Alternating bundle branch block together with alternation of PR intervals: advanced Mobitz II atrioventricular block.

Certain electrocardiographic patterns are well recognized for their propensity to progress to more severe forms of atrioventricular (AV) block, thus requiring pacemaker implantation. This article presents an infrequently recognized and deceptively benign pattern of conduction abnormality with great potential for advancement to severe AV block and necessity for pacemaker therapy. The pattern consists of alternating bundle branch block (BBB) in unison with alternation of the PR interval so that the right BBB beats and left BBB beats appear to be wedded to their own but different PR intervals. It is hoped that the publication of these cases will contribute to the awareness of practicing cardiologists and electrophysiologists to the existence of this potentially serious form of AV conduction abnormality and hasten the implantation of pacemakers in patients who require them.