Orthostatic tests after a 4-day confinement or simulated weightlessness

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Function
• Space Medicine

Background:

• Cardiovascular deconditioning is a known consequence of spaceflight and weightlessness.
• Inactivity, in addition to microgravity, is suspected to contribute to this deconditioning.
• Understanding the specific effects of inactivity and simulated weightlessness is crucial for astronaut health.

Purpose of the Study:

• To compare the cardiovascular effects of 4-day head-down bed rest (HDBR) versus 4-day confinement (C) on orthostatic stress responses.
• To assess changes in blood pressure, heart rate variability (HRV), and spontaneous baroreflex slope (SBS) after HDBR and C.
• To investigate the role of inactivity in cardiovascular deconditioning and orthostatic intolerance.

Main Methods:

• Eight male subjects underwent HUT and LBNP protocols before and after 4-day HDBR and 4-day C.
• Cardiovascular parameters including blood pressure, HRV, and SBS were measured.
• Analysis focused on autonomic nervous system indicators (parasympathetic and sympathetic activity).

Main Results:

• HDBR significantly reduced orthostatic tolerance, with increased instances of orthostatic hypotension.
• Post-HDBR, reductions in RR interval and parasympathetic indicators (PNS) were observed, alongside a decreased SBS.
• Confinement also led to reduced RR interval and PNS, and a decreased SBS, though effects were less pronounced than HDBR.

Conclusions:

• Four-day HDBR impairs baroreflex function and alters autonomic balance, contributing to orthostatic intolerance.
• Inactivity during confinement shows similar, albeit less significant, patterns of autonomic nervous system changes.
• Both simulated weightlessness and inactivity play roles in cardiovascular deconditioning.