• From the Divisions of Clinical Pharmacology and Cardiology, Departments of

The relationship of apparent steady-state serum concentrations of amiodarone and its metabolite, desethylamiodarone, to therapeutic and toxic effects was assessed in 127 patients who had treatment-resistant ventricular or supraventricular arrhythmias or were intolerant to other agents. After at least 2 months (mean, 9.8) of treatment with daily maintenance doses of 200 to 600 mg, arrhythmias were effectively suppressed in 7 8 % of patients. Arrhythmias recurred in 4 7 % of patients with serum amiodarone concentrations of less than 1.0 mg/L, whereas only 14 % of patients with higher concentrations had recurrences (p < 0.005). Side effects, most of them mild, occurred in 57%; only 9 patients required discontinuation of drug therapy. The risk of developing adverse reactions was related to serum amiodarone concentrations (p < 0.0001). Adverse reactions were common in patients with serum values exceeding 2.5 mg/L, although pulmonary complications did occur at lower concentrations. Monitoring serum amiodarone concentrations may differentiate failure of drug therapy from suboptimal dosing and reduce the incidence of concentration-related side effects. T H E EFFECTIVENESS of oral amiodarone hydrochloride in the treatment and prevention of cardiac arrhythmias has been established over the last decade (1-4). This agent appears to be useful in the management of previ-ously drug-resistant ventricular arrhythmias, including ventricular fibrillation, as well as paroxysmal supraven-tricular tachycardia and atrial fibrillation. Unfortunately