MULTI-SYSTEM PHYSIOLOGIC RESPONSE TO VARYING LEVELS OF PARTIAL GRAVITY

Spaceflight induces detrimental changes in most organ systems with both acute changes and chronic adaptations. For example, acute fluid shifts associated with weightlessness (also called - improperly - microgravity) cause initial rapid cardiovascular alterations, whereas the chronic changes are more reflective of cardiovascular deconditioning and cerebrovascular/ocular adaptations (Pavy-Le Traon et al., 2007). Long-duration stays in weightlessness have resulted in ocular structural and functional adaptations in some astronauts, which has been termed the Spaceflight Associated Neuro-ocular Syndrome (SANS ; see review in Lee et al., 2018). The leading hypothesis is that ocular changes are the consequence of chronic exposure to the weightlessness-induced headward (cephalad) fluid shift experienced by all astronauts. Countermeasures targeted to reverse this fluid shift, including lower body negative pressure and veno-occlusive thigh cuffs, have been proposed and tested in ground-based studies using a head-down tilt model of the cephalad fluid shift. However, the amount of fluid shift reversal required to prevent the development of SANS has not been investigated or determined. Similarly, artificial gravity through centrifugation has been proposed as a SANS countermeasure, given its ability to reverse headward fluid shifts and its efficacy as a countermeasure to long-duration bed rest-induced cardiovascular adaptations (long-duration bed rest is widely employed to simulate the effects of microgravity on various physiological systems). However, the minimum level of artificial gravity required also has not been investigated. The primary objective of this study is to characterize cardiovascular, cerebrovascular, and ocular changes across a range of gravity levels to identify a threshold of gravitational load that can serve as a countermeasure to SANS during future spaceflight missions.