The addition of remifentanil during the propofol induced anesthesia facilitates the insertion of laryngeal mask airway (LMA) with minimal adverse hemodynamic disturbances. This study predicted the optimal effect-site concentration of remifentanil for LMA insertion with propofol target-controlled infusion (TCI).

In 25 adults patients, aged 18-60 years, anesthesia was induced with propofol TCI at the effect-site concentration of 4µg/ml. Two minutes later, the predetermined effect-site concentration of remifentanil was started. The remifentanil concentration was determined using modified Dixon's up-and-down method (0.5 ng/ml as a step size). The first patient was tested at 2.0 ng/ml remifentanil. The insertion of LMA was attempted 3 min after remifentanil TCI began. The response of the patients to the insertion of LMA was classified as either 'movement' or 'no movement'.

The remifentanil concentration at which there was a 50% probability of successful LMA insertion (EC50) was 3.18 ± 0.53 ng/ml. From probit analysis, the EC50 of remifentanil was 3.10 ng/ml (95% confidence limits, 2.55-4.11 ng/ml), and the EC95 was 4.31 ng/ml (95% confidence limits, 3.64-11.11 ng/ml).

The predicted effect-site concentration of remifentanil for facilitating LMA insertion was 3.18 ng/ml in 50% of adults during propofol TCI at an effect-site concentration of 4µg/ml without premedication.