Posture and the circulation: the age effect

The primary instigator of circulatory response to the upright posture is the rapid displacement of about 10% of blood volume from the thorax to the lower body. The resultant hemodynamic deficit induces postural intolerance, especially orthostatic hypotension, in elderly over 70 years of age and in some young subjects after exposure to weightlessness. In this review, our objectives have been: 1) to describe in the normal subject the hemodynamic consequences of the headup posture, as well as lower body negative pressure, the compensatory responses intended to cope with these stresses, and their mechanisms; 2) to outline the effect of age on the circulatory responses to these stresses; and (3) to analyze and compare the tests currently used to assess circulatory tolerance. Our ability to design effective countermeasures to orthostatic circulatory intolerance is severely handicapped by our inadequate knowledge of the basic hemodynamic events incident to normal and abnormal orthostatic tolerance. We believe that better understanding and standardization of the postural tests, better experimental design to include greater emphasis on inter and intra-individual variability, and wider application of currently available noninvasive circulatory techniques would greatly improve the prospects for success in this research area.