Fetal blood and tissue PO2 during maternal oxygen breathing

The effect of 100% oxygen breathing on fetal blood and tissue PO2 was examined in chronically catheterised fetal sheep of 12 pregnant ewes. Blood PO2 was monitored with intravascular polarographic electrodes and tissue PO2 with galvanic electrodes implanted in various tissues. In non-acidotic healthy fetuses, oxygen breathing increased fetal PO2 by 30% within 3 min in arterial blood and within 5 min in tissues. Thereafter blood PO2 increased by a maximum of 40% at 9 min and tissue PO2 rose by a maximum of 52% at 12 min. Blood PO2 returned promptly to control values after 15 min of oxygen breathing but washout of oxygen from tissues was slower and tissue PO2 remained significantly higher in the post-oxygen versus the pre-oxygen period. Continuation of oxygen breathing for 30 min did not lead to any further increase in tissue PO2. Oxygen breathing did not prevent a decrease in fetal PO2 associated with spontaneous myoelectrical activity of the uterus. It is concluded that maternal oxygen breathing leads to a significant increase in both blood and tissue PO2 in the healthy fetus. The majority of the increase occurs within the first 3-4 min and no further increase in either blood or tissue PO2 is observed after 12 min.