Antiarrhythmic Drugs

Amiodarone does not fit within any single Vaughan Williams class — it spans all four. It blocks sodium channels (Class I effect: slows conduction in fast-response tissue), blocks beta-adrenergic receptors (Class II effect: slows sinoatrial rate and atrioventricular nodal conduction), blocks potassium channels (Class III effect: prolongs action potential duration and effective refractory period throughout the heart), and blocks calcium channels (Class IV effect: reduces atrioventricular nodal conduction and sinus rate).

This multi-channel profile explains two clinically important properties: first, it makes amiodarone effective across virtually all arrhythmia types; second, the calcium channel blockade counteracts the pro-early-afterdepolarization effect of prolonged action potential duration, resulting in a torsades de pointes incidence below 1% — far lower than any other Class III agent — despite producing substantial QT prolongation.

Amiodarone contains iodine (approximately 37 percent of its molecular weight), which drives both thyroid toxicity and erratic gastrointestinal absorption. It is extremely lipophilic, accumulating massively in adipose tissue, lung, liver, and myocardium. This tissue accumulation has two major consequences: loading doses are required to achieve effective concentrations, because simply starting a maintenance dose would take months to reach steady state; and the elimination half-life is 40 to 55 days, meaning that effects and drug interactions persist for weeks to months after the drug is discontinued.

Because of this extraordinarily long half-life, any toxicity that develops during amiodarone therapy will not resolve quickly after stopping the drug. Monitoring protocols must begin at initiation and continue through and after discontinuation.

For ventricular arrhythmias, intravenous amiodarone is first-line for hemodynamically stable ventricular tachycardia and for shock-refractory ventricular fibrillation in Advanced Cardiac Life Support protocols, where it is superior to lidocaine. For atrial fibrillation rhythm control, amiodarone is the most effective available agent and the one of two options (along with dofetilide) that is safe in patients with heart failure with reduced ejection fraction or significant structural heart disease. Given its toxicity burden, it is generally reserved for patients where other agents have failed or are contraindicated.

Amiodarone's iodine content and extreme lipophilicity drive toxicity across multiple organ systems. All toxicities increase with cumulative dose and treatment duration, making the lowest effective maintenance dose an active clinical goal.

Thyroid: The most common toxicity. Hypothyroidism is more frequent and is treated with levothyroxine supplementation without necessarily stopping amiodarone. Hyperthyroidism is less common but more dangerous; one form involves destructive thyroiditis requiring corticosteroids. Thyroid function testing every six months is mandatory for all patients on amiodarone.

Pulmonary: Interstitial pneumonitis occurs in 1 to 5 percent of patients per year and is potentially fatal. Annual chest radiograph and pulmonary function testing are required. New respiratory symptoms or radiographic infiltrates warrant computed tomography and pulmonary evaluation; confirmed pulmonary toxicity requires discontinuation and corticosteroids in severe cases.

Hepatic: Transaminase elevation is common; cirrhosis is rare. Liver function tests every six months; discontinue if transaminases rise significantly above the upper limit of normal.

Ophthalmic: Corneal microdeposits are nearly universal and are not a reason to stop the drug. Optic neuropathy is rare but vision-threatening and requires immediate discontinuation if it develops.

Dermatologic: Photosensitivity occurs in a majority of patients — sun protection is mandatory from the start of therapy. Blue-gray skin discoloration develops with prolonged use and is usually irreversible.

Cardiac: Bradycardia and atrioventricular block can occur. Torsades de pointes is rare (below 1%).

Amiodarone produces several clinically critical drug interactions that require active management:

Warfarin: Amiodarone substantially increases warfarin effect, raising the international normalized ratio by 30 to 50 percent. Reduce the warfarin dose by approximately one-third when amiodarone is started, and monitor the international normalized ratio closely for weeks. The interaction persists after amiodarone is stopped because of its long half-life.

Digoxin: Amiodarone reduces digoxin elimination, approximately doubling digoxin plasma levels. Halve the digoxin dose when amiodarone is added and recheck digoxin levels.

Statins (particularly simvastatin and lovastatin): Amiodarone inhibits the enzyme responsible for statin metabolism, raising statin levels and increasing the risk of myopathy and rhabdomyolysis. Avoid high-dose simvastatin; prefer pravastatin or rosuvastatin in patients on amiodarone.

Dofetilide is a highly selective potassium channel blocker with no sodium channel, calcium channel, or adrenergic receptor activity. Its pharmacological selectivity produces predictable, dose-proportional QT prolongation. It is one of only two antiarrhythmic agents — along with amiodarone — demonstrated to be safe for rhythm control in patients with heart failure with reduced ejection fraction, based on trial data showing no increase in mortality in this population.

Dofetilide is eliminated primarily by the kidneys, and strict dose adjustment based on creatinine clearance is mandatory. Drugs that interfere with renal cation transport (including verapamil, cimetidine, and trimethoprim) increase dofetilide levels and are contraindicated.

All patients must be initiated on dofetilide in a facility capable of continuous cardiac monitoring for a minimum of three days, with the corrected QT interval checked after each dose. If the corrected QT interval exceeds 500 milliseconds at any point during initiation, the dose is reduced or the drug discontinued. This mandatory in-hospital initiation applies to every new course of dofetilide, including restarts after interruption.

Ibutilide is available only for intravenous administration and is used exclusively for pharmacologic cardioversion of recent-onset atrial fibrillation or atrial flutter to sinus rhythm. It is not used for maintenance of sinus rhythm. Its mechanism prolongs action potential duration rapidly and potently, terminating re-entrant atrial circuits.

Ibutilide carries the highest torsades de pointes risk of any Class III agent, at 4 to 8 percent. All patients must be monitored continuously for a minimum of four hours after administration or until the corrected QT interval returns to baseline, and resuscitation equipment must be immediately available. It is contraindicated in patients with a prolonged corrected QT interval at baseline, hypokalemia, hypomagnesemia, or a history of torsades de pointes.