Low-flow anaesthesia at a fixed flow rate

This study attempts to assess the safety of low-flow anaesthesia (LFA) at fixed flow rates with particular reference to the incidence of a decline in FiO(2) below safe levels of 0.3 and to determine whether LFA can be used safely in the absence of an FiO(2) monitor.

A total of 100 patients undergoing procedures under general anaesthesia at fresh gas flows of 300 ml/min of O(2) and 300 ml/min of N(2)O were monitored while maintaining the dial setting of isoflurane at 1.5% for 2 h. The changes in gas composition were analysed and even a single recording of FiO(2) of <0.3 was considered sufficient to render the technique unsafe in the absence of gas monitors.

The lowest recorded value of FiO(2) was 31% (v/v%). There was no incidence of adverse events necessitating the conversion from low flows to conventional flows.

We conclude that low flows of 300 ml/min of N(2)O and 300 ml/min of oxygen can be used safely for a period of 2 h without the use of monitors for gas analysis of oxygen and agent in adult patients weighing between 40 and 75 kgs.