DECREASED MINIMUM ALVEOLAR CONCENTRATION OF ISOFLURANE IN PREGNANT HUMANS

Background: Minimum alveolar concentration (MAC) is decreased in pregn ant animals, but this change has not been demonstrated in humans, prob ably because of ethical considerations. It is less problematic to dete rmine MAC in pregnant women undergoing termination of pregnancy, howev er, and therefore we compared the MAC of isoflurane in these women wit h the MAC in matched nonpregnant women. Methods: Patients underwent in halational induction of anesthesia with isoflurane and tracheal intuba tion. MAC was determined in each patient by testing the response to a 10-s, 50-Hz, 80-mA transcutaneous tetanic electrical stimulus to the u lnar nerve at varying concentrations of isoflurane. The end-tidal conc entration of isoflurane was kept constant for 10 min before each stimu lus and the concentration of isoflurane ultimately varied in steps of 0.05% until we obtained a sequence of three alternate responses (move, not move, move) or (not move, move, not move). MAC for each patient w as taken as the mean of the two concentrations just permitting and jus t preventing movement. MAC for the group was taken as the median of th e individual MAC values. A blood sample was taken immediately before i nduction of anesthesia for measurement of progesterone concentrations. Data were compared between groups by the Mann-Whitney test. Results: The median (range) MAC for isoflurane in the pregnant group, 0.775% (0 .675-0.825), was less than that in the nonpregnant group, 1.075% (1.02 5-1.175) (P < 0.001). The median (range) plasma progesterone concentra tion in the pregnant group, 63.4 (0.8-106) nM, was greater than that i n the nonpregnant group, 8.4 (0.7-66) nM (P < 0.02). Conclusions: The MAC of isoflurane was reduced by 28% in pregnant women at 8-12 weeks' gestation compared with that of nonpregnant controls.