304 THE RELATIONSHIP OF PLACENTAL OXYGEN UPTAKE AND TRANSFER TO UTERINE BLOOD FLOW

Abstract

Although several theoretical models have been proposed to explain the relationship of placental oxygen uptake and transfer to uterine blood flow, there is no supportive experimental data in a chronic, non-stressed animal preparation. To provide this evidence, uterine blood flow was decreased by mechanical arterial occlusion in 5 ewes. Each ewe was studied at her normal uterine blood flow and at 3 to 7 different uterine blood flow levels. Uterine and umbilical blood flows were measured with antipyrine by application of the Fick Principle. Oxygen tension (PO2) and oxygen content were measured in the uterine artery and vein and in the umbilical artery and vein. In each animal, we found that fetal and placental oxygen uptake remained constant to at least a 50% reduction in uterine blood flow before decreasing. The PO2's decreased in the uterine vein and in the umbilical vessels as uterine blood flow decreased, but the PO2 difference between uterine and umbilical venous blood remained constant. We conclude that (1) variability in uterine blood flow has no effect on fetal and placental oxygen uptake over a wide range, and (2) the placenta does not become a more efficient respiratory organ as uterine blood flow decreases.