MECHANISMS IN RECIPROCAL RHYTHM

Reciprocal rhythm is defined and the mechanisms described in 2 cases. Reciprocal rhythm can occur with impulses of atrial, A-V nodal, or ventricular origin. Two separate A-V nodal pathways are an essential prerequisite. The two pathways may communicate with each other in the upper or lower parts, or both, of the A-V node to form upper or lower common pathways, or both. Reciprocal rhythm can occur in the presence of both upper and lower common pathways; it can also occur in the absence of a common pathway. The time of inscription of the retrograde" P′ deflection does not necessarily reflect the return level of the reciprocal mechanism. An atrial bridge is not an essential link in the reciprocal circuit. Unidirectional block in one of two pathways is necessary for the initiation of reciprocal rhythm. The nomenclature is reviewed. © 1969."