VEBs in Bigeminy

Report:

Sinus rhythm 67 – 71/min 1

Ventricular pacemaker 1

VVI mode 4

Ventricular fusion beats 2

VEBs 1

Bigeminal 0.5

Uniform 0.5

Comment:

The interesting question is whether the atrial deflections following VEBs are blocked sinus P waves or retrogradely conducted waves of ventricular origin. Unfortunately, they fall almost exactly half-way between the sinus P waves preceding the paced beats; one has to rely on their deceptive polarity11 rather than their timing. Given the location, they could even, conceivably, be atrial fusion beats. I think they are sinus. I have been wrong before.

Another interesting question is whether this is an atrial-sensing pacemaker. The pacing spikes all follow the sinus P waves and there is an obvious (once looked for) variation in the paced rate. This is an illusion: the pacing spikes do follow the P waves, but the P-spike intervals vary. Not only that: the second and third paced complexes are smaller and thinner (in the rhythm strip) than the first or fourth paced beats; the last (sixth) paced beat is the fattest of them all. The longer the P-spike interval – the greater the opportunity of native AV conduction – the thinner the resulting QRS! There are fusion beats. Perhaps all the paced beats have a “tad” of fusion.

Why, then, does the paced QRS rate vary? If it is not following the P waves, what different drummer does it hear? It hears, with constant escape intervals befitting a good pacemaker, the preceding QRS complexes – the VEBs. It is the VEBs which introduce the rate change, through variation in their coupling intervals. The fixed coupling of uniform VEBs is not always absolutely fixed; the coupling interval variation here approaches 0.08”. If it were any greater, one would contemplate parasystoles instead of extrasystoles.