A COMPARISON OF THE EFFECTS OF EPINEPHRINE AND VAGAL STIMULATION UPON THE REFRACTORY PERIODS OF THE A-V NODE AND THE BUNDLE OF HIS.

Abstract

In dog hearts perfused from donor animals, recording and stimulating electrodes were located in the right atrium and in the proximal portion of the bundle of His. The minimal interval between two His responses propagated from the atrium (A-H F.R.P.) was determined, and the recovery of excitability of the bundle was directly measured at the same basic driving frequency. In the absence of autonomic influences the minimal interval between two His responses propagated from the atrium was longer than the total refractory period (absolute plus relative) of the bundle. The possibility that a decrease of the action potential of the lower elements of the node was responsible for long delays or block in the activation of the bundle was tested and found unlikely. During vagal stimulation the recovery of excitability of the bundle and its diastolic threshold were not modified, but the A-H F.R.P. was increased. During epinephrine administration it was possible on occasion to show that the A-H F.R.P. was equal to the H F.R.P. estimated by stimulating the bundle directly with strong electrical shocks. These results suggest that in the absence of autonomic influence and during vagal stimulation the weakest link for propagation is located within the A-V node rather than at the junction of node and bundle. During strong adrenergic influences, the functional refractory period of the bundle may be a limiting factor in the propagation of impulses from atrium to the bundle of His.