MINIMUM ALVEOLAR CONCENTRATION OF HALOTHANE AND ENFLURANE ARE DECREASED IN EARLY-PREGNANCY

Background: Minimum alveolar concentration (MAC) of isoflurane is decr eased in early pregnancy but it is not known whether this occurs to th e same extent with other inhalational anesthetics. The MAC of halothan e and enflurane were compared in pregnant women undergoing elective te rmination of pregnancy and in nonpregnant women. Methods: We studied 1 6 pregnant women scheduled for termination of pregnancy at 8 to 13 wee ks gestation and 16 nonpregnant patients undergoing laparoscopic steri lization. Eight patients in each group received halothane and the othe rs received enflurane. After inhalational induction of anesthesia and tracheal intubation, MAC was determined in each patient by observing t he motor response to a 10-s, 50-Hz, 80-mA transcutaneous electric teta nic stimulus to the ulnar nerve at varying concentrations of either ha lothane or enflurane. The end-tidal concentration of inhalational anes thetic was kept constant for at least 15 min before each stimulus and the concentration was varied ultimately in steps of 0.05 vol% (halotha ne) or 0.10 vol% (enflurane) until a sequence of three alternate respo nses (move, not move, move) or (not move, move, not move) was obtained . Minimum alveolar concentration for each person was taken as the mean of the two concentrations just permitting and just preventing movemen t, and MAC for the group was the median of individual MAC values. Conf idence intervals were calculated for the percentage decrease in MAC fo r pregnant women compared with nonpregnant women. Results: The median (range) MAC of halothane, 0.58 vol% (0.53 to 0.58), and enflurane, 1.1 5 vol% (0.95-1.25), in the pregnant women were less than those in the nonpregnant women, 0.75 vol% (0.70 to 0.78), P = 0.0005 and 1.65 vol%: (1.45 to 1.75), P = 0.0007, respectively. The percentage decrease (95 % CI) in MAC for pregnant women was 27% (20 to 27%) for halothane and 30% (24 to 31%) for enflurane. Conclusions: The MAC of halothane and e nflurane were reduced by a similar degree in pregnant women at 8 to 13 weeks gestation compared with nonpregnant women.