Influence of maternal hyperoxia or hypercarbia on the hemodynamics of the placenta and fetal brain.

Pregnant rabbits were subjected to inhalation of different gases, and the changes in placental blood flow (PBF), fetal heart rate (HR), and fetal cerebral blood flow (CBF) associated with the changes in maternal blood gas levels were studied. The results are given below. In maternal hyperoxia, maternal blood pressure (BP) was not much influenced and PBF remained unchanged or was slightly decreased when the PCO2 level was not varied or when it was lowered. In contrast, not only maternal BP but also PBF was increased when the PCO2 level was elevated. In the absence of a conspicuous increase in PCO2, neither fetal HR nor CBF varied, regardless of PBF. Both maternal BP and PBF were increased in mild to moderate maternal hypoxia (PO2 greater than 40 mmHg) and decreased in severe hypoxia (PO2 less than 30 mmHg). The decreasing trend of fetal HR or CBF was strengthened as maternal hypoxia was intensified. An obvious decrease in either parameter was observed in severe maternal hypoxia (PO2 less than 30 mmHg). Fetal HR and CBF were well maintained. in hypoxic dams with increased PBF as compared with those with unchanged or decreased PBF. The higher the PCO2 level or the lower the pH value, the more was fetal bradycardia that was apt to occur, even in a mildly hypoxic state.