Ventricular Function Curves in the Exercising Dog

Ventricular function curves of cardiac output and stroke volume versus left ventricular end-diastolic pressure were obtained during rapid infusion of Tyrode's solution in dogs standing at rest and during near-maximal exercise both before and after administration of propranolol. Cardiac output and stroke volume ventricular function curves were expressions of first-order processes as indicated by the close fit of data points to computer-derived exponential curves. At near-maximal levels of exercise, the rapid infusion of Tyrode's solution increased stroke volume by an average of 12 ml, indicating that at near-maximal exercise the pumping ability of the heart may be limited by venous return rather than by ventricular performance. The heightened sympathetic activity during exercise increased myocardial contractility, since the plateaus of the ventricular function curves were significantly higher during exercise than at rest. Propranolol negated the increase in contractility associated with exercise and the plateaus of ventricular function curves were then not significantly different from those obtained at rest.