A wide "gap" in retrograde conduction through a concealed accessory atrioventricular pathway depending on ventricular pacing sites.

We present a 57-year-old man with Wolff-Parkinson-White syndrome who exhibited a wide "gap" in retrograde conduction through a concealed atrioventricular accessory pathway. The appearance of the wide "gap" depended on the ventricular pacing sites. While ventricular extrastimuli at a basic cycle length of 600 msec from the right ventricular outflow tract consistently conducted to the atria, retrogradely through the accessory pathway, those from the right ventricular apex repeatedly revealed disappearance of the retrograde conduction at the wide coupling intervals from 550 to 380 msec. The mechanisms of this rare "gap"-like phenomenon are discussed in this paper.