Background and Goal of Study: Cough on anesthetic emergence should be prevented considering dangerous complications. Target-controlled infusion (TCI) of remifentanil can reduce emergence cough effectively, and the sex-related differences in effect-site concentrations (Ce) of remifentanil was evaluated in young patients. We determined the Ce of remifentanil for preventing emergence cough following extubation in males and females among elderly patients. 
Materials and Methods: Twenty-three male and 22 female elderly patients aged between 60 and 75 years were enrolled. Anesthesia was maintained with sevoflurane and remifentanil TCI. The Ce of remifentanil for preventing emergence cough was determined for each sex using the isotonic regression method with a boot­strapping approach, following Dixon's up-and-down method. 
Results and Discussion: The Ce of remifentanil for preventing emergence cough in 50 % (EC50) and 95 % (EC95) of the population were significantly lower in females than in males. Isotonic regression revealed the EC50 (83 % confidence interval) of remifentanil was 1.67 (1.55-1.83) ng/mL in females and 2.60 (2.29-2.91) ng/mL in males. The EC95 (95 % confidence interval) of remifentanil was 2.30 (2.02-2.62) ng/mL in females and 3.41 (3.27-3.58) ng/mL in males. Mean arterial pressure and heart rate were not different between the sexes, but those in males were significantly lower at the end of operation compared to baseline. The sex-related differences in opioid efficacy may contribute sex-related differences in basal pain perception and sensitivity to opioid. Although being conflicting, there is a greater opioid efficacy in female patients. In current study, Ce value of remifentanil for preventing emergence cough in elderly patients may be affected by Minto pharmacokinetic model that was applied to infusion pump used for remifentanil TCI. Minto model takes height, weight, and age into account as covariates.
Conclusion(s): The remifentanil require­ment for preventing emergence cough following extubation was lower in females than in males, and the sex-related differences in Ce of remifentanil existed in elderly patients. Both sex and age should be considered to using of remifent­anil TCI for preventing emergence cough.

Sex-related differences in the effect-site concentration of remifentanil for preventing cough following extubation during anesthetic emergence in elderly patients