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Russian program also employs lower body negative pressure (LBNP) with salt loading in the last weeks of flight to improve orthostatic tolerance. LBNP is produced when a rigid chamber encloses the lower half of the astronaut's body and 10 to 50 mm Hg of suction is applied to shift blood footward, producing fluid shifts similar to what occurs with the standing position in 1 g. There is no evidence that salt loading itself prior to reentry increases intravascular volume, with or without LBNP stimulation. Early shuttle era data suggested that salt loading improved orthostatic tolerance postflight, but measurements were often made several hours after landing, and not all subjects were tested.71 The important question is whether the countermeasure improves blood pressure and orthostatic tolerance during reentry and in the first few minutes after landing, when an emergency egress might be required. Virtually no data are available from these time frames. A recent study measured multiple hemodynamic variables including cardiac outputs within 4 hours of landing and was unable to document a difference in orthostatic tolerance between those who did or did not fluid load prior to reentry to 1 g.72 A brief bout of maximal exercise performed just before reentry has been suggested as an intervention to promote fluid and salt retention, but this hypothesis has not been adequately tested.73 The mineralocorticoid fludrocortisone acetate (Florinef) showed promise for promoting fluid retention and elevating blood pressure in some bed-rest studies.74 However, when it was administered to a few astronauts in-flight to produce salt and water retention, the results were disappointing. Further studies have not been conducted.
Liquid-filled cooling garments used during EVA decrease thermally induced increases in skin blood flow and could be used during reentry to maintain cardiac output and blood pressure.7576 Modified anti-g suits, similar to those used in high-performance military aircraft, have been employed to decrease the lower extremity and abdominal venous pooling that occurs during reentry hypergravity, and their use appears to produce the desired hemodynamic improvement. In addition to decreasing venous pooling, these suits raise systemic vascular resistance and blood pressure significantly when used at standard operating pressures.7778 The Russians wrap the lower body tightly with inelastic strapping (karkas) to achieve the same effect as the anti-g suit.
The current exercise countermeasures are broadly applied and do not specifically address muscle atrophy, bone demineralization, aerobic deconditioning, or orthostatic intolerance. It is likely that more efficient, better-focused regimens could be devised if each problem were initially addressed separately, with attention paid to underlying physiological mechanisms. The current Russian exercise countermeasure regimen is enormously expensive in terms of crew time and the amount of additional food and water that must be supplied to support the metabolic requirements associated with daily, prolonged, high-intensity exercise. Also, major differences related to age and gender exist among crew members with respect to bone and muscle metabolism, as well as orthostatic tolerance and aerobic capacity. These differences would seem to require astronaut-specific countermeasure "prescriptions." In addition, current Russian and NASA flight rules require rigid adherence to inadequately tested antiorthostatic countermeasures. This confounds testing of any new interventions and impedes assessment of both current and future countermeasures.
Future Directions
Over the last few decades, a number of space physiology reviews and reports have identified important research questions and recommended studies to address them. The cardiopulmonary data discussed above, obtained during a series of Skylab, Salyut, Mir, and Spacelab Life Sciences missions, provide a fairly comprehensive, systems-level view of the nature and time course of cardiovascular and pulmonary changes that occur when humans are exposed to microgravity for periods ranging from a few days to 13 months. Many of the more observational research questions have been answered at least
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