Effect of site, summation and asynchronism of inputs on atrioventricular nodal conduction and refractoriness

The impulses coming from the sinus node synchronically penetrate the A V node via the crista terminalis and inter-atrial septum. Studies in superfused rabbit AV preparations suggest that the crista terminalis is a more effective input than the inter-atrial septum, and that the summation of both inputs facilitates AV nodal conduction. The aim of this study was to verify the hypothesis in a more physiological model, such as the whole rabbit heart perfused by a Langendorff system. Fifteen rabbit hearts were studied in a Langendorff perfusion system with six bipolar extracellular electrodes: two for stimulating (crista terminalis and inter-atrial septum) and four for recording (crista terminalis, inter-atrial septum, His bundle electrogram and right ventricle). Seven hearts (Group I) were consecutively paced at the crista terminalis, inter-atrial septum and both sites simultaneously, to determine the AV nodal Wenckebach cycle length and effective refractory period under basal conditions and after acetylcholine (0.75 × 10−6 M). In eight hearts under 0.75 × 10−6 M acetylcholine (Group II), the crista terminalis and inter-atrial septum were simultaneously (delay = 0 ms) or sequentially (delay = 2,4, 6,8,10,12,14,and 16 ms) stimulated to calculate the AV nodal effective refractory period and the AH interval at an atrial coupling interval 5 ms longer than the AV nodal effective refractory period, for each delay tested. There were no basal differences in AV nodal parameters during crista terminalis pacing, inter-atrial septum pacing or simultaneous stimulation in both sites in Group I; after acetylcholine, the AV nodal Wenckebach cycle length and effective refractory period tended to be shorter during crista terminalis pacing (crista terminalis = 188 ±33 and 147±34; inter-atrial septum = 195±35 and 158±35; both sites = 195±34 and 154±36; values expressed in cycle length of pacing-ms), although the differences did not reach statistical significance. In Group II, the AH interval tended to prolong slightly on increasing the delay between crista terminalis and inter-atrial septum stimulation (delay 0 = 119±31, 2 = 125±29, 4 = 129±33,6 = 129±29,8 = 128±30,10 = 134±34,12 = 132±35,14 = 129±32,and 16 = 131 ±31 ms),but again the differences did not reach statistical significance; the A V nodal effective refractory period did not change when the delay was varied. Conclusions: (1) Neither the input site nor the synchronous summation of inputs plays an important role in A V nodal conduction. (2) These results suggest that A Vnodal response during atrial tachyarrythymias depends more on atrial rate than on shiftings in site and time coupling of inputs.