Hematologic responses to hypobaric hyperoxia.

Study of the effects of hypoxia, activity, and G forces on human hematopoiesis in an attempt to elucidate these phenomena more precisely. Eight subjects were exposed to an atmosphere of 100% O2 at 258 mm Hg for 30 days, and thereafter immediately exposed to transverse G forces, simulating the Gemini flights' reentry profile. All subjects displayed a significant continuous decline in red cell mass during the exposure period, as measured by the carbon monoxide-dilution method. The Cr51 method also indicated a decline in red blood corpuscle mass. The decrease in red cell mass was due to suppression of erythropoiesis and to hemolysis. After exposure to hyperoxia, all subjects exhibited elevated plasma hemoglobin levels, decreased reticulocyte counts, and decreased red cell survivals. CO production rates and urine erythropoietin levels were unchanged. Two hours after termination of exposure to hyperoxia, all subjects exhibited increased reticulocyte counts which were sustained for longer than two weeks. The progressive decrease in red cell mass was promptly arrested on return to ground level atmospheres. Within 116 days after exposure to hyperoxia, the hematologic parameters of all eight subjects had returned to control levels.