The Relationship between Calculated Effect-site Sevoflurane Levels and Awakening from Anaesthesia

Abstract

We have previously described a system that displays real-time estimates of effect-site sevoflurane concentrations. Estimates of effect-site levels should be similar to minimum alveolar concentration (MAC) values, which are determined after allowing time for equilibrium. This study aimed to determine estimated effect-site sevoflurane concentrations at awakening from routine anaesthesia and to compare this with published estimates of MAC-awake. If these values were similar, this would validate our approach to the calculation of effect-site concentration. Sixty-five patients undergoing a variety of surgical procedures were observed. Prior to disconnection from the breathing circuit, forward estimates of effect-site sevoflurane were recorded. Patients were observed in the postanaesthesiacare unit and the time at which they responded to comm and wasrecorded. Age-adjusted effect-site sevoflurane at the time of awakeningwas determined. Correlation with patient, surgical and anaestheticfactors includingage, gender, ASA status and intraoperative opioid usage were explored. Mean age-adjusted calculated effect-site concentration at awakening was 0.59 (SD 0.27) vol%. This value is within the range of values determined for MAC-awake of sevoflurane. There was no correlation with any of the demographic or anaesthetic factors, but patients undergoing major surgery woke at a significantly lower mean sevoflurane level. These results support the use of effect-site sevoflurane concentration to guide administration of anaesthesia.