Halothane uptake and nitrous oxide concentration

The effect on halothane uptake of changing the nitrous oxide concentration during the first few minutes of a general anaesthetic for Caesarean section was investigated. In 10 mothers anaesthesia was maintained with halothane 0.4%, nitrous oxide 33% and oxygen 66%. In 10 others the sole difference in anaesthetic technique was that the ratio of nitrous oxide to oxygen was reversed for the first 3 minutes only. Serial maternal arterial blood samples showed a significant increase in halothane levels in the group where the nitrous oxide was increased, with p less than 0.02 at 1 and 2 minutes and p less than 0.05 at 3.4 and 5 minutes. After 5 minutes there was no significant difference between the two groups. Cord blood concentrations between the two groups were comparable. The difference in halothane levels is a demonstration of the influence of the concentration effect of nitrous oxide on the uptake of halothane, the second gas effect. The relevance of anaesthetic uptake to obstetric anaesthesia and awareness is discussed.