Cardiovascular effects of simulated zero-gravity in humans

Area of Science:

• Cardiovascular Physiology
• Human Physiology
• Space Medicine Analogues

Background:

• Head-out water immersion (HOWI) and head-down tilted bedrest are used to simulate microgravity effects.
• Understanding the physiological responses to these simulations is crucial for astronaut health.

Purpose of the Study:

• To compare the effects of head-out water immersion (HOWI) and head-down/head-up tilted bedrest on cardiovascular parameters and plasma catecholamines.
• To investigate the differential impact of these simulated microgravity conditions on sympathetic nervous activity.

Main Methods:

• Subjects underwent 6-hour periods of either HOWI or 5-degree head-down/head-up tilted bedrest.
• Measurements included cardiac output, blood pressure, forearm blood flow, vascular resistances, hematocrit, hemoglobin, plasma volume, and plasma catecholamines.

Main Results:

• HOWI led to decreased total peripheral resistance (TPR), forearm vascular resistance (FVR), mean arterial pressure (MAP), hematocrit (Hct), and plasma volume (PV).
• Plasma catecholamines were 30-50% lower during HOWI compared to both bedrest conditions.
• Head-down tilt elevated MAP and maintained catecholamine levels, with slow decreases in TPR and FVR.

Conclusions:

• HOWI is a more potent stimulus than -5 degree bedrest, likely due to greater central venous pressure elevation and peripheral vein emptying.
• The observed reduction in catecholamines during HOWI indicates a distinct modulation of sympathetic nervous activity compared to bedrest.
• Bedrest may permit venous distension, potentially increasing sympathetic nervous activity, contrasting with the effects of HOWI.