Some Characteristics of Transmembrane Potentials of AV Nodal Cells during Propagation of Premature Beats

Transmembrane potentials of AV nodal cells in isolated rabbit hearts were recorded during propagation of premature responses. With selected patterns of stimulation it was possible to obtain conduction block at different levels inside the AV node. When block occurred, action potentials recorded just proximal to the sites of block were brief. When propagation was barely successful, action potentials obtained from the same cells were prolonged and showed a hump in the repolarizing limb. Assuming that the AV node behaves as a functional syncytium, these phenomena can be explained as follows. (1) When conduction block occurs, local circuits established between active and inactive regions provide repolarizing current to the distal active region. This current will shorten the action potential of such cells. (2) When propagation is barely successful, the impulse may stop and resume its travel after a delay. While the impulse is arrested, repolarizing current will tend to shorten the action potential of the active region, but when propagation starts again, the newly excited tissue will provide depolarizing current to the previously active region, causing a delay in repolarization. Parallel changes in the refractory phase were also demonstrated.