The gating mechanism in the distal atrioventricular conducting system.

AN IMPULSE propagating from the atrioventricular (A-V) node to ventricular muscle must traverse a length of conducting tissue consisting of the bundle of His, bundle branches, peripheral ramifications of the branches, and subendocardial Purkinje network. Electrophysiologic studies of the properties of these tissues have provided a body of information on the nature of normal and abnormal conduction, functional block, and the genesis of the arrhythmias. Recording of transmembrane action potentials from single cells with intracellular microelectrodes has shown that the refractory period of individual cells after a depolarization is primarily related to the duration of the action potential-that is, to the time required to achieve a certain level of repolarization of the membrane' (fig. 1). The refractory period is prolonged as the duration of the action potential increases. It has further been demonstrated that action potential durations in the conduction system increase from the bundle branches distally.2 An area of maximum action potential duration occurs about 2 mm proximal to the termination of Purkinje cells in muscle,3 and then the durations progressively decrease through a sequence of Purkinje cells, transitional cells, and muscle cells4 (fig. 2). Figure 3 demonstrates the significance of the pattern of these changes. The drawing