Postpartum changes in the minimum alveolar concentration of isoflurane

Minimum alveolar concentration (MAC) is decreased in pregnancy, but it is not known how quickly after delivery MAC returns to normal. We measured the MAC of isoflurane in a group of women undergoing elective tubal ligation after delivery.

After delivery, 20 patients underwent inhalational induction of anesthesia with isoflurane and tracheal intubation. MAC was determined in each patient by observing the response to a 10-s, 50-Hz, 80-mA transcutaneous tetanic electric stimulus to the ulnar nerve at various concentrations of isoflurane. The end-tidal concentration of isoflurane was kept constant for at least 10 min before each stimulus, and the concentration of isoflurane was ultimately varied in steps of 0.05 vol% until we obtained a sequence of three alternate responses: move-not move-move or not move-move-not move. The MAC for each subject was taken as the mean of the two concentrations just permitting and just preventing movement. A venous blood sample was taken immediately before induction of anesthesia for measurement of progesterone concentration. MAC was compared with time after delivery and plasma progesterone concentrations by Kendall's rank correlation.

There was a positive correlation between MAC and the time after delivery (P < 0.001). The median MAC of isoflurane was 0.775 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 plasma progesterone concentration was almost statistically significant (P = 0.060).

The MAC of isoflurane was reduced in women 24-36 h postpartum and gradually increased to normal values by 72 h postpartum.