Nitrous oxide: use in low-flow systems/economics

The development of rebreathing systems and the history of the rebreathing technique in anaesthesia are closely related to the use of nitrous oxide (N2O) as an anaesthetic gas. For many years a mixture of oxygen and N2O has been used as the carrier gas for delivering inhalational agents, with no thought being given to its true value or disadvantages. Several contra-indications and undesirable effects as well as the need to reduce pollution of the workplace and the atmosphere by this gas are strong arguments for consistently omitting N2O. This has become all the more apparent as clinical experience has revealed that the use of this anaesthetic gas only subtly, if at all, alters the course of inhalation anaesthesia and the development of untoward outcomes. This chapter focuses on the more technical and practical aspects of N2O-free inhalation anaesthesia. When N2O is used as a component of the carrier gas, with a fresh gas flow reduced to 0•5 l/min, i.e. with minimal flow anaesthesia, generally the limits of safe performance of conventional anaesthetic machines are reached. Consistent omission of N2O, however, considerably facilitates the routine use of fresh gas flows that are even lower than 0•5 l/min. The fresh gas flow can be reduced to just that amount of oxygen taken up by the patient, thus making it possible to even perform closed system anaesthesia with conventional anaesthetic machines. The missing anaesthetic and analgesic effects, resulting from the omission of N2O, necessitates only a minor increase in the anaesthetic agent concentration and in the dose of supplementarily applied opioids. Nevertheless, if consistently judicious use is made of the low-flow anaesthetic technique an increase in costs can be avoided.