Abstract: The volume of blood pumped by an artificial ventricle or ventricular assist device is extracorporeally monitored as a function of the volume of fluid used to drive the ventricle. The ventricle is divided into two chambers by a flexible diaphragm. Fluid is alternately supplied to one chamber during systole and then removed from that chamber during diastole. In the preferred embodiment, during systole the rate of fluid flow is sensed by establishing a differential pressure between two points in the fluid's flow path. The differential pressure is converted to a proportional voltage which may be integrated to provide the desired volume over a given time interval, which corresponds to the cardiac output within .+-.5-10%.