Placental diffusing capacity for carbon monoxide and oxygen in unanesthetized sheep.

We measured placental diffusing capacity for carbon monoxide (DpCO) in 10 unanesthetized sheep with catheters chronically implanted in maternal and fetal vessels. Serially measured DpCO during the last third of gestation averaged 0.55 +/- 0.02 (SEM) ml.(min.Torr.kg fetal wt)-1, essentially the same value as that previously reported in anesthetized, acutely operated animals. While placental CO diffusing capacity increased as a function of gestational age when expressed as ml.(min.Torr)(-1), it remained constant when calculated per kg fetal wt. In eight experiments, DpCO showed no change following the administration of several drugs which combine with cytochrome P450. We interpret this as evidence against the carrier hypothesis for placental CO exchange. We calculated the placental diffusing capacity for oxygen to be 0.68 ml.(min.Torr.kg)-1 and the mean maternal-to-fetal capillary Po2 difference to be 10 Torr. Caculations predict that maternal and fetal oxygen partial pressures equilibrate in end-capillary blood during the course of a single transit in placental exchange vessels; thus placental oxygen exchange is not limited by diffusion.