Idioventricular Rhythm & AV Block

Report:

Sinus rhythm 84/min

Second degree AV block, Möbitz 1 (Wenckebach)

Idioventricular rhythm 43/min

AV dissociation

Fusion beats (4th beat in the top strip, last in second strip)

VEBs, small, possibly also fusion beats (bottom strip)

Comment:

During the (much slower) idioventricular rhythm, all the sinus P wave are blocked. Yet, this is not truly a third degree AV block, despite a temporarily complete AV dissociation - a point eloquently stressed by Marriot116. When is an AV block complete? Without the ventricular escape rhythm 3:2 and 2:1 conduction indeed takes place.

The small blips, resembling P waves and yet not resetting them, are either movement artefact or VEBs. In the latter category, an electronic pacemaker should be considered, but it would have to be malfunctioning, with intermittent failure to both sense and pace. Sinus-VEB fusion is possible, but the AV conduction is not exceeding 3:2 ratio anywhere else and the maximum PR interval increment should be between the first and the second beat. The putative fusion beats would have to represent 4:3 conduction. On the other hand, the morphology and the timing is right for fusion. The participating VEBs need not be the same shape as the AIVR beats, but should be basically similar in polarity, despite obviously shorter coupling intervals. This is where the money is.

The alternative, movement artefact, could hardly be expected to show constant coupling to a sinus beat. It was mentioned only in order to provide a differential diagnosis – an electrocardiographic essential.