Uterine and whole body oxygen extractions in the pregnant rabbit under chronic steady-state conditions

Seventeen pregnant rabbits were studied under conscious, unstressed conditions after catheterization of the right ventricle (RV), a femoral artery (A), and a uterine vein (UV). Respiratory gas tensions, pH, and oxygen saturations and contents were determined serially throughout the latter half of gestation. The uterine coefficient of oxygen extraction increased with gestational age (R = 0.86) and became 60.6% +/- 0.3% during the last 4 days of pregnancy. To compare uterine perfusion with whole body perfusion in relationship to oxygen demands, the (Ao2 - UVo2/Ao2 - RVo2) ratio was computed. The ratio was greater than or equal to 1 after 20 days of gestation, which demonstrated that in regard to oxygen demands the rabbit uterus is relatively underperfused compared to the rest of the maternal organism in the last part of pregnancy. A comparison with analogous data in other species demonstrates that the pregnant rabbit, like the guinea pig, has a much lower rate of uterine blood flow than does the pregnant sheep. These interspecies differences in the perfusion rate and oxygen extraction of the pregnant uterus are related to differences in placental structure.