Periodic variation in atrioventricular conduction time: mechanisms of initiation, maintenance and termination of periods of long PR intervals.

Five patients with periodic variation in atrioventricular (AV) conduction time are reported in whom periods of comparatively short PR intervals alternated with periods of comparatively long PR intervals. In all patients, the transition from a period of long PR intervals to a period of short PR intervals usually occurred without a blocked P wave. In 3 patients, however, the period of long PR intervals was occasionally terminated by a blocked P wave. The mechanisms of initiation, maintenance and termination of periods of long PR intervals are explained by the use of dual AV pathways consisting of fast and slow pathways. It is suggested that the effective refractory period of the fast pathway was abnormally prolonged by increased vagal tone and that, during the period of long P-R intervals, sinus impulses anterogradely passed only through the slow pathway and later retrogradely excited the fast pathway. Apparently, when vagal tone was decreased, the effective refractory period of the fast pathway was shortened, resulting in abrupt improvement in atrioventricular conduction without a blocked P wave.