Role of Positive Feedback in the Atrioventricular Nodal Wenckebach Phenomenon

In the atrioventricular (AV) nodal Wenckebach phenomenon, there is a progressive increase in the P-R interval and a concomitant reduction in the R-P interval. Two series of experiments were conducted on anesthetized dogs to ascertain whether the changes in the P-R interval depended on the associated changes in the R-P interval. In one series of experiments, the atrium was paced at various basic cycle lengths, and a single test cycle was interposed about every 100 beats. When the test cycle was very short, the increase in the P-R interval sometimes exceeded the associated reduction in the R-P interval. During the subsequent basic cycle, although the P-P interval was much greater than it was during the test cycle, there was a further reduction in the R-P interval. When this change in the R-P interval was appreciable, AV conduction failed despite the increase in cycle length. In the second series of experiments, the AV nodal Wenckebach phenomenon was induced by rapid atrial pacing. When the stimulation mode was suddenly altered to maintain a constant R-P interval, the Wenckebach rhythm immediately ceased, although the mean heart rate had not changed significantly. Therefore, it appears that above a critical heart rate a positive feedback loop is established in which an increase in the P-R interval evokes a cor- responding decrease in the R-P interval. In turn, the curtailed R-P interval that re- flects a less complete recovery of AV nodal excitability elicits a still greater increase in the P-R interval. This cycle continues until an impulse is ultimately blocked. Clamping the R-P interval opens the feedback loop and terminates the arrhythmia.