Brain uptake of meperidine in the fetal lamb.

The uptake of meperidine by the brain of the fetal lamb was investigated by determining the concentration of meperidine in the fetal brachiocephalic artery and sagittal vein after intravenous and intramuscular administration to the mother. A positive arteriovenous gradient across the fetal brain existed for 10 minutes after intravenous administration, and for 20 to 25 minutes after intramuscular administration, thus indicating the uptake of meperidine by the fetal brain. The peak concentration of unbound meperidine in the fetal brain, as estimated from the plasma concentration of free meperidine at equilibrium, was three to four times greater after intravenous administration than after intramuscular administration. The findings suggest that, during labor, the route of administration of meperidine will determine the time course of meperidine in the fetal brain and, consequently, the time action of effects seen in the neonate. The lag time required for plasma-brain equilibration may explain the lack of correlation between the incidence of neonatal respiratory depression and the levels of meperidine in the cord after intramuscular administration.