Echocardiography and circulatory response to progressive endurance exercise.

Cardiac ultrasound techniques have provided an abundance of empirical information regarding normal circulatory responses to dynamic exercise. These data are consistent with a schema by which alterations in peripheral resistance, effected by arteriolar dilatation, facilitate blood flow to exercising muscle and match these responses to increasing metabolic demand. In this model, cardiac responses are governed by quantity of systemic venous return, matched by increases in heart rate to maintain a constant ventricular filling volume. In a progressive test in the upright position, stroke volume rises early, then plateaus as work intensity rises. The initial increase in stroke volume reflects refilling of the heart from the sequestration of blood in the dependent extremities by gravity that occurs when assuming the upright position. Well documented improvements in both inotropic and lusitropic function during progressive exercise therefore serve to maintain constancy of stroke volume and ventricular filling, respectively, with progressive shortening of systolic and diastolic time periods as work intensity rises. During exercise, then, the circulatory system appears to act like an arterial venous fistula, with peripheral resistance serving as the principal factor facilitating and controlling blood flow. Observations in subjects with altered circulatory dynamics during exercise (patients with cardiac disease, highly trained endurance athletes) can be understood within the context of this physiological model.