Another Paced Bigeminy

Report:

Sinus rhythm 88/min 1

AV block, unspecified 1

DDD pacemaker rhythm 62/min 1

Alternating atrial sensing and pacing 2

Failure to sense alternate P waves 2

Atrial trigeminy 1

Ventricular bigeminy 1

Posterior electrode position 1

Comment:

The sensed AV delay is quite long, 0.24 sec; the paced AV delay is obviously shorter, 0.15 sec. This is how the pacemaker is programmed, for reasons known only to the programmer. The bigeminy is predicated on atrial trigeminy, which in turn depends on every second P wave being ignored, activating the atrial pacing.

The long and the short (no pun intended) of the problem is a very long PVARP, presumably set at over 0.5 sec in order to prevent recognition of retrograde P waves and endless loop problems. This was deduced from the tracing itself and confirmed by checking the patient’s record22. I would have thought that a shorter sensed AV interval would have been better than longer PVARP – but one cannot argue with a chart.

In V1 the morphology is more like RBBB than the expected LBBB. Her RV was dilated and the electrode tip was placed more posteriorly (from the apex) than usual. The vector of the paced QRS would thus spread in the direction of V1, i.e., anteriorly, inscribing a dominant R wave. Other mechanisms may also be responsible23.

Below (Fig 28a) is a tracing taken at another time, with slower sinus rate and fever “blocked” (ignored) P waves. The atrial-paced beats evoke sinus node depression – the post-ectopic sinoatrial depression – that slows the sinus rate temporarily. When it speeds up enough, another P wave falls within the PVARP and is “blocked” and another atrial-paced P wave slows the SA node again.