Model for the administration of low-flow anaesthesia

A general (multiple-gas) three-compartment mass-balance model of the circle-absorber breathing circuit with intermittent positive-pressure ventilation has been developed. We propose it as a tool to determine flowmeter and vapourizer settings for inhalation anaesthesia by low-flow methods (less than 1 litre min-1 total fresh gas flow). This model reproduces the results of various previously-published mass-balance models, but it appears to underestimate slightly the delivery of fresh gases to the inspired limb of the breathing circuit when tested with clinical data from surgical cases. Low-flow dosing schedules for 35% inspired oxygen and 1% inspired halothane were computed with the model and tested in vitro with a circle-absorber breathing circuit and an active gas-exchange lung at nine values of simulated patient gas exchanges. The mean inspired oxygen concentration over all trials was 32.8% (SD = 1.9%), while the mean inspired halothane concentration was 1.2% (SD = 0.3%). The flow meter and vapourizer settings calculated from the model appear to have sufficient accuracy to be useful in the clinical setting in conjunction with active oxygen and anaesthetic agent monitoring.