Orthostatic tolerance and hormonal changes in women during 120 days of head-down bed rest

Background: Women will be included as mission specialists in the upcoming i nternational Space Station program. This paper describes the changes in vol ume-regulating hormones and determines the degree of degradation in orthost atic tolerance in a group of women after 120 d of bed rest. The aim of this study was to test a countermeasure program to be used by women during long -duration spaceflights. Methods: For 120 d of -6 degrees head-down bed rest (HDBR), eight healthy women were assigned either to a no-countermeasure (N o-CM, n = 4), or to a countermeasure (CM, n = 4) group. In the countermeasu re group, exercise began after 2 wk, pharmacological agents were given duri ng the ist and 3rd mo, and the "Centaur" suit was worn on the last day of b ed rest and during the day time for several days after bed rest. Diet suppl ements were taken during the 1st and 4th mo of HDBR. Tilt tests were run be fore and after HDBR. Results: After the HDBR, none of the CM subjects, had pre-syncopal or syncopal symptoms during tilt tests: BP was well maintained in the CM group, while heart rate and BP changed in the No-CM group. In pl asma, atrial natriuretic peptide (ANP) increased in both groups and remaine d high throughout HDBR, while aldosterone increased and remained elevated i n the No-CM group. Natriuresis was decreased during HDBR. Conclusion: The C M protocols used during this study were efficient and prevented orthostatic intolerance for the four CM subjects. It would be necessary to obtain more data regarding this set of CM protocols on female subjects to lead to stat istical and formal conclusions.