Physiology of Atrioventricular Transmission

This paper describes records of the transmembrane action potential of fibers from different parts of the specialized conducting system and electrograms recorded directly from these fibers in situ. On the basis of these records it is possible to describe certain physiologic mechanisms for conduction delay, block, and supernormal conduction. In general, impaired conduction is associated with a reduced level of membrane potential. This may be caused by incomplete repolarization or partial depolarization. In the normal conducting system, local differences in, action-potential duration and local pacemaker activity most frequently are the cause of a low membrane potential. In disease states, on the other hand, many other factors may be operative. At the atrial margin of the atrioventricular (A-V) node, local anatomic and electrophysiologic properties of the fibers normally cause a very low conduction velocity. The safety factors for conduction here appear to be quite low, and delayed transmission or block often does not result from refractoriness or partial depolarization of nodal fibers. Supernormal conduction, at least in Purkinje fibers, seems to result from the high level of membrane potential reached at the end of repolarization. Whether other factors are responsible for supernormality within the A-V node remains to be seen.