Body volume changes during simulated microgravity. II: Comparison of horizontal and head-down bed rest.

Changes in segmental fluid volume were measured in 10 male subjects during 7 d of either horizontal (0 degrees) or antiorthostatic (-6 degrees) head-down bed rest as part of the Joint U.S./U.S.S.R. Hypokinesia Program. Impedance plethysmography (IPG) was used to measure leg, pelvic, thoracic, and arm volume changes prior to, during, and following bed rest. Results of this study demonstrate that antiorthostatic bed rest produces a larger and more persistent thoracic fluid shift than horizontal bed rest. Segmental volume responses found during this investigation are compared to those obtained during other simulated or actual space-flight experiments. Antiorthostatic thoracic volume changes from this study were found to closely approximate those estimated to take place in space while leg volume changes produced by either 0 degrees or -6 degrees bed rest were much less than those observed by others in the microgravity environment.