Amiodarone is the most broadly effective antiarrhythmic drug in clinical use and the only agent with demonstrated survival benefit in specific high-risk populations. Its extraordinary pharmacokinetic complexity and multi-organ toxicity profile require systematic understanding for safe prescribing.³

Amiodarone does not fit neatly within any single Vaughan Williams class. Its actions span all four classes:³ Class I (Na+ channel block): Tonic INa blockade slows conduction in fast-response tissue. Less use-dependent than Class Ic agents. Class II (β-adrenergic block): Non-competitive β-blockade reduces heart rate and AV nodal conduction, contributing to rate control in AF. Class III (K+ channel block): Blocks IKr, IKs, IK1, and IKAch, producing marked APD and ERP prolongation in all cardiac tissues. Class IV (Ca²⁺ channel block): L-type Ca²⁺ channel blockade reduces AV nodal conduction velocity and slows the sinus rate.

This multi-channel profile accounts for amiodarone's efficacy across virtually all arrhythmia types and its low risk of torsades de pointes despite marked QT prolongation. L-type calcium current (ICaL) blockade counteracts the EAD-promoting effect of APD prolongation by reducing the inward trigger current for early afterdepolarizations (EADs).³

Amiodarone has the most unusual pharmacokinetics of any commonly used drug. Its extreme lipophilicity drives accumulation in virtually every tissue compartment:^3,4 Oral bioavailability is highly variable at 22 to 86%, reflecting erratic gastrointestinal absorption and making plasma level titration unreliable. The volume of distribution is 60 to 100 L/kg, among the highest of any drug, reflecting massive accumulation in adipose tissue, lung, liver, and myocardium; loading doses are therefore required to achieve effect. Protein binding exceeds 96%, accounting in part for its interaction with warfarin through albumin displacement. The elimination half-life of 40 to 55 days means that effects and drug interactions persist for weeks to months after discontinuation. Hepatic metabolism via CYP3A4 (cytochrome P450 3A4) and CYP2C8 produces desethylamiodarone (DEA), an active metabolite with comparable pharmacological activity; this metabolic pathway generates multiple clinically significant drug interactions. Time to steady state without loading takes months, making loading protocols essential for any acute indication.

Because steady-state plasma levels take months to achieve, loading doses are mandatory when rapid antiarrhythmic effect is required:³ IV loading (acute VT/VF, hemodynamically unstable AF): 150 mg IV over 10 minutes, then 1 mg/min for 6 hours, then 0.5 mg/min maintenance. Maximum 2.2 g in 24 hours. Administer via central line if possible (phlebitis risk with peripheral IV). Oral loading: 400–600 mg three times daily for 4–8 weeks (total load ~10 g), then reduce to maintenance 100–200 mg daily. Higher maintenance doses substantially increase toxicity risk. Maintenance: Use the lowest effective dose (100–200 mg daily for most indications). Effective tissue levels persist for weeks after discontinuation, providing a buffer if doses are missed but complicating toxicity management.

VT/VF: First-line IV agent for hemodynamically stable VT and pulseless VT/VF (ACLS, ALPS trial). Superior to lidocaine for shock-refractory VF. AF rhythm control: Most effective agent for maintaining sinus rhythm, including in structural heart disease where Class Ic agents are contraindicated. Reserve for patients where other agents have failed or are contraindicated due to toxicity profile. ICD recipients with frequent shocks: Adjunct to ICD therapy to reduce VT/VF burden and shock frequency (OPTIC trial data).

Perioperative AF prophylaxis: IV or oral amiodarone reduces incidence of post-cardiac surgery AF.

Amiodarone's iodine content (37% by weight) and extreme lipophilicity drive toxicity in multiple organ systems. Toxicity risk increases with cumulative dose and duration.^4,5 Thyroid dysfunction is the most common toxicity: hypothyroidism occurs in 6 to 10% of patients and is treated with levothyroxine without necessarily stopping amiodarone; hyperthyroidism occurs in 2 to 3% and may require corticosteroids for type 2 amiodarone-induced thyrotoxicosis (AIT), which involves destructive thyroiditis rather than iodine-driven hormone synthesis. Thyroid function tests should be checked every 6 months. Pulmonary toxicity, interstitial pneumonitis, bronchiolitis obliterans, or acute respiratory distress, occurs in 1 to 5% per year and requires annual chest X-ray and pulmonary function testing; high-resolution CT is indicated for new symptoms; confirmed toxicity requires drug discontinuation and corticosteroids in severe cases. Hepatotoxicity manifests as transaminase elevation in 15 to 25% of patients; cirrhosis is rare but reported with high cumulative doses; liver function tests should be checked every 6 months and the drug discontinued if transaminases exceed three times the upper limit of normal. Corneal microdeposits are nearly universal and are not an indication to stop the drug; optic neuropathy is rare (<1%) but vision-threatening and requires immediate discontinuation. Neurologic effects including peripheral neuropathy, ataxia, tremor, and sleep disturbance occur in 3 to 5% and often respond to dose reduction. Dermatologic effects include photosensitivity in up to 75% of patients (sun protection is mandatory) and a distinctive blue-gray skin discoloration that is usually irreversible. Cardiac toxicity including bradycardia and AV block is comparatively rare; TdP incidence is less than 1%, much lower than other Class III agents.

