Nitrous oxide concentrations in maternal and fetal blood during Caesarean section

Abstract

Background and objective: There are little data on nitrous oxide (N2O) concentrations in neonatal blood at delivery. We investigated the effects of the time elapsing between the induction of anaesthesia and delivery (the I-D interval) on umbilical blood N2O concentrations. Methods: Maternal and neonatal blood N2O concentrations were measured in 27 patients undergoing Caesarean section under N2O 67% anaesthesia. The duration of N2O administration (range 2-50 min) was arbitrarily divided into three groups (each n = 9): short (2-9 min), medium (9.1-14 min) and long duration (14.1-50 min). Results: Compared with a rapid increase in the maternal arterial N2O concentration (48.9 ± 4.7%), the umbilical venous N2O concentration (17.9 ± 8.3%) rose slowly in the short duration group, whereas the N2O concentrations became more similar (61.6 ± 4.3 and 43.2 ± 10.0%, respectively) in the long duration group. The ratio of umbilical vein to maternal artery N2O concentrations correlated with the duration of N2O anaesthesia (r = 0.739), resulting in ratios of 0.37 ± 0.18, 0.61 ± 012 and 0.70 ± 0.13 for the short, medium and long duration groups, respectively. The Apgar score at 1 min correlated inversely with the duration of anaesthesia and with the umbilical vein N2O concentration (r = −0.457 and −0.423, respectively). Conclusions: The data suggest that placental N2O transfer during Caesarean section is time-dependent and slower compared with maternal N2O uptake. They also suggest that the Apgar score is less affected by N2O administration when the I-D interval is shorter.