The estimation of inspired isoflurane concentration in a low-flow system

We have examined the predictability of inspired isoflurane concentration during low-flow anaesthesia using a to-and-fro breathing system. Twenty one adult patients requiring mechanical ventilation of the lungs during surgery took part in this study. Using a fresh gas flow of 2 l.min-1, the ratio of inspired isoflurane concentration to isoflurane vaporizer setting was found to be approximately 4/5th after 5 min of anaesthesia. The ratio was maintained throughout the procedure, except for a few minutes following each change in vaporizer setting.