Determinants of the Duration of the Refractory Period of the Atrioventricular Nodal System in Man*

Abstract

A technique was devised for measurement of the refractory period of the atrioventricular conduction system (AVRP) in man. A bipolar electrode catheter was placed against the right atrial wall and stimulated the right atrium at a constant rate just above the sinus rate. Extra impulses were interposed between the regular stimuli; the time intervals between these extra impulses and the preceding stimuli were then shortened progressively. The time interval at which AV conduction just failed was taken as the AVRP. In 20 unanesthetized patients studied in the basal state, the AVRP averaged 350 msec. In 9 patients atrial tachycardia, induced by rapid electrical stimulation of the right atrium prolonged the AVRP. In 7 patients muscular exercise shortened the AVRP by an average of 10 msec. Isoproterenol (7 patients) and atropine (11 patients) resulted in reductions of the AVRP similar to those observed during exercise. Elevations of arterial pressure with methoxa-mine or phenylephrine in 8 patients prolonged AVRP by an average of 252 msec. Since this prolongation could be prevented by atropine, it was considered to result from reflex vagal stimulation. Thus, in any given patient the AVRP is affected profoundly by many influences, including sympathetic and parasympathetic stimuli.