Pretreatment with small-dose ketamine reduces predicted effect-site concentration of propofol required for loss of consciousness and Laryngeal Mask Airway insertion in women

Study objective: To investigate the effect of small-dose ketamine on the predicted effect-site concentration of propofol required for loss of consciousness (LOC) and Laryngeal Mask Airway (LMA) insertion.

Design: Randomized, double-blinded study.

Settings: Operating room.

Patients: 50 ASA physical status 1 and 2 women scheduled for elective breast or gynecological surgery.

Interventions: Patients were randomly allocated to a ketamine group or a control group. Thirty seconds before propofol injection, ketamine group patients received ketamine 0.2 mg/kg, while control group patients received saline. Propofol was given in a target-controlled infusion and target blood concentration was gradually increased until LOC. The effect-site concentrations for attempting LMA insertion was predetermined by modifying Dixon's up and down method. LMA insertion was attempted without muscular blocking agents.

Measurements: Pain scores on propofol injection, effect-site concentrations at LOC, hemodynamic variables, and patient movement or side effects on LMA insertion were recorded.

Main results: The effect-site concentration of propofol required for LOC was 2.14 μg/mL for the control group and 1.66 for the ketamine group (P = 0.0082). The predicted effect-site concentration of propofol alone at which 50% of patients did not move with LMA insertion (EC(50)LMI) was 3.59 μg/mL (95% CI: 3.18 ∼ 4.19 μg/mL). Pretreatment with ketamine 0.2 mg/kg decreased EC(50)LMI from 3.59 (3.18 ∼ 4.19) to 2.39 (1.22 ∼ 2.99).

Conclusions: Pretreatment with ketamine 0.2 mg/kg reduced the propofol concentration required for both LOC (22%) and LMA insertion (33%) in women.