Halothane requirement during pregnancy and lactation in rats.

Near-term pregnancy is associated with a decrease in the minimum alveolar concentration (MAC) for halothane in ewes. Although increased progesterone levels might account for this change, a correlation between MAC and the known variations of progesterone levels which occur throughout gestation and the postpartum period has not been performed. Therefore, MAC for halothane was determined in nonpregnant, 10 days pregnant, term, and postpartum lactating rats. MAC values were significantly decreased by 19 per cent on the tenth day of pregnancy, and by 16 per cent at term, but they returned to control values 5 days postpartum. These changes did not correlate with the known changes in progesterone levels during pregnancy and lactation, and the authors conclude that progesterone is not responsible for the reduction in halothane MAC.