Abstract
Effects of lidocaine on blood pressure, heart rate, electrocortical activity, pH, and blood gases were studied in chronically catheterized sheep fetuses in utero. Lidocaine infused at a constant rate of less than I mg. · min.−1 · Kg.−1 into a fetal femoral vein did not cause changes in fetal blood pressure, heart rate, pH, or blood gases. Infusion rates above 1 mg. · min.−1 · Kg.−1 caused sudden, phasic increases in blood pressure in the majority of the fetuses. The heart rate increased in association with each phasic increase in blood pressure. In some fetuses the heart rate decelerated after the initial acceleration. Fetal electrocortical recordings showed that each phasic change in blood pressure and heart rate was secondary to epileptiform activity. The concentration of lidocaine in fetal arterial blood was 11.6 ± 3.8 μg per milliliter when the first phasic increase in blood pressure occurred. Tracheal pressure recordings showed deep fetal breathing movements with each epileptiform activity. It is concluded that lidocaine causes convulsions in the fetal sheep and that the cardiovascular changes are due to central stimulation during these convulsions. In newborn lambs lidocaine produced similar epileptiform activity associated with tonic-clonic convulsions and cardiovascular changes. Fetal arterial pH was not affected until the phasic changes occurred, after which the pH fell rapidly in the majority of the fetuses. In three paralyzed fetuses the pH also decreased at the onset of phasic increases in blood pressure. This indicated that muscular activtiy was not the main cause of the acidemia during fetal epileptiform activity. Bolus injections of lidocaine in doses of 3.0 to 22.2 mg. per kilogram of fetal weight caused an immediate decrease in blood pressure and heart rate. These changes were related to the dose of lidocaine. The convulsive dose of lidocaine as a bolus injection ranged from 8.0 to 22.2 mg. per kilogram.
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