Friday, May 17, 2013

PVC or Aberrant Conduction? (Another Guest Post from Dr. Wang)!



Question: This tracing shows (choose one from the list below)

               a) atrial bigeminy with aberrant conduction

               b) ventricular bigeminy















 Annotated ECG:


Answer:   a) atrial bigeminy with aberrant conduction.


Discussion: Every other QRS is wide, different looking from the sinus beats and, at a quick glance, the tracing seems to show ventricular bigeminy. However, careful observation reveals a prematurely occuring P wave (↓) in front of these wide QRSs and this is an examle of atrial bigeminy with aberrant conduction. Why does aberrant conduction ever occur? It is simply because the two bundle branches (BBs) have different length of refractory periods. If an impulse occurs when one BB is still refractory while the other has recovered from the refractory period, the impulse will conduct thru the recovered BB, bypasssing the refractory BB: aberrant conduction results. Of course, if an impulse occurs at a time when both BBs have recovered from the refractory period, it will conduct normally, while if both BBs are refractory, it won't conduct at all (non-conducted PAC). This was well diagrammed and explained in the Medscape ECG of the week posted on 3/7/2012.



5 comments:

  1. Nice post again! It seems there's a slight delay through the AV node as well? Is one of the two ventricles generally more likely to have a longer refractory period than the other? We see both LBBB and RBBB patterns in aberrancy, but what's most common?

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    1. K. Wang answers that: The right bundle branch more frequently has a longer refractory period than the left, and thus aberrant conduction more often manifests as a RBBB pattern.

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  2. If you look at the ST segment in lead lead III it looks like there a small inverted T wave that is followed by a small positive upstroke. I have seen these small positive upstrokes before but have not called them dropped complexes, just assumed that they were part of the repolarization cycle since they were part of the ST segment.
    Anyway, this really helps out because I have been at a loss as to what to call these "PVCs" with P waves. I thought they were some kind of fusion complexes.

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  3. But there is qR in V1 and also R>R´ in V1+2 (mostly V2) as well as R/S < 1 in V6 during the "extra"-beats, which if it was about VT vs. SVT with aberrancy would all suggest ventricular origin. How can this not hold true?
    Thanks - S.

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    1. These are all consistent with RBBB with old MI present (Q-waves). R/S is often < 1 in V6 in RBBB.

      Steve Smith

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