Body fluid alterations during head-down bed rest in men at moderate altitude

Area of Science:

• Space physiology
• Environmental physiology
• Cardiovascular regulation

Background:

• Simulated microgravity, such as head-down bed rest (HDBR), induces fluid shifts. Hypoxia is a common environmental stressor.
• Understanding fluid balance under combined stressors is crucial for spaceflight and high-altitude adaptation.

Purpose of the Study:

• To investigate the impact of hypoxia on fluid balance regulation during simulated zero-gravity (HDBR).
• To compare fluid and hormonal responses to HDBR in hypoxia versus normoxia.

Main Methods:

• Six subjects acclimatized to high altitude (5,400 ft) underwent 8 days of -5° head-down bed rest (HDBR) at a higher altitude (10,678 ft).
• A crossover design included a control period (CON) at the higher altitude without HDBR.
• Measurements included plasma volume, diuresis, natriuresis, and key hormones (ANP, ADH, aldosterone, catecholamines).

Main Results:

• HDBR in hypoxia resulted in more pronounced fluid balance changes than previously reported at sea level.
• Plasma volume loss was greater (-19%), and diuresis and natriuresis doubled in magnitude and were sustained longer compared to normoxic conditions.
• Transient increases in plasma atrial natriuretic peptide (ANP) and catecholamines were observed in both HDBR and CON, suggesting their role in enhanced fluid shifts during hypoxia and HDBR.

Conclusions:

• Hypoxia exacerbates fluid loss and hormonal responses during simulated zero-gravity (HDBR).
• Elevated ANP and catecholamines appear to be key mediators of these enhanced fluid shifts under combined hypoxic and HDBR conditions.
• These findings have implications for astronaut health during space missions and for individuals exposed to high altitudes.