Retrograde (ventriculoatrial) conduction that reaches the atrioventricular node simultaneous with, or just before an atrial impulse can facilitate subsequent
anterograde conduction. However, a spontaneous or programmed ventricular extrasystole during atrial fibrillation is generally followed by a compensatory pause indicating subsequent delayed anterograde transmission. This characteristic response was used as a model to study the mechanism of atrioventricular nodal behavior during atrial fibrillation. In eight medically-treated patients with chronic atrial fibrillation and a relatively slow but random
ventricular response, single premature right ventricular stimuli were delivered after
every eighth spontaneous R wave during at least 1 hour. A fixed coupling interval of the extrastimulus, considerably shorter than the shortest spontaneous RR interval, was used. The histograms of the postextrasystolic intervals were compared with those of the spontaneous
noninterrupted RR intervals. The average postextrasystolic interval was 180 to 300 msec longer than the mean control RR interval, and in six of eight patients, the shape of the histogram of the postextrasystolic cycles was insignificantly different from that of the spontaneous
RR intervals. This suggests that in those six patients, the retrograde impulse had reset the random timing cycle of atrioventricular nodal discharge during atrial fibrillation. This observation
is compatible with the hypothesis that electrotonically-mediated propagation across
a weakly coupled junctional area within the atrioventricular node, rather than decremental conduction and extinction of anterograde atrial impulses at different levels within the node, may be the mechanism of atrioventricular transmission in atrial fibrillation