Respiratory Function of the Placenta as Determined with Carbon Monoxide in Sheep and Dogs*

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Abstract
A technique is described for studying the respiratory function of the placenta using carbon monoxide, a gas whose exchange across the placenta between the maternal and fetal circulations is limited by diffusion rather than blood flow. During the steady state before the introduction of CO, the normal concentration of carboxyhemoglobin in the ewe, [COHb]M, is approximately 0.90%, and that in the fetus is 2.9%, the ratio [COHb]F/[COHb]M being 3.2. In dogs the corresponding values are 1.9%, 4.8%, and 2.4%. After the introduction of CO into the mother animal, CO diffused across the placenta slowly with an equilibration half-time of approximately 2 hours. The average carbon monoxide diffusing capacity (DPco) of the placenta during maternal to fetal exchange was 0.54 ml per (minute × mm Hg × kg fetal weight) (SD ± 0.13) in sheep and 0.57 ml per (minute × mm Hg × kg) (SD ± 0.18) in dogs. The fetal to maternal placental diffusing capacity in two sheep was 0.54 ml per (minute × mm Hg × kg). Calculations considering the relative rates of reaction of O2 and CO with red cell hemoglobin and the relative rates of diffusion of the two gases suggest that the true DPo2 should be about 1.2 to 2 times greater than the DPco or 0.65 to 1.1 per (minute × mm Hg × kg). This is about 5 times greater than the reported value of DPo2 calculated from measurements of PO2 in the mixed uterine and umbilical venous blood. With a diffusing capacity of this magnitude the maternal and fetal placental end capillary PO2 would approach equilibrium, becoming too small to measure, and the calculation of DPo2 would be unreliable. We suggest that the apparent end capillary Po2 gradients of 15 to 20 mm Hg, obtained from sampling uterine and umbilical venous blood, result from a combination of uneven distribution of maternal and fetal placental blood flow and from placental oxygen consumption.