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 exhibi ted a wide "gap" in retrograde conduction through a concealed atrioventricu lar accessory pathway. The appearance of the wide "gap" depended on the ven tricular pacing sites. While ventricular extrastimuli at a basic cycle leng th of 600 msec from the right ventricular outflow tract consistently conduc ted to the atria, retrogradely through the accessory pathway, those from th e right ventricular apex repeatedly revealed disappearance of the retrograd e conduction at the wide coupling intervals from 550 to 380 msec. The mecha nisms of this rare "gap"-like phenomenon are discussed in this paper.