A 77-year-old man with aortic stenosis and preserved left ventricular ejection fraction
underwent transcatheter aortic valve replacement (TAVR) for symptomatic severe aortic
stenosis. After the procedure, he had a transient left bundle branch block that resolved.
Two months later, he presented to the emergency department with syncope. Electrocardiography
showed normal intervals (Fig. 1
A). Subsequent telemetry revealed paroxysmal complete atrioventricular block (PAVB).
At first glance, it appears that PAVB is induced consistently by a conducted junctional
extrasystole (Figure 1
B, single asterisk) that delays the subsequent sinus P wave. However, on a closer
inspection, it is evident that the subsequent sinus P wave occurs on time, buried
within the ST segment of the junctional premature extrasystole; however, not conducted
to the ventricle which could be due to the retrograde conduction of premature junctional
extrasystole (Fig. 1
B, single asterisk) into the AV node (concealed conduction) (Fig. 1
B, caret) and blocking the sinus P wave. The sinus P wave that follows now conducted
via the atrioventricular (AV) node to the His bundle and gets blocked because of the
relatively prolonged H-H interval, ie, a pause-dependent block. Pause-dependent, or
phase 4, atrioventricular block (AVB) occurs in the diseased His-Purkinje system (HPS)
because of spontaneous depolarisation of the diseased conduction system rendering
it refractory to subsequent impulses (Fig. 1
B). An appropriately timed ventricular extrasystole resets the membrane potential
of the HPS to baseline and allows conduction to resume (Fig. 1
B, double asterisks). The patient underwent uncomplicated dual-chamber pacemaker implantation.