Cardiovascular response to altered gravity in healthy adults: Insight from graded tilt testing.

Microgravity exposure during spaceflight induces a thoracocephalic fluid shift that affects the cardiovascular system both during flight and after return to Earth. As the proportion of female astronauts increases, it is essential to understand how altered gravity impacts cardiovascular function across sexes. In this study, we examined sex differences in central hemodynamics, vascular morphology of the common carotid artery and internal jugular vein (IJV), and IJV pressure during graded head-up to head-down tilt (+45° to -45° in 15° increments) in healthy participants (12 female and 12 male adults). A strong gravitational dependence on almost all variables was observed, except for oxygen consumption. Only a few variables showed significant sex differences, and these include cardiac output, total peripheral resistance, rate pressure product, oxygen consumption, and sympathovagal balance (LF/HF ratio). Overall, hemodynamic, vascular morphology, and IJV pressure responses to tilt were largely similar between sexes. The additional female gravitational dose-response curves augment our previous, male-only database of cardiovascular responses to tilt. Together, these results provide a unique and more comprehensive normative baseline to support the development of spaceflight countermeasures as well as other terrestrial clinical applications, such as surgery in Trendelenburg or prone positioning.