Refractoriness in Cardiac Muscle

A preparation consisting of a papillary muscle and attached bundle of Purkinje fibers has been employed to study refractoriness of single cardiac fibers of the dog heart. Transmembrane stimulation of single fibers and records of the transmembrane potential have been used to compare the stimulating efficacy of applied current pulses and normally propagated action potentials. The absolute and effective refractory period and full recovery time of the single cardiac fiber are outlined in a similar manner by both applied cathodal stimuli and the propagated action potential. Two factors, a local response and a change in the local action potential, have been shown to contribute to the latency of response to stimuli applied during the relative refractory period. These studies have also demonstrated a considerable safety factor in propagation in cardiac muscle.