Estimation of effect-site concentration of propofol for laryngeal mask airway insertion using fentanyl or morphine as adjuvant

Background and Aims: Insertion of laryngeal mask airway (LMA) requires adequate depth of anaesthesia, which provides jaw relaxation and suppression of upper airway reflexes. Propofol can provide these conditions especially when combined with narcotics. This study had been designed to find out the effect-site concentration (EC50) of propofol using target controlled infusion (TCI) when fentanyl or morphine is added as an adjuvant. Methods: Patients satisfying inclusion criteria were divided into fentanyl and morphine groups. Intravenous glycopyrrolate 0.2 mg was given 15 min before induction. Patients were given either intravenous fentanyl (1 mu g/kg) or morphine (0.1 mg/kg) before propofol infusion depending on the group. Patients in either groups were induced by continuous infusion of propofol at an EC of 6 mu g/mL by TCI with Schneider pharmacokinetic model. The LMA supreme of appropriate size was inserted 1 min after achieving target concentration. Patient movement at LMA insertion or within 1 min of insertion was classified as failure. For subsequent patients, the target EC was increased/decreased depending on previous patients' response. Dixons up and down method was used to determine the EC50. The EC50 is defined as the mean of crossover midpoints in each pair of failure to success. Results: The EC50 of propofol in the fentanyl group for LMA insertion was 5.95 +/- 0.6 mu g/ml and morphine group was 5.75 +/- 0.8 mu g/ml. No significant difference in insertion conditions was noticed between the two groups (P = 0.3). Conclusion: We conclude that there was no significant difference in propofol EC50 for insertion of LMA and insertion conditions were similar when fentanyl or morphine was used as an adjuvant drug.