The effect of gender on EC50 of remifentanil to prevent pain during injection of microemulsion propofol

BACKGROUND Propofol injection pain is an unpleasant experience to patients and its prevalence can be influenced by age and gender. We determined the half maximal effective concentration (EC(50)) of remifentanil for preventing the microemulsion propofol injection pain in the male and female adult groups. METHODS After institutional review board approval, a total of 60 patients were assigned into 2 groups depending on their gender: group M (male, 20-65 yr) and group F (female, 20-65 yr). Anesthesia was induced with propofol and remifentanil, by a target-controlled infusion. Target effect-site concentration (Ce) of propofol and remifentanil for the first patient started at 4.0 ug/ml and 4.0 ng/ml. Ce of remifentanil for each subsequent patient was determined by the response of the previous patient by the Dixon's up-and-down method (DUDM) with an interval of 0.2 ng/ml. After equilibration of plasma and effect site remifentanil concentration, propofol was administered, and the pain responses were observed. RESULTS The remifentanil EC(50) was 3.8 ± 0.2 and 2.7 ± 0.2 ng/ml in groups M and F, respectively, by DUDM. From Probit regression model, the remifentanil EC(50) was 3.7 (3.0-4.3) and 2.7 (1.8-2.9) ng/ml in groups M and F, respectively. CONCLUSIONS The remifentanil EC(50) for preventing the moderate to severe injection pain of propofol was higher in males than in females.