The minimum alveolar concentration of xenon in the elderly is sex-dependent

Background: The minimum alveolar concentration (MAC) of xenon in the elderly has not been determined. Moreover, because xenon inhibits the activity of the N-methyl-D-aspartate receptors, and because N-Methyl-D-aspartate receptor antagonists such as ketamine and MK-801 exert sex-dependent actions, we hypothesized that the MAC of xenon would also be sex-dependent. Methods: Forty-eight patients of both sexes (24 patients of each sex), who were aged 65 yr or older and were undergoing elective laparotomy, were anesthetized with inhalational induction of xenon. Those who demonstrated marked agitation received supplemental propofol intravenously. After tracheal intubation, the end-tidal concentration of xenon was maintained at 45 (women only), 50, 55, 60, 65, 70, or 75% (men only) for at least 15 min before skin incision. These concentrations were randomly allocated to four patients of each sex. Each patient was monitored for the presence or absence of any purposeful bodily movement for I min following skin incision. The MAC of xenon was calculated separately for men and women using logistic regression analysis. Results: The MAC of xenon was 69.3% (95% Cl, 63.0-75.6%) for men and 51.1% (44.6-57.6%) for women. The two 95% confidence intervals did not overlap, indicating a statistically significant difference (P < 0.05). Conclusions: The MAC of xenon in the elderly is higher in men than in women.