Effect of changes in inputs to atrioventricular node on AV conduction

The effect of different inputs into the atrioventricular node (AVN) on orthograde impulse transmission was studied. In rabbit AVN preparations, two stimulating electrodes were placed posteriorly in the crista terminalis (P) and anteriorly in the interatrial septum (A). Electric stimuli were given individually (P,A) or simultaneously (P + A). Premature stimulation, when properly timed (e.g. 70-100 msec at the basic cycle length of 400 msec) achieved AV conduction only in P + A but not in either P or A. Action potentials (APs) in AVN in this situation revealed an incomplete depolarization in P and A, but became a fullsized AP in P + A. This might be explained by summation. The site of simmation appeared to be in the upper node, because APs distal to the middle node as well as APs in the His bundle behaved in an "all or none" fashion. Time intervals between the stimulus artifact and the His bundle AP upstroke (S-H) were shorter in P + A was enhanced with the decrease of the S-H difference between P and A suggested an accelerated conduction by summation in AVN. A hyperpolarizing current flowing through the suction electrodes which were attached in the vicinity of AVN caused different amounts of S-H shortening between P and A input. The above data support the view that AVN has a summating property engendered by the slow inactivation of ionic channel on which the upstroke of AP depends and by the limited number and sites of atrial inputs which might connect with some preferential paths within the AVN.