THE EFFECTS OF REDUCED CARDIAC SYMPATHETIC TONE ON MYOCARDIAL FUNCTION*

Abstract

The ventricular responses to the increased filling pressure produced by tilting into the head-down position were studied in 10 vagotomized dogs treated with gallamine. Before administration of a ganglionic blocking agent there were only slight changes in heart rate, cardiac output, stroke volume and external work of the ventricles in response to the rise in pressure. After cardiac sympathetic tone was reduced by administration of a ganglionic blocking drug (trimethidinium) heart rate, cardiac output, stroke volume and ventricular work fell. Increasing atrial pressure caused significant increases in output and work, but the levels present before trimethidinium were not achieved. Restoration of the heart rate by electrical stimulation of the atrium with the animals in the head-down position caused no further increase in cardiac output or work. It is suggested that increased atrial pressure in the presence of an intact sympathetic nervous system causes little change in the work output of the heart because of reflex withdrawal of sympathetic tone to the myocardium. Pharmacologic denervation of the heart appears to remove this sympathetic influence which might otherwise modify the effect of an increased filling pressure. The denervated heart responds more clearly to an increase in filling pressure. In this respect its performance resembles that of a heart-lung preparation.