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The areas of maximum action potential duration, or gates, of the isolated canine distal A-V conducting system were studied by microelectrode techniques to determine the characteristics of their responses to premature impulses and to rapid rates of stimulation. The results indicate that premature responses will be conducted at the same velocity as driven responses until the coupling interval between the driven and premature impulses approaches the refractory period of the gate. When delayed conduction of the premature impulse develops, it occurs over a segment of the conducting system incorporating the gate. The magnitude of the delay which can develop at coupling intervals just in excess of the functional refractory period tends to be small in healthy tissue and larger in depressed tissue. Suitable delays allowed premature impulses having coupling intervals shorter than the refractory period at the gate to be conducted distally. The refractory period in response to rapid rates of stimulation was discordant in the several false tendons in six of nine preparations studied. Both delayed conduction and 2:1 conduction were shown to occur near the limit of response to rapid stimulation. The data presented suggest certain functional analogies between the A-V node and the gating mechanism.

Functional Characteristics of the Gating Mechanism in the Canine A-V Conducting System