This (NASA) study investigates the cardiovascular responses to: changes in intraventricular hydrostatic pressure; intravascular hydrostatic pressure; and to a limited extent, extravascular and pedcardial hydrostatic pressure. A complete hydraulic model of the cardiovascular system has been built and flown aboard the NASA KC-135, and a computer model developed and tested in simulated microgravity. Results obtained with these models confirm that a simple lack of hydrostatic pressure within an artificial ventricle causes a decrease in stroke volume. When combined with the acute increase in ventricular pressure associated with the elimination of hydrostatic pressure within the vasculature, and the resultant cephalad fluid shift with the models in the upright position, stroke volume increases in the models. Imposition of a decreased pedcardial pressure in the computer model, and in a simplified hydraulic model, increases stroke volume. Physiologic regional fluid shifting is also demonstrated by the models.

The unifying parameter characterizing the cardiac response is diastolic ventricular transmural pressure (DVDELTAP). The elimination of intraventricular hydrostatic pressure in O-G decreased DVDELTAP stroke volume, while the elimination of intravascular hydrostatic pressure increased DVDELTAP and stroke volume in the upright posture, but reduced DVDELTAP and stroke volume in the launch posture. The release of gravity on the chest wall and its associated influence on intrathoracic pressure, simulated by a drop in extraventricular pressure, increases DVDELTAP and stroke volume.