Sinus Bigeminy: Sick Sinus Syndrome

Report:

Sinus rhythm 1

Bigeminy, probably due to 3:2 sinoatrial exit block 2

Demand pacemaker, bigeminal fusion beats 7

Comment:

The differential diagnosis for this rhythm includes sinus extrasystoles, with post-ectopic SA depression, and atypical sinus arrhythmia. There is probably not enough room for blocked SVEBs: they’d have to be very early and, at any rate, visible in the ST segment of the preceding sinus beat. Pacemaker tracks the P waves – no pause is long enough for it to produce escape beats – and the pre-implantation rhythm (below) is preserved.

There is only a “tad of fusion” here – the fusion QRS is more distorted by the spike decay than by fusion per se. The repolarisation difference confirms the presence of true fusion. Often this is the most sensitive test for the presence of fusion. Even here, the T wave difference can be due to different cycle length – but we know this not to be the case because of “control” sinus rhythm below.

Fig 97b. Sinoatrial exit block, with characteristic acceleration before the pause. There is a tendency to another Wenckebach block, in the AV node, but the SA node (invisible) beat beats it to it. SA node beat? Why not?52

Fig 98. 80 year old lady with temporary pacemaker inserted at thoracotomy for aortic vavle replacement.