Acclimatization Response of the Pregnant Ewe and Fetal Lamb to Diminished Ambient Pressure

¹ From the Departments of Obstetrics and Gynecology, Medical School and of Animal Husbandry, Institute of Agriculture, University of Minnesota, Minneapolis, Minnesota, and the Department of Anatomy and the Aeromedical and Physical Environment Laboratory, University of Illinois, Chicago, Illinois

Eight pregnant ewes were exposed to diminished ambient pressure from about the 95th to the 105th day of gestation and then operated upon. At 345 mm Hg, one ewe died and two of the survivors had recently dead fetuses. At 385 mm Hg all the ewes and fetuses survived. The ewes were found to have a marked reduction in blood oxygen saturation and pCO₂ and a rise in hemoglobin from prepregnancy levels. Plasma electrolyte distributions were essentially normal, since the reduction in plasma bicarbonate was compensated by a rise in chloride. One ewe was found to have elevated plasma lactate and pyruvate levels. A circulating erythropoietic substance was demonstrated in the ewe plasma. The surviving fetuses, despite a very low pO₂ in umbilical vein blood and an increased hemoglobin, had oxygen saturations nearer to normal than those of the ewe and a normal umbilical arteriovenous oxygen difference. The fetal plasma electrolyte distributions were similar to those found in the normal fetus in our laboratory, except that there appeared to be a decrease in total concentration in the fetuses at 385 mm Hg. In one fetus a very large increase in plasma lactate and blood pyruvate was found, apparently compensated by a profound hypochloremia. The placental oxygen gradient at 385 mm Hg was estimated to be less than one half of normal.