Placental gas exchange depends on the pressure gradient of the gases and is brought about by a simple diffusion of gases across the placental membrane.
The rate of gas exchange on the species, the placental membrane is probably lower than that of the lung. The rate varies depending on the species, the stage of gestation, the blood flow and the functional accommodation to tension of environmental gas. (metcalfe et al, 1967; Shapiro et al, 1967; DeReuck et al, 1967;
Bartels et al, 1967; Tominage and Page, 1966).
The fetus adjusts to the environment physiologically by accelerating gas transport in hypoxic states or by reducing transport in hyperoxic states. However, whenever placental gas exchange is disturbed, the health or even the life of the fetus becomes endagered.
Many studies have been made to investigate the mechanism of placental gas transport in order to prevent and correct fetal hypoxia, the main cause of fetal distress.
The present experiment was undertaken to evaluate the effect of oxygen and carbon dioxide on placental gas exchanges in the pregnant rabbit.
Health rabbits of 30 days gestation and their fetuses which weighed an average 2,200gm. and 26gm. were employed in this experiment.
The respiration of the pregnant rabbits was mechanically controlled in such a way that the maternal blood pressure and beart rate were maintained within normal ranges throughout the experiment.
The experimental group were divided into four groups depending on the composition of gas mixtures used: 1) air, 2) ninety-three per cent oxygen plus seven per cent nitrogen, 3) seven per cent carbon dioxide plus twenty-one per cent oxygen and seventy-two per cent nitrogen, and 4) seven per cent carbon dioxide plus
ninety-three per cent oxygen.
Blood samples were taken from the maternal femoral artery before and after mechanical ventilation for 10 minutes and from the umbilical vein after mechanical for 10 minutes.
The blood samples were analyzed for the partial pressure of oxygen and carbon dioxide, pH, and lactic acid. Plasma bicarbonate and base excess in the samples of blood were calculated.
The following results were obtained:
1. In the group of rabbits ventilated with high oxygen (Group Ⅱ), a slight rise of oxygen tension in the umbilical wein and of carbon dioxide tension gradient between fetal and maternal bloods was observed without changes of acid-base balance of umbilical venous blood.
2. In the group of rabbits ventilated with seven per cent carbon dioxide plus twenty-one per cent oxygen (Group Ⅲ), a slight rise of oxygen tension in umbilical vein and a significant reduction of carbon dioxide tension gradient between fetal and maternal bloods were observed. However, there was no change in plasma bicarbonate concentration in spite of the lower pH and higher carbon dioxide tension in umbilical vein.
3. In the group of rabbits ventilated with seven per cent carbon dioxide plus ninety-three per cent oxygen (Group Ⅳ), a tendency for the oxygen tension in the umbilical vein to rise was noted, and the carbon dioxide tension gradient between the fetal and maternal bloods increased significantly when compared with the group ventilated with seven per cent carbon dioxide plus twenty-one per cent oxygen (Group Ⅲ). A lower pH, an elevation of carbon dioxide tension and no change of plasma bicarbonate in umbilical vein were noted.
The above results suggest that high oxygen ventilation had a little inhibiting effect on placental gas exchange while carbon dioxide definitely accelerated the exchange. The effect of carbon dioxide on placental gas exchange was significantly inhibited by a high oxygen content in the breathing mixture of gases.