Placental diffusing capacity in unanesthetized rabbits

The purpose of this study was to determine the placental diffusing capacity of CO in unanesthetized rabbits. A standard dose of CO was administered to 18 near-term rabbits, and at time intervals varying from 5 min to 4 h maternal and fetal blood samples were analyzed for carboxyhemoglobin saturation (HbCO). Following administration of CO, the maternal HbCO rose rapidly within the 1st min and remained nearly constant for the remainder of the experiment. The fetal HbCO after CO introduction to the mother rose as CO crossed the placenta from maternal to fetal blood. After 15 min the fetal HbCO rose to equal the maternal HbCO averaging 18.5%. The fetal HbCO after 1 h averaged 33% saturation, 1.7-fold greater than the maternal HbCO at 1 h. Thereafter there was little further fetal rise and apparently equilibrium was reached. The average placental diffusing capacity for CO was 2.3 ml/(min times mmHg times kg), a value fourfold higher than has previously been reported in anesthetized sheep and dogs. Possible reasons are species difference, effects of posture, influence of anesthesia and surgery, and effects of an open versus intact uterus.