Adaptation of physiological systems to weightlessness

Abstract

Exposure to weightlessness affects most physiological systems in the human body. Usually, the clinical manifestations of the so-called space adaptation syndrome are relatively benign. The intensivist will be mostly concerned about the cardiovascular deconditioning experienced by astronauts, which is marked by vascular remodeling and changes in blood volume, cardiac performance, autonomic nervous system and hormonal regulation. The new physiological status reached after the adaptation phase, rather than a pathological state, is likely to correspond to a new equilibrium which matches reduced cardiac loading conditions. After a space mission, the return to gravity leads to another transitioning phase, which is sometimes complicated by potentially serious manifestations. Among those, orthostatic intolerance and reduced exercise capacity represent the most serious operational risks. During and immediately after a space flight, any additional acute condition involving a risk of hemodynamic compromise (absolute or relative hypovolemia, cardiac function alteration, general anesthesia, mechanical ventilation) would most likely lead to a challenging situation.

Research focusing on the question of anesthesia and critical care remains scarce, because no terrestrial model on Earth is able to accurately reproduce the changes induced by microgravity. During the history of manned space flight, severe medical events (excluding spacecraft failures) have remained uncommon, mostly thanks to an in-depth medical selection process. In the close future, the onset of a space tourism era with both more people enter zero gravity and a potentially less medically focused customer selection, could see a shift in this pattern and an increase in severe medical events.