Pacemaker: Escape-Capture Bigeminy

Report:

Sinus tachycardia 100/min

First degree AV block (PR 0.28")

Second degree AV block.

Right bundle branch block

Pacemaker (escape) beats

Escape-capture bigeminy

Comment:

Before Bradley and Marriott's classical description6, this rhythm was called pseudoreciprocal rhythm, as the P wave between the escape and the capture beat simulates a reentry (reciprocal beating).

The bigeminy is maintained by a coincidental relationship of the two cycle lengths, an “arithmetical caprice”. The effective sinus cycle length is, in this example, twice the observed one. This is of course because of the second degree AV block rendering only alternate P waves known to the ventricles; the result would be the same for sinus bradycardia 50/min.

It is possible to diagnose the RBBB from this rhythm strip even without knowing it to be V1 or MCL1: the paced beats have LBBB morphology except for the slurred S descent indicating their ventricular origin. One would still get full marks for calling them ventricular escape beats – the pacing spike is not exactly obvious.

The QR RBBB morphology may be due to old anterior infarction or lead placement, but the late delay is, obviously, opposite of that seen in the paced beats.