Effects of heart rate and isoproterenol on the functional refractory period of the AV node

The effects of heart rate and l-isoproterenol on the functional refractory period (FRP) of the atrioventricular (AV) node were analyzed in anesthetized dog. A rectangular hyperbola was adopted to express the relationship between the AV conduction time of the premature beats and the atrial coupling intervals. FRP was shown to be expressed as FRP = E + 2 square root of C - C/(T - E). Thus, it is a function of the effective refractory period (E), basic cycle length (T), and C, a constant. A decrease in the basic cycle length caused a decrease in the effective refractory period as well as FRP; the minimum conduction time remained unchanged. As expected from the above equation, relationship of FRP to the basic cycle length was curvilinear. The above equation also indicates that changes in FRP produced by a change in the basic cycle length were to be ascribed to the change in the AV conduction time of the basic beats. Therefore, FRP under this condition cannot be equated with the refractory period in its true sense. In the paced hearts, l-isoproterenol hydrochloride caused a decrease in the effective refractory period, FRP, C and the minimum conduction time, and the changes in FRP were shown to be correlated with the changes in the effective refractory period, indicating that the change in FRP attained in a given fixed cycle length may be due to a change in the effective refractory period.