Toxicity of lidocaine in adult, newborn, and fetal sheep.

The relative central nervous system and cardiovascular toxicity of lidocaine was compared in adult, newborn, and fetal sheep during continuous infusion of lidocaine into the jugular vein at the rate of 2 mg x kg-1 x min-1. An identical sequence of toxic manifestations occurred in the adult, newborn, and fetus: convulsions, hypotension, respiratory arrest, and circulatory collapse. Doses necessary to produce these manifestations were highest in fetuses and lowest in adults. For example, in order to elicit convulsions, 5.8 +/- 1.8 mg/kg of lidocaine was required in the adults, 18.4 +/- 2.2 in the newborns, and 41.9 +/- 6.0 in the fetuses. Measurements of lidocaine concentrations in blood demonstrated that these toxic symptoms occurred at levels which were not significantly different among the three groups. The results indicate that fetal and newborn lambs are no more sensitive to lidocaine toxicity than are adult sheep. The fact that the highest doses were required in the fetuses is probably related to the placental clearance of the drug into mothers and better fetal maintenance of arterial Po2 despite convulsions and respiratory arrest (cessation of breathing-like movements).