A new model of cardia sudden death in dogs will be used to determine the role of neural reflexes and daily exercise on the amelioration of ventricular fibrillation. Previous studies have indicated that removal of the left stellate ganglion will reduce the incidence of ventricular fibrillation in dogs with an anterior myocardial wall infarction. New preliminary data suggest that daily exercise will also reduce the episoded of ventricular fribrillation in this model. The mechanisms underlying these protective efforts will be investigated in conscious dogs instrumented to measure coronary flow, aortic pressure, left ventricular pressure, left atrial pressure, heart rate and in some cases regional myocardial segment length. The specific aims are directed toward determining the involvement of coronary collateral flow, peripheral resistance, neural reflexes, and control of heart rate in the reduction of ventricular fibrillation. Coronary collateral flow will be determined using the injection of radioactive micrsopheres before and after the removal of the left stellate ganglion. Baroreceptor control of heart rate and peripheral resistance will be determined by altering arterial pressure using vaso-active drugs and aortic constriction. Neural reflex will be assessed during transient myocardial ischemia and submaximal exercise. Daily exercise will be used to alter heart rate control and possibly ventricular function in these studies. Alterations in ventricular function will be assessed by the measurement of left ventricular pressure and regional myocardial segment length. In several of the studies pharmacological blockade will be used to investigate the separate roles of the two divisions of the autonomic nervous system in control of heart rate and ventricular function. The long term goals of this study are to define the mechanisms responsible for the reduction in the incidence of ventricular fibrillation by removal of the left stellate ganglion and daily exercise. The mechanisms may have a similar physiological basis. Cardiac sudden death following myocardial infarction still represents a major problem that has not been resolved. Selection of patients at high risk for ventricular has not proven to be adequate nor has the drug therapy normally used been totally effective. Physiological alteration of the autonomic nervous system by daily exercise may prove to be of tremendous benefit in reducing the incidence of sudden death in this patient population.