Fresh gas flow (FGF) influences the speeds of induction and emergence. In general, emergence protocol involves a stepwise decrease in the vaporizer setting at fixed FGF, which causes anesthetic overuse and contaminates operating rooms. This study was designed to compare the decreasing patterns of sevoflurane concentration among groups with a similar course but with different FGFs.

One hundred patients scheduled for elective operation were randomly allocated to 3 groups (FGF 1 L/min, FGF 2 L/min, FGF 4 L/min). After induction with thiopental sodium 5 mg/kg and rocuronium 0.9 mg/kg or vecuronium 0.1 mg/kg for tracheal intubation, anesthesia was maintained at 1.5% of end-tidal sevoflurane concentration at an inflow of 4 L/min (N₂O 2 L/min and O₂ 2 L/min). Ten minutes prior to the estimated operation end point, we changed FGF and vaporizer settings to the following 3 different emergence protocols: changing inflow in the FGF 1 L/min group (N₂O 0.5 L/min and O₂ 0.5 L/min) with turning vaporizer off, changing inflow in the FGF 2 L/min group (N₂O 1 L/min and O₂ 1 L/min) with a two-step decrease in the vaporizer setting (1.0 vol% for first 5 minutes then with the vaporizer off), and maintaining the inflow in the FGF 4 L/min group with a three-step decrease in the vaporizer setting (1.0 vol% for first 5 minutes and 0.6 vol% for next 5 minutes then with the vaporizer off). In each group, inspiratory and end-tidal sevoflurane concentrations were recorded every minute for 16 minutes, while end-tidal CO₂, mean arterial pressure, heart rate, and inspired oxygen fraction were recorded every two minutes for 16 minutes.

End-tidal concentrations of sevoflurane were similar in the 3 groups at the 6th, and 7th minutes and continuously increasing differences in sevoflurane concentrations from the 11th to 16th minute were observed in the FGF 1 L/min and FGF 2 L/min groups versus the FGF 4 L/min group. The concentration curves for the FGF 1 L/min group showed a smoother decreasing pattern than those of the other groups.

The use of low FGF without vaporizer during emergence reduces sevoflurane washout within anesthetic machines and the exhausting of anesthetics into operating rooms, and also offers an easier means of controlling anesthetic depth.