Oxygenation of the human fetus as a function of hemoglobulin concentration.

We studied the relationships between fetal hemoglobin concentration, acid base status, and lactate concentration in umbilical venous and fetal heart blood in 157 fetuses affected by blood group incompatibility who had been exposed for 214 fetal blood sampling procedures (cordocentesis in 153 and intracardiac puncture in 61 cases). All blood samplings were obtained before fetal blood transfusions were administered. The results indicate that the human fetus can maintain a normal acid-base status until a 50% reduction of the hemoglobin concentration. A further reduction of hemoglobulin is associated with an accumulation of lactate both in umbilical venous and fetal heart blood. The partial oxygen tension and the oxygen saturation in umbilical venous blood remained virtually unchanged with decreasing hemoglobin concentration (r = -0.11, P = 0.21; r = 0.09, P = 0.31, respectively), whereas these parameters decreased significantly (r = 0.33, P = 0.02; r = 34, P = 0.02) in blood obtained from the fetal heart. The partial carbon dioxide tension of umbilical vein blood decreases significantly with a reduced hemoglobin concentration (r = 0.25 P = 0.008). We speculate that these alterations in acid-base status in umbilical vein and fetal heart blood reflect a circulatory transition from a high to a low cardiac output as the hemoglobin concentration decreases.