Studies on ventriculo-atrial conduction and the reentry phenomenon.

SUMMARY Ventriculo-atrial conduction and the reentry phenomenon was studied during right ventricular and bundle of His pacing in the intact dog heart. Bipolar electrogram recordings were obtained from the sinus node, Bachman's bundle, the right and left atria, the bundle of His, and the ventricle. Retrograde conduction resulted in a specific change in the sequence of atrial activation. The recording of both antegrade and retrograde His deflections provided a more accurate means of comparing A-V and V-A conduction times. In all experiments retrograde conduction time exceeded antegrade conduction time. The A-V nodal region was found to be the zone of retrograde conduction delay and block. Increasing the driving rate of the heart, vagal stimulation, and isoproterenol, affected V-A conduction similar to the way in which these maneuvers affect A-V conduction. Reentry (ventricular echoes) commonly occurred when right ventricular or bundle of His pacing resulted in the reverse Wenckebach phenomenon. A-V dissociation Isoproterenol Reverse Wenckebach Reciprocal beating Ventricular echoes Retrograde block IT HAS BEEN known for a long time that the conduction system of the heart is capable of transmitting impulses in both an antegrade and retrograde direction.1-5 Failure to transmit impulses in one direction does not necessarily mean that the conduction system is unable to transmit them in the opposite direction. A number of clinical reports have demonstrated intact retrograde conduction in the presence of complete or near complete failure of antegrade conduction.-9 The functional properties of atrioventricular conduction have been extensively studied in the experimental animal and man using recordings from specialized conducting fibers.'0-'7 These studies have shown that the major site of delay and block of antegrade impulses is in the A-V node. However, conduction delay and block have also been demonstrated in the His-Purkinje system. Most studies concerned with ventriculo-atrial conduction in the intact heart have utilized single atrial and ventricular electrogram recordings, and the site of retro-grade conduction delay has been less precisely defined. Microelectrode experiments on isolated cardiac tissue of the rabbit have demonstrated that in addition to the A-V node, a major location of retrograde conduction delay and block is within the ventricular specialized conduction system.18 It has been repeatedly demonstrated in clinical electrocardiograms that the occurrence of retrograde conduction delay and block are important considerations in the understanding of complex cardiac arrhythmias.19-20 It will be 423