A 65-year-old man is seen in the emergency room for an episode of tachycardia that began 30 minutes ago. He had a myocardial infarction three years ago. Since then, he has had recurrent bouts of tachycardia diagnosed as ventricular tachycardia Treatment with quinidine, procainamide, disopyramide, and several beta-adrenergic blocking drugs was ineffective in controlling the bouts of tachycardia. Recently he has begun taking amiodarone, currently at a dosage of 800 mg per day.