the Lead MCL-1 rhythm strip in the tracing shown in Figure-1.
  • Is this a run of VT?
  • How certain are you of your diagnosis?

Figure 1 - What is the cause of the run of beats #6-thru-#14?

INTERPRETATION: The underlying rhythm (as suggested by the first 5 beats) — is sinus tachycardia at ~105/minute (regular, narrow QRS with R-R interval of just under 3 large boxes; P waves with fixed PR interval precede each QRS for beats #1-5 in this right sided MCL-1 lead). Sinus rhythm is then interrupted by a run of a WCT (Wide-Complex Tachycardia). Despite QRS widening — the run of beats #6-thru-#14 in Figure 1 is not ventricular tachycardia.

The 3 possibilities for conduction of PACs (Premature Atrial Contractions) are shown in Figure-2:
  • Premature Impulse A — occurs so early as to fall within the ARP (Absolute Refractory Period). Because the entire conduction system is still in an abolute refractory state — premature impulse A is "blocked" (ie, non-conducted to the ventricles).
  • Premature Impulse C — occurs after the refractory period is over.  As a result — a PAC occurring at Point C will conduct normally (with a narrow QRS that looks identical to other sinus beats on the tracing).
  • However — Premature Impulse B occurs at an intermediate point during the RRP (Relative Refractory Period).  A PAC occurring at Point B will therefore conduct aberrantly (ie, with QRS widening) — because only part (but not all) of the ventricular conduction system has recovered. Most often PACs that occur during the RRP will conduct with some form of bundle branch block and/or hemiblock (reflecting that part of the conduction system which has not yet recovered).

Figure-2: Absolute and Relative Refractory Periods (ARP & RRP) — explaining why beat A is blocked — and beat B is conducted with aberration.

Returning to the Questions in this Case:  
It is important to emphasize that by far, the most common cause of a WCT is ventricular tachycardia. That said — there are times when one can definitively exclude VT from consideration.  This is one of those times. The best way to diagnose aberrant conduction is by identifying a premature P wave at the onset of the tachycardia. The RED arrow in Figure-3 does just this. Note notching in the T wave of beat #5 just before the run begins. None of the sinus-conducted beats at the beginning of the tracing show such notching, confirming that this finding is real and indicative of a PAC.

Figure-3: Answer to Figure 1.

Two additional findings consistent with a supraventricular etiology for beats #6-thru-#14 are: 1) that QRS widening though present is not marked; and2) that the initial deflection (both the small, slender r wave and the downslope of the S wave) is very similar in morphology to the initial part of the QRS of sinus beats.
  • Final POINT: Note that there is atrial activity during the run in the form of a small point at the peak of the T wave (Note thin RED circles in the T wave of beats #6,7). Because VT may sometimes manifest 1:1 retrograde conduction back to the atria — the atrial activity that we see in Figure 3 is not helpful diagnostically in distinguishing between VT and SVT with aberration.

Sinus tachycardia for the first 5 beats. This is interrupted by a PAC (in the T wave of beat #5) — and followed by a wide tachycardia that is supraventricular with aberrant conduction. This is not VT.

NOTE: See also ECG Blog Review #14 -


  1. Why atrial activity, T wave, QRS morphology and R-R interval in beats #6,8,10,12 are slightly different from those in beats #7,9,11,13?

    1. Highly astute observation (!) - that I did not notice before. It would be nice to have a longer rhythm strip (I don't) to confirm your observation - but I agree that P wave amplitude seems to alternate - and both S waves and T waves of alternate beats, as well as the R-R interval seem to vary (very slightly - but nevertheless vary). This is electrical alternans - which is VERY common with reentry SVT rhythms - so not surprising that we see it for beats #6-thru-14.

      My ECG Blog #83 reviews the phenomenon of Electrical Alternans in detail. Please GO TO - -

      THANK YOU again for your excellent comment!

  2. Mostly SVT start with Premature beat accompanied by a pause that pause is not visible here ?

    1. There IS a premature beat that initiates the run of SVT (RED arrow in Figure-3). There does NOT have to be a pause before the onset of the run of SVT — :)

  3. Can you differentiate AVNRT from AVRT basing on this strip?