Fetal anesthetic requirement (MAC) for halothane.

We asked whether the anesthetic requirement (MAC) of fetal lambs is lower than that of pregnant ewes. In five pregnant ewes anesthetized with a subarachnoid block, a fetal foot was withdrawn through a hysterotomy. The ewe then breathed 1.5% halothane and a clamp was applied to the fetal foot at 2-min intervals. We concomitantly obtained arterial blood from previously implanted catheters. When fetal movement in response to clamping the foot ceased, halothane was discontinued and the stimulus and sampling continued until the fetus began to move. Anesthesia was again resumed and continued until movement stopped. Anesthesia was then deepened and MAC was determined in the mother (stimulus--ear clamp). The fetal blood concentrations of halothane at MAC were 48 +/- 28 mg/L; they were 133 +/- 5 mg/L in the mother. This difference was highly significant (P less than 0.001). Calculated end-tidal concentrations were 0.33% and 0.69%, respectively. In two animals delivered by cesarean section, MAC increased progressively over the first 12 h of life. Progesterone levels concomitantly decreased.