DIAGNOSIS: Atrial Fibrillation with Right Bundle Branch Block (RBBB) and Left Posterior Fascicular Block (LPFB).

EXPLANATION: This wide complex tachycardia at a rate of 125 bpm is clearly very irregularly irregular. There are no visible waves in an undulating baseline; only atrial fibrillation can look like this.

What to make of the wide complexes? The marked right axis of 120 degrees with small r waves in leads I and aVL suggests LPFB. The predominantly positive complexes in V1 with a duration of at least 120 msec suggests RBBB. Of note, LPFB is about 30 times less frequent than LAFB and almost always occurs together with RBBB.

It is interesting that the combination of RBBB with either LPFB or LAFB so distorts the morphology of the complexes in V1 that they become indistinguishable from beats of ventricular origin. You may remember that in the last two ECGs, V1 morphology was shown to be the most reliable way to distinguish wide complex supraventricular beats from those of ventricular origin. I am presenting this tracing to show that V1 morphology becomes useless with bifascicular block; one must then use other clues, if present, to ascertain the origin of such wide complex beats.

In this example, fortunately, the irregularly irregular nature of the underlying rhythm makes the diagnosis of atrial fibrillation, and thus the supraventricular origin of the tachyarrythmia, readily apparent.

Editor: Sol Nevins MD FACEP. Faculty, Emergency Medicine Residency, Morristown Memorial Hospital, Morristown, N.J.