Junctional Extrasystoles with RBBB Conduction

Report:

Sinus rhythm 81/min 1

Atrial-sensing ventricular pacemaker 3

AV interval 0.10 sec 1

Junctional premature beats with RBBB conduction 4

ST/T changes consistent with ischæmia 1

Comment:

Like in the previous case, there are large ventricle-pacing spikes, probably denoting a unipolar pacemaker. Digital ECG recorders simulate the pacing spikes as high-frequency activity and the size of the spike no longer has the same diagnostic implications with respect to the electrode being unipolar or bipolar7. Nevertheless, the unipolar spikes still tend to be bigger.

The spikes follow the sinus P waves and there is some slight variation in cycle lengths, reflecting a sinus arrhythmia. The AV (i.e., P to spike) interval is very short, for unknown reasons. Perhaps it was arrived at using echocardiographic measurements of the stroke volume8. Because it was programmed into the pacemaker, it has nothing to do with LGL conduction or other causes of short PR intervals. At any rate, the differential diagnosis should include isorhythmic AV dissociation, where AV intervals would be slightly variable and the paced cycles all of the same length.

The premature beats have typical RBBB morphology and are almost certainly of junctional origin: there is no preceding atrial activity. The pause induced is slightly longer than fully compensatory and depends on the sinus P wave response (if any) rather than the pacemaker (whose spikes, all the same, make the measurements easy). As Schamroth put it, the length of the pause is a feeble reed to lean on in telling the origin of ectopic complexes! Anyway, this patient had RBBB in sinus rhythm prior to AVR (below, Fig 6a)).

The ST changes, ischæmic-looking as they may be, probably reflect the patient’s LVH from severe aortic stenosis.