Toxicity of Lidocaine in Adult, Newborn, and Fetal Sheep

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Abstract
Relative CNS 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/kg per min. An identical sequence of toxic manifestations occurred in the adult, newborn and fetus as follows: convulsions, hypotension, respiratory arrest and circulatory collapse. Doses necessary to produce these manifestations were highest in fetuses and lowest in adults. To elicit convulsions, 5.8 .+-. 1.8 mg/kg of lidocaine was required in adults, 18.4 .+-. 2.2 in newborns and 41.9 .+-. 6.0 in fetuses. Measurements of lidocaine concentrations in blood demonstrated that these toxic symptoms occurred at levels which were not significantly different among the 3 groups. Fetal and newborn lambs were no more sensitive to lidocaine toxicity than adults. The fact that the highest doses were required in fetuses was probably related to placental clearance of the drug into mothers and better fetal maintenance of arterial PO2 [O2 partial pressure] despite convulsions and respiratory arrest (cessation of breathing-like movements).