Relationship of the Functional Refractory Period to Conduction in the Atrioventricular Node

The functional refractory period of atrioventricular (AV) transmission has been accepted as a measure of AV nodal refractoriness and has been assumed to be determined solely by conduction of interpolated extrasystoles through the AV node when it is partially refractory. In the present study, we found an important effect of the conduction time of the regular beats by measuring AV nodal conduction times of atrial extrasystoles from the His bundle of isolated, blood-perfused dog hearts. We separated three independent components that determine AV nodal conductivity: (1) a "basal conduction time" measured with a "postmature" extrasystole at low heart rates (<120 /min), (2) a rate-dependent increment in conduction time (previously called "fatigue") that affects both normal and premature cycles equally and (3) an exponential change in conduction time that depends entirely on the immediately preceding interval and, therefore, is not further affected by heart rate. The functional refractory period is one point defining this continuous exponential function. We showed that an important cause of the decrease in the functional refractory period that occurs when heart rate is increased is the change in the conduction time of the regular beats.