Hydrogen Ion and Hemoglobin Concentration, Carbon Dioxide and Oxygen Content of Blood of the Pregnant Ewe and Fetal Lamb

¹ From the Department of Obstetrics and Gynecology, the Medical School, and Department of Animal Husbandry, Institute of Agriculture, University of Minnesota, Minneapolis, Minnesota

Sixty-five pregnant ewes and 85 of their lambs were subjected to blood sampling under local anesthesia in acute experiments at 70, 88, 106, 124 and 142 days of gestation. ph and oxygen, carbon dioxide and hemoglobin contents were determined. The ewes were all in a state of mild hypoxia, presumably due to the prone position, accompanied by mild compensated respiratory alkalosis. The fetuses exhibited a lower ph and a higher pCO₂ than the ewes at all stages studied. Hemoglobin gradually rose until it reached adult values at term. The oxygen content of umbilical vein blood reached about 11 vol. % at 88 days and remained at that level thereafter, with a concomitant drop in oxygen saturation. Twinning did not appear to alter these results except insofar as it necessitated uterine manipulation to obtain cord blood samples in two fetuses. It is concluded that our present knowledge of fetal-maternal gas relationships is based on observations under conditions of artifactitious hypoxia. With this reservation, the gradual rise of the over-all fetal-maternal pCO₂ gradient as pregnancy advances suggests limiting factors in the efficiency of exchange. The corresponding increasing tissue hypoxia does not appear to be a stimulus sufficient to produce fetal hemoglobin higher than adult levels over the duration of sheep pregnancy. The analogy made between the state of the fetus and that of the adult respiring at high altitude holds only for oxygen and not ph, pCO₂ or hemoglobin conditions.