Abstract
The cardiac sympathetics and parasympathetics were directly and reflexly activated in dogs having complete heart block. Activation of the vagus produced a very slight decrease in ventricular rate and in blood pressure. The vagus had no direct effect upon the ventricles. It was concluded that vagal fibers may supply the bundle of His and its branches but not the ventricular muscle. Contrasted to the almost negligible effects of the vagus was the striking positive inotropic and chronotropic effects of stimulation of the cardiac sympathetics. Activation of the carotid sinus (carotid hypertension) reflexly induced a ventricular negative inotropic and chronotropic effect. This effect was mediated by reflex inhibition of the cardiac sympathetics, and the vagus played no role. It was concluded that the ventricle in complete A-V block is primarily under the control of the sympathetics. Ventricular asystole was produced in dogs with complete heart block by activation of the carotid sinus and by cessation of ventricular pacemaking. Similar mechanisms nay be active in the production of Adams-Stokes attacks in man.
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