Uptake of halothane and isoflurane by mother and baby during caesarean section

Twenty-three patients undergoing Caesarean section received either 0.5% halothane or 0.8% isoflurane to supplement nitrous oxide-oxygen anaesthesia. We studied the rate of uptake of the agents by the mother and fetus by measuring partial pressures in maternal arterial (Pa) and fetal umbilical venous (Puv) blood. Mean induction-delivery interval did not differ between the halothane (10.8 min) and isoflurane (11.7 min) groups. There were no differences in maternal heart rate, arterial pressure, pH and blood-gas tensions and fetal pH, blood-gas tensions or Apgar scores between the two groups. Isoflurane uptake by the mother was more rapid than halothane; at delivery, mean Pa of isoflurane as a fraction of the inspired partial pressure (Pl) was 0.44 compared with 0.35 for halothane (P < 0.05). Mean Puv as a fraction of maternal Pa at delivery was 0.71 for both agents; thus placental transfer was the same for both agents. Consequently mean Puv/Pl was greater for isoflurane (0.32) than halothane (0.26) (P < 0.05). We conclude that both halothane and isoflurane are suitable agents for general anaesthesia for Caesarean section. The rate of uptake of isoflurane by the mother during Caesarean section was more rapid than halothane. The rate of uptake by the fetus from the mother was the same for halothane and isoflurane, so that fetal partial pressure as a fraction of the inspired partial pressure was greater for isoflurane than halothane.