Modulation of atrioventricular conduction by isometric exercise in human subjects.

In humans, arterial baroreceptow depress atrioventricular (AV) conduction through vagal influences on the AV node but not on the His-Purkinje fibers. We investigated the influence on AV conduction of an excitatory neural influence, i.e., isometric exercise (handgrip). In subjects with normal AV conduction, blood pressure was measured by an intra-arterial catheter, R-R interval by an electrocardiogram, and A-H and H-V intervals (representing conduction, respectively, through the AV node and the His-Purkinje fibers) by His bundle recording. Handgrip raised blood pressure markedly, it shortened R-R interval, and caused no change in A-H and H-V intervals. Because prior studies had demonstrated that autonomic influences over AV conduction can be quantified accurately only when heart rate is constant, the subjects also were studied during atrial pacing. Under this condition, handgrip again caused a marked pressor response but also induced a marked shortening in the A-H interval (30%) whereas the H-V interval still wax unaffected. The handgrip-induced shortening in the A-H Interval was less pronounced after atropine but it was also Impaired by propranolol. Thus AV conduction normally is modulated by inhibitory influences but also by powerful excitatory stimuli. Like the inhibitory influences, the excitatory ones are seen mainly at constant heart rate and are limited to the AV node with no effect on the His-Purkinje fibers. Unlike the Inhibitory Influences, however, the excitatory modulation is substantially mediated via the cardiac sympathetic nerves.