Placental diffusing capacity for carbon monoxide in unanesthetized guinea pigs

In unanesthetized pregnant guinea pigs 5-7 ml of maternal blood which had been equilibrated with carbon monoxide (CO) was reinfused into the sow's carotid artery. Three serial blood samples were withdrawn from the sow and a single terminal fetal sample obtained for determination of CO content and hemoglobin concentration. Transplacental CO uptake (VCO) was determined as the product of fetal blood CO content and fetal CO space (11.8% of fetal weight). Placental diffusing capacity (DPCO) was calculated by dividing VCO by the mean partial pressure difference between maternal and fetal blood. DPCO (ml-min(-1)/torr) increased significantly with gestational age: 45-50 days = 0.0413, 51-57 days = 0.1092 and 58-68 days = 0.1858. This increase paralleled fetal weight but was not related to placental weight.