Plasma levels of lidocaine after intramuscular administration

Lidocaine has proved to be an effective ventricular antiarrhythmic agent at plasma levels of 1 to 1.5 μg/ml. Since serious ventricular arrhythmias are most likely to occur within the first hour after myocardial infarction, lidocaine administered before the patient reaches the hospital may be of prophylactic value. Plasma levels of lidocaine were determined in 69 patients with proved or suspected myocardial infarction after the administration of 200 mg of lidocaine intramuscularly as either a 6 percent (3.3 ml), 8 percent (2.5 ml), 9 percent (2.2 ml) or 10 percent (2 ml) solution. Injection sites were deltoid, lateral thigh or buttocks. Slightly higher plasma levels were achieved after 6 percent injection than with more concentrated solutions. Deltoid injection led to higher levels than lateral thigh injection which in turn led to higher levels than buttocks injection. Deltoid injection of 6, 8 or 9 percent solution gave levels above 2 μg/ml at 10 minutes. Therapeutic levels persisted for 1 hour or more. The hypertonicity of more concentrated solutions and the affinity of fatty tissue for lidocaine may explain differences in plasma levels after intramuscular injection.