Physiologic Evidence for a Dual A-V Transmission System

A study of the transmission of early premature contractions between atria and ventricles and in the retrograde direction in the dog heart suggests the existence of two parallel A-V conduction pathways communicating with each other over one or more branches. The evidence is based on the excessive delay of very early premature responses in traversing the node, suggesting that a slowly conducting pathway recovers earlier than the normal "fast" pathway; on the echoing back to the chamber of origin of early premature responses; and on ventricular electrograms of "abnormal" configuration obtained during early premature responses. These observations and the hypothesis to which they lead provide a natural explanation for reciprocal rhythm and nodal paroxysmal tachycardia.