Pharmacokinetics of lidocaine in fetal and neonatal lambs and adult sheep.

The pharmacokinetics of lidocaine were studied in fetal and neonatal lambs and in pregnant and nonpregnant adult sheep. Catheters were implanted in the femoral vessels and in the urinary bladders of animals prepared for chronic study. Lidocaine, 5--10 mg/kg, was injected intravenously either into the fetus or newborn lamb or into nonpregnant adult sheep. Serial samples of arterial blood and urine were obtained over four hours and analyzed for unchanged lidocaine using a gas chromatographic technique. The elimination half-lives of lidocaine in the bloods of nonpregnant ewe, neonate and fetus were 31, 51 and 33 min, respectively. Total-body clearances in the neonate and adult were 53 and 41 ml/min/kg. The metabolic clearances of lidocaine were the same in both, and approximated hepatic blood flow. Renal clearance was greater in the neonate, which was attributed to differences in urinary pH values and extents of protein binding. Thus, despite differences in half-lives, the newborn lamb is as capable as the adult of clearing lidocaine.