Exercise-provoked distal atrioventricular block.

Abstract Spontaneous or pacing-induced nonfunctional atrioventricular (AV) block occurring distal to the His bundle recording site is associated with a high probability of subsequent syncope or progression to high grade AV block. 1 Accordingly, a reliable noninvasive method that could potentially unmask distal His-Purkinje block would be helpful in identifying patients in whom invasive conduction system studies should be undertaken. We report 3 patients in whom the occurrence of second-degree AV block during exercise testing unmasked serious underlying conduction system disease, which would have otherwise gone undetected by routine ambulatory monitoring and resting electrocardiograms. Subsequent evaluation of their conduction systems by electrophysiologic studies revealed marked prolongation of the HV interval and AV block distal to the His bundle in response to rapid atrial pacing.