Supernormal conduction in accessory atrioventricular connections.

Prospective electrophysiologic evaluation of 74 patients with ventricular preexcitation revealed 4 patients who had supernormal anterograde conduction over an accessory atrioventricular pathway. In each patient, anterograde conduction over the accessory pathway was present at relatively slow sinus rates, but the accessory pathway anterograde effective refractory period was substantially prolonged. During premature atrial stimulation, a "window" of supernormal conduction was identified at which closely coupled atrial extrastimuli conducted over the accessory pathway, whereas longer premature intervals resulted in accessory pathway block. Causes other than supernormal conduction to explain this phenomenon, for example, phase 4 block, were unlikely in each case. Although the mechanism of supernormal conduction in humans is unknown, the electrophysiologic findings in our patients are similar to those reported in patients with supernormal conduction in the His-Purkinje system.