Amniotic gas values and acid-base status during acute maternal hyperoxemia and hypoxemia in the early fetal sheep

Objective: This study was undertaken to determine amniotic fluid gas values and acid-base balance during maternal hyperoxemia and hypoxemia in early pregnancy. Study Design: Anesthetized sheep (n = 12) in early and mid pregnancy (0.3 to 0.5 gestation) were subjected to hyperoxemia followed by hypoxemia. Amniotic fluid Po2, pH, Pco2, bicarbonate concentration, and base excess were monitored continuously with a multiparameter Paratrend (Diametrics Medical Inc, St Paul, Minn) sensor and compared between 0.3 and 0.5 gestation. Results: During maternal normoxemia all parameters were constant. At all gestational ages maternal hyperoxemia caused no changes apart from a rapid increase in amniotic fluid P02 (P< .001). Maternal hypoxernia led to a reduced amniotic fluid Pot (P< .001), whereas the Pco2 and the bicarbonate concentration increased (P< .001). Changes in amniotic fluid gas values and acid-base balance were more pronounced at 0.3 gestation than at 0.5 gestation. Amniotic fluid Pot responded earlier to maternal hyperoxemia than to hypoxemia (9.5 vs 14.3 minutes; P< .001). During hypoxemia maternal Paco2 changed faster than did amniotic fluid Pool (P< .001). Conclusions: Acute maternal hypoxemia during early pregnancy was quickly reflected in amniotic fluid gas values and acid-base balance, whereas hyperoxygenation induced the quickest changes in amniotic fluid Pco2. The metabolic effects of maternal hyperoxemia and hypoxemia were also more pronounced during early pregnancy than at mid pregnancy.