Supernormal Conduction in a Case of Mobitz Type II Atrioventricular Block

A 52-year-old woman exhibited Mobitz type II atrioventricular block with right bundle branch block and 1:1 atrioventricular conduction at or slower than 80 beats/min. Electrophysiologic study revealed transient HV interval block followed by recovery from the block at shorter coupling intervals without prolongation of the H1H2 and H2V2 intervals, suggesting true supernormal conduction. Isoproterenol enhanced the supernormal conduction, with shortening of blocked intervals and recovery of atrioventricular conduction, while atropine caused their less marked enhancement. Linking (ie, retrograde concealment of the impulse to the distal His bundle region through the blocked right bundle branch) is considered a possible mechanism of supernormal conduction in this case.