Isorhythmic Dissociation of a VVI Pacemaker

Report:

Sinus rhythm 70/min 1

Ventricular pacemaker rhythm 70/min 4

AV interval 0.36 sec 5

Comment:

The sinus P waves are not very well shown in the frontal leads, but there’s no doubting them in the chest leads V1-3. The AV interval is long, but constant: it looks like an atrial-sensing ventricular pacemaker. The problem is, it isn’t: it’s a VVI unit implanted 11 years previously, following episodes of atrial fibrillation.

There are two diagnostic possibilities: the P waves are in fact retrograde, with long first degree VA block, or else there are two independent rhythms at the same rate, fortuitously associated in what the French called isorhythmic AV dissociation38. Brief episodes are, traditionally, called accrochage; longer ones synchronization.

Of all people, I should know that retrograde P waves may, rarely, be upright39; but the long retrograde block 0.44 sec makes this theory even less likely. The simpler explanation – isorhythmic dissociation – should win the day.

Fig 62. 79 year old man in ICU following successful CABGs.