As, in normal practice, the decision whether to include of nitrous oxide in the anaesthetic mixture is largely discretionary on the part of the anaesthetist, and therefore allocation to either arm of the protocol is consistent with routine practice, it is intended that the patients will be approached for consent on admission to hospital for their surgery. Standard patient monitoring will be used including BIS and a 2 mL sample of blood will be taken to assess blood gas content lung function and optimise lung ventilation.