Blood levels of lidocaine after various infusion rates in patients with acute myocardial infarction.

Blood levels of lidocaine were estimated following two different infusion rates in patients with acute myocardial infarction. Forty-one patients received lidocaine as a bolus injection of 75 mg. directly followed by an infusion. The infusion rate was, in 16 patients, 2 mg. per minute. Blood levels of lidocaine were determined at different times up to 180 minutes after the start of drug administration. The levels in the 2 mg. per minute group were lower than had been expected and the mean level plus or minus S.E. was, after three hours, only 1.1 mug per milliliter. Many of the 2 mg. per minute petients did not, during the time period observed, reach 1.2 mug per milliliter, which has been considered as the lowest effective therapeutic level. In the 4 mg. per minute group, mean level of lidocaine already at 15 minutes was significantly higher than in the 2 mg. per minute group (1,1 plus or minus 0.14 and 0.6 plus or minus 0.05, respectively; p less than 0.05). After three hours, the mean blood level in the 4 mg. per minute group was 2.6 mug per milliliter. Although the blood levels were not followed after three hours, a careful observation did not reveal any severe toxic effects in the 4 mg. per minute group during next 21 hours. In five cases, however, lidocaine side effects were probable. These symptoms disappeared rapidly after cessation of the infusion. In order to reach therapeutic blood levels of lidocaine within a reasonable time, the infusion rate of 4 mg. per minute is recommended at least for the first three hours.