Fetal oxygen extraction: comparison of the human and sheep

To estimate fetal oxygen extraction in the human, the inverse, linear relationship between umbilical arterial Po 2 and oxygen extraction was calculated from measurements made on cord blood samples obtained after vaginal (n = 12) and cesarean (n = 16) delivery and used to predict oxygen extraction in utero. Comparisons with similar data from fetal lambs in utero and during labor and delivery indicate that extraction in the human and sheep fetus is 47% and 27%, respectively, at Pa o 2 = 20 mm Hg. For the human fetus to have extraction similar to that in the fetal lamb, vascular Po 2 would have to be substantially higher. This would lead to a higher umbilical venous oxygen content in the human, compensating for the lower umbilical blood flow and yielding a rate of oxygen delivery comparable with that observed in the fetal sheep. These data indicate that there may be significant quantitative differences in the fetal oxygen delivery systems in the human and sheep.