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The spread of excitation wave fronts over the atrial septum of puppies, adult dogs, and rabbits was studied in vitro by extracellular measurements with a 50μ-diameter electrode. Wave front spread through the AV node of puppies and rabbits was determined, and the functional location of the junctional region between atrial muscle and the AV node was evaluated during antegrade atrial and retrograde His bundle pacing. For dogs of all ages, the pattern of spread over the septum was greatly affected by the location of the pacemaker site simulating a high or low sinus node position. For all sinus pacing positions, wave fronts spread over the crista terminalis to form a posterior input to the AV node, while the anterior septal wave fronts formed another input. No functional evidence could be found for narrow, specialized inteniodal tracts of fixed location. Rather, wave fronts spread over broad areas creating patterns of simultaneous, multiple wave fronts which corresponded in extent to the gross anatomical landmarks of the septum. Only in this fashion were the findings consistent with the idea of three general routes of intemodal conduction in the dog and two general routes in the rabbit. The position of the functional boundary between atrium and AV node could be accounted for only by overlapping of the two tissues of this region; the boundary shifted between antegrade and retrograde conduction. Antegrade wave fronts within the AV node accelerated from the superior border of the node to the exit at the His bundle; retrograde excitation wave fronts from the His bundle decelerated through the AV node. The amplitude of the atrial wave forms as well as the status of die inputs from the posterior and anterior regions of the atrial septum were found to be important factors in AV node conduction.

Excitation Sequences of the Atrial Septum and the AV Node in Isolated Hearts of the Dog and Rabbit