The anesthetic contribution of magnesium sulfate and ritodrine hydrochloride in rats.

The anesthetic effects of the tocolytic agents, magnesium sulfate and ritodrine hydrochloride, were investigated by determining their effect on the minimal alveolar anesthetic concentration (MAC) of halothane in male and in pregnant and nonpregnant female rats. Magnesium and ritodrine were administered by continuous intravenous infusion to mechanically ventilated rats anesthetized with halothane. The tail-clamp technique was used to establish the MAC of halothane before and then again during the infusion of either magnesium or ritodrine. Ritodrine produced no change in halothane MAC. Increasing magnesium dosages and magnesium plasma levels were associated with nonlinear reductions in halothane MAC that were unrelated to sex or pregnancy. The alveolar halothane MAC concentration in pregnant rats (0.85 +/- 0.02) was not significantly different from the halothane MAC in nonpregnant female or male rats. At the highest plasma magnesium concentrations (15.8 +/- 1.57 mg/dl) achieved in the pregnant rats, the alveolar halothane MAC was 0.36 +/- 0.13, a 61.6% reduction in MAC. The anesthetic effects of magnesium were not attributable to cardiovascular, respiratory, or neuromuscular depression. Major decreases in blood pressure occurred only in the pregnant rats with the highest magnesium concentrations.