Warfarin: Amiodarone inhibits CYP2C9 (warfarin metabolism) and displaces warfarin from protein binding. international normalized ratio (INR) increases by 30–50%; reduce warfarin dose by one-third to one-half and monitor INR closely for 4–8 weeks after starting or stopping. Digoxin: Amiodarone inhibits P-glycoprotein and reduces digoxin renal clearance. Digoxin levels increase ~70–100%; halve the digoxin dose and recheck levels. Statins (simvastatin, lovastatin): CYP3A4 (cytochrome P450 3A4) inhibition raises statin levels, increasing myopathy/rhabdomyolysis risk. Avoid simvastatin doses >20 mg; prefer pravastatin or rosuvastatin. QT-prolonging combinations: Avoid concurrent use with other QT-prolonging agents (dofetilide, sotalol, macrolide antibiotics, fluoroquinolones, antipsychotics).

Dofetilide is a pure, highly selective IKr blocker (Class III) with no adrenergic, calcium channel, or sodium channel activity. Its selectivity produces predictable, dose-proportional QT prolongation with a risk of torsades de pointes of approximately 1–3%.7 Pharmacokinetics: Oral bioavailability ~96%; 80% renal elimination unchanged; 20% hepatic via CYP3A4. Half-life ~10 hours. Strict renal dose adjustment: CrCl >60 mL/min → 500 mcg BD; CrCl 40–60 → 250 mcg BD; CrCl 20–40 → 125 mcg BD; CrCl <20 mL/min → contraindicated. Indications: Cardioversion of AF/AFL to sinus rhythm; maintenance of sinus rhythm. One of only two antiarrhythmic agents (with amiodarone) safe for rhythm control in HFrEF (DIAMOND-CHF trial demonstrated no excess mortality). Mandatory in-hospital initiation: All patients must be initiated on dofetilide in a facility capable of continuous cardiac monitoring for at least 3 days (in the US, prescribers must be certified through the Tikosyn In Pharmacy System, TIPS). QTc must be <440 ms (≤500 ms in bundle branch block) to initiate; check QTc 2–3 hours after each dose during initiation; reduce dose or discontinue if QTc exceeds 500 ms.

Drug interactions: Drugs that inhibit renal cation transport (verapamil, cimetidine, trimethoprim, ketoconazole, megestrol) increase dofetilide levels and are contraindicated.

Ibutilide is an IV-only Class III agent used for pharmacologic cardioversion of recent-onset AF or AFL. Its mechanism includes IKr blockade but also activation of a slow, sustained inward Na+ current that prolongs APD. The combination produces rapid, potent APD prolongation effective for acute cardioversion.⁷ Dosing:¹ mg IV over 10 minutes; repeat once if arrhythmia persists at 10 minutes. Reduce to 0.01 mg/kg for patients <60 kg. Efficacy: Cardioversion rates ~40–60% for recent-onset AF; higher (~65–70%) for AFL. More effective than IV procainamide for acute cardioversion in several trials. risk of torsades de pointes: 4–8% incidence of sustained TdP requiring treatment; patients must be monitored continuously for at least 4 hours after administration or until QTc returns to baseline. Resuscitation equipment must be immediately available. Contraindications: QTc >440 ms before administration; hypokalemia; hypomagnesemia; prior TdP or LQTS; severe LV dysfunction (relative).

Amiodarone carries the lowest risk of torsades de pointes (<1%) of any Class III agent and is safe in HFrEF; no renal dose adjustment is required; its key advantages are broadest efficacy and IV availability, offset by multi-organ toxicity and extensive drug interactions. Sotalol carries a risk of torsades de pointes of 2 to 4%, should be avoided in EF below 40%, and requires mandatory renal dose adjustment and in-hospital initiation; its advantage is combined beta-blockade and Class III activity. Dofetilide carries a risk of torsades de pointes of 1 to 3%, is one of only two agents safe in HFrEF (DIAMOND-CHF data), requires strict renal dosing and in-hospital initiation with prescriber certification (TIPS program); it is available only as an oral agent. Ibutilide carries the highest risk of torsades de pointes at 4 to 8%, is restricted to IV use for acute cardioversion, requires at least 4 hours of post-administration monitoring, and should be used with caution in severe LV dysfunction. Dronedarone has low-to-moderate risk of torsades de pointes, no renal adjustment needed, but is absolutely contraindicated in EF below 35% or recently decompensated HFrEF (ANDROMEDA) and in permanent AF (PALLAS); it is appropriate only for paroxysmal or persistent AF with preserved or mildly reduced EF.