The quotient end-tidal/inspired concentration of sevoflurane in a low-flow system.

STUDY OBJECTIVE: To investigate the effect of two different fresh gas flows on inspired and end-tidal sevoflurane concentration for a given vaporizer setting in a low-flow anesthesia system.DESIGN: Prospective clinical study.SETTING: Department of Anesthesiology of a university teaching hospital.PATIENTS: 56 ASA physical status I and II patients without systemic diseases, having elective surgery with an expected anesthesia time of at least 120 minutes.INTERVENTIONS: Patients were randomly assigned to receive either 1.0 or 2.0 L/min fresh gas flow with the vaporizer setting fixed at 2% sevoflurane. The inspired (In), end-tidal (Et), and Et/In ratio sevoflurane concentrations were estimated.MEASUREMENTS AND MAIN RESULTS: After 120 minutes of sevoflurane anesthesia the inspired and end-tidal sevoflurane concentration were 1.45 +/- 0.10% versus 1.28 +/- 0.12% (p < 0.001) in the 1.0 L/min group and 1.64 +/- 0.08% versus 1.46 +/- 0.11% (p < 0.001) in the 2.0 L/min group. The ratio end-tidal and inspired concentrations/vaporizer setting was 0.64 +/- 0.06 and 0.73 +/- 0.05 in the 1.0 L/min group versus 0.73 +/- 0.05 and 0.82 +/- 0.04 in the 2.0 L/min group. For the ratio inspired and end-tidal/vaporizer setting there were significant difference between the groups (p < 0.001). The estimated ratio end-tidal/inspired was 0.88 +/- 0.04 in the 1.0 L/min group versus 0.89 +/- 0.04 in the 2.0 L/min group (ns).CONCLUSION: After 120 minutes of sevoflurane anesthesia at a vaporizer setting of 2% there is a significant difference between fresh gas flow of 1.0 and 2.0 L/min for inspired and end-tidal concentrations, but not for the ratio end-tidal/inspired.