Relation between the minimum RR interval during atrial fibrillation and the functional refractory period of the AV junction

The relation between the minimum RR interval during atrial fibrillation and the functional refractory period of the AV junctional tissues was studied in isolated perfused dog hearts. In each heart, the functional refractory period, the minimum ventricular interval obtained by regular pacing of the atria at increasing frequencies, and the minimum RR interval during atrial fibrillation were compared. It appears that the length of the minimum RR interval during atrial fibrillation is proportional to both the functional refractory period and the minimum RR interval obtained by atrial pacing. These values are nearly equal in preparations where the functional refractory period is short. However, in cases where the functional refractory period is long, the minimum RR interval during atrial fibrillation is appreciably longer than the two other parameters. It is suggested that this difference may be explained in terms of a decreased safety factor between the efficiency of the incoming atrial wavefront as a stimulus and the level of excitability of the atrioventricular junctional tissues. Alternatively, these results may also imply that the conduction velocity of a propagated impulse across the AV junction is prolonged in hearts with longer functional refractory periods.