Atrial Pacemaker: Failure to Sense

Report:

Sinus rhythm 80/min 1

Atrial pacemaker 2

Failure to sense 2

Blocked atrial P waves of pacemaker origin 4

Left bundle branch block 1

Comment:

There is a profusion of pacemaker spikes and LBBB complexes, giving an initial impression of variable atrial and AV pacing. The next impression is that the pacemaker failed – its spikes just march through the sinus rhythm. The final impression is – or should be – that the pacemaker is atrial, responds appropriately to sinus pauses and merely fails to sense subsequent sinus P waves.

Sinus pause is a convenient but not very precise term. One should always look for the usual suspects: blocked SVEBs, AV blocks, SA blocks or sinus arrest. The “commonest causes of pauses” are, of course, blocked SVEBs28. One has to look carefully for any suspicious wrinkles or bumps following the last conducted QRS complex. Here, the onset of ST segment has just that in both complexes preceding the pauses; furthermore, the shallow dip is too early to be a blocked sinus P wave. What it could be, and very likely is, is atrial wave caused by the preceding pacemaker spike – itself a blocked SVEB of sorts!

If there were no bumps, the diagnosis would be sinoatrial exit block, Möbitz 1 (Wenckebach), since the rate appears to accelerate before the pause. This is fallacious: the acceleration is in fact decremental post-ectopic SA depression following the blocked “SVEB” and the atrially paced P wave.