Effects of atrial pacing site on rate-dependent AV nodal function in rabbit hearts

The functional origin of the changes in atrioventricular (AV) nodal function with the atrial pacing site was studied in isolated rabbit heart preparations. The rate-dependent AV nodal properties of recovery, facilitation, and fatigue were characterized with premature stimulation protocols repeated for each of three atrial pacing sites (upper atrium, low crista terminalis, and low interatrial septum). The effects of the atrial pacing site, reference site from which the beginning of nodal activation is measured (low crista and low septum) and stimulation protocol on nodal conduction and refractory parameters, were assessed with multifactorial analyses of variance. The changes in nodal parameters with the stimulation protocol did not differ significantly with the pacing site, indicating that the rate-dependent nodal properties are not affected by the atrial origin of the impulse. Only the baseline value of nodal parameters varied with the atrial pacing and reference site. However, the comparison of data obtained while the low crista was the pacing and reference site to those obtained while the low septum was the pacing and reference site yield no statistically significant differences, thus indicating that changes in perinodal activation were largely responsible for the observed changes in baseline. Upper atrial and low crista pacing yielded very similar data. We conclude that 1) the atrial pacing site affects perinodal activation but not rate-dependent nodal function, 2) the two inputs are equally effective in activating the AV node, and 3) input summation is a minor factor in rate-dependent nodal function.