Transient loss of complete bundle branch block patterns during exercise

In patients with various forms of heart disease and in some normal subjects, the high heart rates that occur during exercise can result in decremental conduction4 that leads to fascicular block,2 BBB,1 or rarely even atrioventricular block.5 On the other hand, it is surprising that disappearance of complete BBB does not result more often from the increased sympathetic activity that accompanies exercise, which could offset the effects of decremental conduction. Some of our observations could be explained by rate-related modulation during exercise of bradycardia-dependent or pseudobradycardia-dependent electrical alternans in the bundle branch system,6 by ventricular fusion complexes, or interestingly, as suggested by a reviewer of this report, by exerciseinduced alternating conduction delay in the opposite bundle leading to more synchronous depolarization of alternating beats. We find no convincing variability of alternate PR intervals on these routine exercise recordings, but measurement precision at these rates is limited. Because none of our patients underwent electrophysiologic study, the present report remains descriptive and the mechanisms underlying exerciseinduced reduction of intraventricular block remain to be clarified.