Placental transmission of nitrous oxide Was studied in 14 patients during elective cesarean section and in 26 patients during vaginal delivery. Duration of ancsthesia varied between 2, and 19 minutes. Randomly-selected patients were hyperor hyporcntilated. Umbilical vein nitrous oxide levels ranged from 55 to 91 per cent of maternal values (average 79 per cent). The fetal-maternal nitrous oxide concentration ratio showed no correlation with incidence of uterine contractions, duration of anesthesia (after the first two minutes), or materanal pH (7.32-7.59) or Paco2 (14-41 torr). Umbilical artery nitrous oxide levels ranged from 31 to 90 per cent of the umbilical vein levels. The umbilical artery-vein nitrous oxide concentration ratio increased progessivcly with increasing durations of anesthesia. The data demonstrate rapid transmission of nitrous oxide across the human placenta as well as rapid uptake by the fetus.
(C) 1970 American Society of Anesthesiologists, Inc.