Abstract
Dry immersion (DI) is a Russian-developed, ground-based model to study the physiological effects of microgravity. It accurately reproduces environmental conditions of weightlessness, such as enhanced physical inactivity, suppression of hydrostatic pressure and supportlessness. We aimed to study the integrative physiological responses to a 3-day strict DI protocol in 12 healthy men, and to assess the extent of multi-system deconditioning. We recorded general clinical data, biological data and evaluated body fluid changes. Cardiovascular deconditioning was evaluated using orthostatic tolerance tests (Lower Body Negative Pressure + tilt and progressive tilt). Metabolic state was tested with oral glucose tolerance test. Muscular deconditioning was assessed via muscle tone measurement.Results: Orthostatic tolerance time dropped from 27 ± 1 to 9 ± 2 min after DI. Significant impairment in glucose tolerance was observed. Net insulin response increased by 72 ± 23% on the third day of DI compared to baseline. Global leg muscle tone was approximately 10% reduced under immersion. Day-night changes in temperature, heart rate and blood pressure were preserved on the third day of DI. Day-night variations of urinary K+ diminished, beginning at the second day of immersion, while 24-h K+ excretion remained stable throughout. Urinary cortisol and melatonin metabolite increased with DI, although within normal limits. A positive correlation was observed between lumbar pain intensity, estimated on the second day of DI, and mean 24-h urinary cortisol under DI. In conclusion, DI represents an accurate and rapid model of gravitational deconditioning. The extent of glucose tolerance impairment may be linked to constant enhanced muscle inactivity. Muscle tone reduction may reflect the reaction of postural muscles to withdrawal of support. Relatively modest increases in cortisol suggest that DI induces a moderate stress effect. In prospect, this advanced ground-based model is extremely suited to test countermeasures for microgravity-induced deconditioning and physical inactivity-related pathologies.
Highlights
MATERIALS AND METHODSSpaceflight induces physiological multi-system deconditioning which may impact astronauts efficiency and create difficulties upon their return to normal gravity (Nicogossian et al, 1993)
The benefit of Dry immersion (DI), compared to more widely-known and traditional head-down bed rest (HDBR) technique, is support unloading (“supportlessness”), a state akin to weightlessness, with water hydrostatic pressure distributed over the body surface, providing conditions similar to complete lack of structural support (Grigor’ev et al, 2004; Navasiolava et al, 2011a)
Diuresis remained unchanged on DI1 despite a 30% reduction in water intake, partial water balance decreased up to 700–800 mL and became negative on the first day of DI
Summary
MATERIALS AND METHODSSpaceflight induces physiological multi-system deconditioning which may impact astronauts efficiency and create difficulties upon their return to normal gravity (Nicogossian et al, 1993). Dry immersion (DI) is one such prolonged microgravity model It accurately reproduces most physiological effects of microgravity, including centralization of body fluids and hypokinesia (Kozlovskaia, 2008; Navasiolava et al, 2011a; Watenpaugh, 2016). DI promotes rapid gravitational deconditioning, exceeding for some systems (i.e. for neuromuscular system) the deconditioning induced by spaceflight itself (Navasiolava et al, 2011a). This DI method, developed and widely used in Russia, is not yet routine elsewhere. This study is the first DI protocol specially conceived to assess integrative aspects of “strict” DI impact
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