Maintained prolongation of AV conduction time by acetylcholine.

Although chronotropic responses to maintained cholinergic activation have been reported to fade during tonic vagal stimulation, discrepancy exists as to whether AV conduction behaves similarly. Since previous studies have examined only low grade AV block, we sought to more fully characterize the dromotropic responses to different degrees of maintained cholinergic activation. We used anesthetized mongrel dogs in which the nutrient artery supplying the AV node region was cannulated and perfused with either autologous blood or oxygenated Tyrode solution containing various concentrations of acetylcholine. Electrograms were recorded from the SA node, atrium, ventricle and His bundle. In ten of eleven animals, perfusion with high concentrations of acetylcholine for five min resulted in the production of complete heart block which was sustained for the duration of the infusion. Moderate concentrations of acetylcholine, perfused via the AV node artery, resulted in maintained second degree AV block in five of eight animals and oscillation between first and second degree block in the other three. Perfusion with lower concentrations of acetylcholine resulted in maintained prolongation of the AH interval, which appeared however, to be dependent upon continuous uninterrupted perfusion of the AV node artery with acetylcholine. These results suggest that, irrespective of the magnitude of the dromotropic response during acetylcholine perfusion, fading is not characteristic of AV conduction during maintained cholinergic activation. Thus acetylcholine affects chronotropic and dromotropic responses differently.