Sites of Impulse Formation within the Atrioventricular Junction of the Rabbit

Spontaneous A-V nodal rhythm was observed in 23 isolated, perfused rabbit hearts during electrophysiologic studies of A-V transmission. In 19 of the 23, the earliest point of activation was determined by recording transmembrane potentials from numerous A-V junctional fibers. Most commonly (15 hearts), the earliest depolarization was found in a specific area of the A-V node near the point where the fibrous A-V ring extending into the coronary sinus joins a portion of A-V ring between right atrial and ventricular free walls. Various observations suggested that this area was the NH region. In the remaining four hearts, the origin of nodal beats was in the AN region. Diastolic depolarization was often seen in and around the earliest point of activation. The A-V interval in these nodal rhythms ranged from -38.7 to +71.6 msec, in contrast to the A-V interval of 93.3 to 135.7 msec during sinus rhythm. In the presence of NH rhythm, different retrograde intranodal conduction time (across the N region) was the major cause for the wide variation of the A-V interval. Wandering of the pacemaker between the S-A and A-V nodes and examples of exit block of A-V nodal impulses were observed.