Women Appear to Have the Same Minimum Alveolar Concentration as Men: A Retrospective Study

Data were retrieved for 258 patients (127 women and 131 men) from previous studies of MAC for desflurane and desflurane with 60% nitrous oxide, 13diethyl ether, 14halothane (several studies), 15,16methoxyflurane, 14sevoflurane (three studies), 17–19and xenon. 15The investigators for each of these had obtained approval for human studies as appropriate for the time period of the study. We limited the data taken from the various studies to patients who had been given 2.1 MAC or less. We also (arbitrarily) excluded data from studies in which either sex constituted less than 29% of all subjects. 20,21 

A MAC value for each of the anesthetics had been calculated and published by the investigators for each of the above reports (table 1). Because the methods for determination of specific MAC values varied from report to report, we recalculated the MAC value for the combined (women plus men) data for each report using logistic regression.

Then, again using logistic regression, we separately determined the MAC for women and men from each report. In a few cases, we could not determine the MAC and variance using logistic regression because the concentrations for movement did not overlap those for nonmovement. In many of these, we could determine MACs and SEMs by nonlinear regression to a sigmoid curve using an iterative Levenberg-Marquardt method with a bootstrapping approach, but in some cases, even this approach did not give reasonable values (e.g. , we could not separately determine the desflurane–oxygen data for women).

All data were normalized by dividing the concentration values for individual patients by the combined MAC for both women and men for that anesthetic for a given report (e.g. , three separate MAC values were used in the case of sevoflurane) as determined by logistic regression. We separated the data for desflurane plus 60% nitrous oxide from the remaining data and did not perform further analyses for the desflurane plus 60% nitrous oxide data because we did not wish to assume a desflurane equivalent for the nitrous oxide used. We combined the remaining normalized data for women and for men and used logistic regression to determine the MAC value (here, MAC becomes a value of 1.0, i.e. , a normalized value) and SEM for men and (separately) women. The difference in the MAC values for gender within a given anesthetic, and the difference in the MAC values for the combined data for each sex were compared by use of an unpaired 2-tailed Student's t  test with no correction for multiple comparisons. We accepted P < 0.05 as indicating a significant difference.

MAC values determined by logistic regression were virtually identical to the previously published values, differing by 0.3 ± 2.9% (mean ± SD for 14 values, table 1; no difference was statistically significant.) In 2 (both sevoflurane) of 10 comparisons, the MAC for men significantly exceeded the MAC for women. In no comparison was the value for men significantly less than the value for women. The MAC for the normalized combined (all) data for women (1.013 ± 0.017; mean ± SEM) did not differ significantly from the normalized combined data for men (1.005 ± 0.009) (fig. 1) (MAC itself would equal a value of 1.000). Considering all data except those for sevoflurane, the normalized MAC for men (0.999 ± 0.009) did not differ from that for women (1.030 ± 0.019). However, considering only the data for sevoflurane, the normalized MAC for men (1.052 ± 0.015) significantly (P < 0.01) exceeded the MAC for women (0.957 ± 0.025), women having a value 9% less than men.

Overall, we found no difference in the MAC for women versus  men. However, consistent with the report by Goto et al. , 1we found two instances in which the MAC for men significantly exceeded the MAC for women (table 1). Both of these were for sevoflurane, 18,19and both were obtained in Japanese patients. The differences between men and women found by Katoh et al.  (12 and 16%) 18,19were smaller than the difference found by Goto et al.  (26%). 1None of the remaining anesthetics demonstrated a significant difference, although the MAC for women given xenon by Cullen et al.  15was 15% less than the MAC for men (table 1).

Overall, for six anesthetics and eight reports, we found no difference between the MAC for women and the MAC for men in a total of 258 patients (fig. 1). Thus, our findings do not support the suggestion that women generally have smaller MAC values than do men. We also do not find that younger women given xenon have a MAC significantly different from men of a similar age.

Are there subsets of patients in which women demonstrate a smaller MAC than men? Do Japanese women have smaller MAC values than Japanese men, and if such a difference exists, does it increase with increasing age? Such questions remain to be answered, but some observations suggest that the answer to both questions is that probably no difference exists. Aging should minimize or eliminate hormonal differences that otherwise might explain a difference between women and men. Many of the North American studies were done in San Francisco, where a substantial fraction of the population comes from the Far East. No obvious difference appeared in these studies, but race was not formally considered in the evaluation of the data.

The authors gratefully acknowledge the several helpful suggestions made by Daniel I. Sessler, M.D., Director, Outcomes Research Institute, University of Louisville, Louisville, Kentucky.