MINIMUM ALVEOLAR ANESTHETIC CONCENTRATION VALUES FOR THE ENANTIOMERS OF ISOFLURANE DIFFER MINIMALLY

Results of in vivo and in vitro studies of the anesthetic potencies of the enantiomers (optical isomers) of isoflurane provide various resul ts ranging from no difference to differences of nearly two fold. A fin ding of a difference in anesthetic requirement in the whole animal has particular relevance to theories of anesthetic mechanisms of action b ecause it suggests that anesthesia may result from a specific anesthet ic-receptor interaction. This led to our decision to redetermine the m inimum alveolar anesthetic concentration (MAC) of (+)-S and (-)-Renant iomers of isoflurane in 12 Sprague-Dawley rats (six per group). The ()-S enantiomer gave a MAC of 0.0144 +/- 0.0012 atm (i.e., 1.44% +/- 0. 12% at 1 atm pressure; mean +/- SD) and the (-)-Renantiomer gave a MAC of 0.0169 +/- 0.0020 atm. Although the 17% greater value for the (-)- R enantiomer is qualitatively consistent with previous results the dif ference is not significant (P = 0.06), and the absolute difference is smaller than that found by a previous study. However, given the small sample size, our power to define a small significant difference is lim ited. Regardless of statistical significance, our results do not confi rm the conclusion that interaction with a specific receptor is importa nt to the mechanism of action of inhaled anesthetics.