Dynamics of human cardiovascular responses in different periods of long-term exposure to weightlessness

Abstract

The purpose of this work was to determine the dynamics of changes in the state of the human cardiovascular system at rest and upon exposure to lower body negative pressure (LBNP) in different periods of short-term (8–25 days) and long-term (126–438 days) space flights (SFs) using the data of ultrasonic examinations and leg occlusive plethysmography. It was established that the changes caused by blood redistribution and hypovolemia development—a decreased left-ventricular filling and stroke volume without an impairment of myocardial contractility, a decreased renal artery resistance, and an increased maximal capacity of leg veins—occurred in the first week of an SF. Over 30–40 days of SF, these changes increased and were followed by the relative stabilization of hemodynamics at rest. Arterial cerebral blood flow was stable; however, the phenomena of venous congestion in this region increased with the SF’s duration. The most dramatic changes were observed in leg vessels, both in arteries (decreased resistance) and veins (increase in maximum capacity). Changes in the venous part of the cardiovascular system were more marked than in the arterial one. Despite the relative stabilization of the hemodynamics at rest, exposure to LBNP revealed a deterioration of gravity-dependent reactions, which changed as a function of the SF duration. In the first month of FS, the downward trend of the femoral artery vasoconstriction was not detrimental to cerebral blood flow. SF extension impaired the regulation of the vascular tone and caused increased blood flow deficiency on exposure to LBNP. In some cases, the hemodynamic response was affected to the extent that could be regarded as a failure to adapt to orthostatic effects.