By Ken Grauer, MD

Professor Emeritus in Family Medicine, College of Medicine, University of Florida

Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.

The ECG in the figure below was obtained from an otherwise healthy 20-year-old man when an irregular heartbeat was noted on routine exam. The patient was asymptomatic. How would you interpret this tracing? Are these multifocal (multiform) premature ventricular contractions (PVCs)?

The underlying rhythm is sinus. Virtually every other beat occurs early and looks different, with at least some degree of QRS widening. That said, these wider beats are not PVCs. Instead, the rhythm is atrial bigeminy in that every other beat is a premature atrial contraction (PAC). The reasons why the different-looking beats in this tracing are not PVCs include: Early beats are preceded by premature P waves (red arrows in lead II) and QRS morphology of the early beats is highly characteristic for aberrant conduction.

Aberrant conduction is most likely to take the form of some type of bundle branch block and/or hemiblock pattern. As a result, attention to QRS morphology sometimes may provide invaluable assistance in distinguishing between aberrantly conducted PACs vs. ventricular beats. The interesting feature about this tracing is the changing QRS morphology seen with every other beat. The most common form of aberrant conduction manifests a right bundle branch block (RBBB) pattern. This is because under normal circumstances, the right bundle branch tends to exhibit the longest refractory period, which means that an early-occurring impulse (i.e., a PAC) has the greatest chance to arrive at the AV node at a time when the right bundle branch still is refractory. That said, any form of conduction defect may be seen with aberrant conduction, depending on the relative length of the refractory period for the various conduction fascicles in a given patient. In this case, beats 9 and 11 in lead V1 show typical RBBB aberration. Consistent with this RBBB pattern, beats 9 and 11 demonstrate a wide terminal S wave in simultaneously occurring lateral lead V6. In contrast to beats 9 and 11, there is only minimal aberrant conduction for beat 13.

In the limb leads, the pattern of RBBB aberration is suggested again for alternate beats by the presence of wide terminal S waves in lateral lead I. Additionally, QRS morphology in leads I, II, and III suggests left posterior hemiblock aberration for beat 5, left anterior hemiblock aberration for beat 7, but no hemiblock aberration for beats 1 and 3.

PVCs do not do what we see here. More than the already diagnostic presence of premature P waves preceding each early beat (best seen in the limb leads), changing QRS morphology of every other beat manifesting multiple variations of highly typical conduction defect morphology establishes with 100% certainty that the rhythm is atrial bigeminy with varying forms of aberrant conduction.

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