This study was conducted to investigate the optimal time interval for tracheal intubation and the effect of adjuvant drugs such as remifentanil and lidocaine during induction and tracheal intubation using sevoflurane inhalation without muscle relaxant.

This study enrolled patients with the age of 20-60 years old and ASA 1 or 2. Patients were randomly assigned into one of 4 groups (S, SR, SRL, SL), in which they were given remifentanil (R) i.v. at a rate of 0.25µg/kg/min, or lidocaine (L) i.v. bolus of 1.5 mg/kg during sevoflurane inhalation (S). Anesthesia was performed as inhalation induction 2 minutes after pre-filling with sevoflurane 8 vol%. The time interval between induction and tracheal intubation was determined using up-and-down method. When satisfied all of the categories of response to tracheal intubation, the case was assigned to 'success', otherwise 'fail'. In each groups, effective time for successful intubation in 50% (ET₅₀) and 95% (ET₉₅) were calculated by probit analysis.

ET₅₀ was 3.90 minutes (95% confidence interval 3.32-4.38) in group S, 3.18 minutes (2.92-3.48) in group SL, 2.83 minutes (2.47-3.07) in group SR, and 2.68 minutes (2.37-2.95) in group SRL. In group S, SL, SR, and SRL, ET₉₅ was 4.52 minutes (4.17-7.95), 3.63 minutes (3.37-4.97), 3.30 minutes (3.06-4.64), and 3.12 minutes (2.89-4.42), respectively.

The optimal time to intubate successfully using sevoflurane without muscle relaxant in 95% patients was 4.5 minutes. The optimal time is reduced to 3.1 minutes by coadministration of remifentanil and lidocaine.