Amniotic gas values and acid-base status during acute maternal hyperoxemiaand 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 p regnancy. STUDY DESIGN: Anesthetized sheep (n = 12) in early and mid pregnancy (0.3 t o 0.5 gestation) were subjected to hyperoxemia followed by hypoxemia. Amnio tic fluid PO2, pH, PCO2, bicarbonate concentration, and base excess were mo nitored continuously with a multiparameter Paratrend (Diametrics Medical In c, St Paul, Minn) sensor and compared between 0.3 and 0.5 gestation. RESULTS: During maternal normoxemia all parameters were constant. At all ge stational ages maternal hyperoxemia caused no changes apart from a rapid in crease in amniotic fluid PO2 (P < .001). Maternal hypoxemia led to a reduce d amniotic fluid PO2 (P < .001), whereas the PCO2 and the bicarbonate conce ntration increased (P < .001). Changes in amniotic fluid gas values and aci d-base balance were more pronounced at 0.3 gestation than at 0.5 gestation. Amniotic fluid PO2 responded earlier to maternal hyperoxemia than to hypox emia (9.5 vs 14.3 minutes; P < .001). During hypoxemia maternal PaCO2 chang ed faster than did amniotic fluid PCO2 (P < .001) CONCLUSIONS: Acute maternal hypoxemia during early pregnancy was quickly re flected in amniotic fluid gas values and acid-base balance, whereas hyperox ygenation induced the quickest changes in amniotic fluid PO2. The metabolic effects of maternal hyperoxemia and hypoxemia were also more pronounced du ring early pregnancy than at mid pregnancy.