13.01.7 Amiodarone

Amiodarone is a unique anti-arrhythmic that has activity in all classes. Thus, amiodarone is a potent K⁺ channel blocker, including blocking the delayed outward rectifying potassium channel. However, amiodarone is also a Na⁺ channel blocker, and a Ca²⁺ channel blocker, mimicking the effect of verapamil. Amiodarone is also a blocker of a- and b-adrenoceptors. Finally, an unwanted mechanism of amiodarone is that it is an analog of thyroid hormone. Thus, amiodarone interacts with nuclear thyroid hormone receptors in an inhibitory way, such that many of the adverse effects of amiodarone mimic those of hypothyroidism.

Amiodarone is used orally in recurrent ventricular tachycardia/fibrillation, particularly when the arrhythmia is resistant to other drugs. Also, amiodarone is used orally to convert atrial fibrillation into normal sinus rhythm. Amiodarone is used intravenously for the termination of ventricular or supraventricular arrhythmias in an emergency.

In clinical trials, it has been shown that preventative oral use of amiodarone, after myocardial infarction or in heart failure, reduces sudden death, which is usually arrhythmic death. However, amiodarone is not standard therapy after myocardial infarction or in heart failure, probably because it has many adverse effects, which means people do not like taking it, and the benefits may not exceed the risks.

Amiodarone is very lipid soluble, and large amounts accumulate in fat tissues, and are only slowly released. The adverse effects associated include pulmonary toxicity in 2-17% of patients and hypothyroidism in 2-10% of patients, but severe arrhythmias are rare.