POSTPARTUM CHANGES IN THE MINIMUM ALVEOLAR CONCENTRATION OF ISOFLURANE

Background: Minimum alveolar concentration (MAC) is decreased in pregn ancy, but it is not known how quickly after delivery MAC returns to no rmal. We measured the MAC of isoflurane in a group of women undergoing elective tubal ligation after delivery. Methods: After delivery, 20 p atients underwent inhalational induction of anesthesia with isoflurane and tracheal intubation. MAC was determined in each patient by observ ing the response to a 10-s, 50-Hz, 80-mA transcutaneous tetanic electr ic stimulus to the ulnar nerve at various concentrations of isoflurane . The end-tidal concentration of Isoflurane was kept constant for at l east 10 min before each stimulus, and the concentration of isoflurane was ultimately varied in steps of 0.05 vol%until we obtained a sequenc e of three alternate responses: move-not move-move or not move-move-no t move, The MAC for each subject was taken as the mean of the two conc entrations just permitting and just preventing movement. A venous bloo d sample was taken immediately before induction of anesthesia for meas urement of progesterone concentration. MAC was compared with time afte r delivery and plasma progesterone concentrations by Kendall's rank co rrelation. Results: There was a positive correlation between MAC and t he time after delivery (P<0.001). The median MAC of isoflurane was 0.7 75 vol% (range 0.675-0.775 vol%) in five women 24-36 h postpartum. MAC was more variable, 0.825 vol% (0.675-0.975 vol%) in nine women 36-72 h postpartum, whereas six patients more than 72 h postpartum had a MAC of 1.125 vol% (1.025-1.175 vol%). The correlation between MAC and pla sma progesterone concentration was almost statistically significant (P =0.060). Conclusions: The MAC of isoflurane was reduced in women 24-36 h postpartum and gradually increased to normal values by 72 h postpar tum.