Metabolic aspects of a hypoxia test for placental sufficiency

A hypoxia test for placental sufficiency was performed on 24 healthy pregnant women nearing term and 23 high-risk pregnant women in advanced stages of gestation. Hypoxia was induced by inhalation of a breathing mixture containing 7% oxygen through a semi-open system which permitted partial rebreathing, thus preventing hypocapnia. The changes in fetal heart rate (FHR) resulting from maternal hypoxia were recorded continuously by external or internal monitoring. The changes observed in FHR were more marked in high-risk cases. Maternal blood gases were tested before and after 10 min of hypoxia in 10 normal and 10 high-risk patients. In no case was maternal acidosis observed. All pregnant patients regarded as high-risk because of hypertensive disorders or chronic renal disease manifested metabolic alkalosis. This finding cannot be explained. It is regarded as being responsible for the more severe FHR changes through a negative Bohr effect, which reduces the dynamic exchange of oxygen in the placenta. Metabolic alkalosis does not appear in high-risk pregnant women exposed to milder hypoxia by breathing 12% oxygen. The changes in FHR observed during severe maternal hypoxia, i.e., 7% oxygen, are probably due more to maternal metabolic alkalosis than to placental insufficiency. Consequently, the "severe hypoxia test" cannot be used as a test for placental insufficiency.