Nitrous oxide concentrations in maternal and fetal blood during caesarean section

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.

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).

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).

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.