Amiodarone
Amiodarone is a potent, broad-spectrum **Class III antiarrhythmic agent** used primarily for severe, refractory ventricular and supraventricular arrhythmias. Because of its complex pharmacokinetic profile and significant potential for toxicity, it is generally reserved for cases where conventional, less toxic therapies have failed. **Clinical Pearls:** * Structurally, amiodarone is an iodinated benzofuran derivative. A 200 mg tablet contains approximately 75 mg of iodine, which can impact thyroid function. * It is highly lipophilic, leading to extensive tissue distribution and accumulation in adipose tissue, which contributes to its exceptionally long and variable half-life. * In veterinary medicine, it is most notably used in Doberman Pinschers with occult dilated cardiomyopathy (DCM) presenting with rapid, wide-complex ventricular tachycardia or syncope. * Due to the high risk of adverse effects (including hepatotoxicity and thyroid dysfunction), rigorous monitoring is essential.
Mechanism: Amiodarone is uniquely versatile, possessing properties of all four Vaughan-Williams antiarrhythmic classes, though it is primarily classified as a **Class III** agent. * **Class III (Primary):** Blocks **potassium (K+) channels** โ prolongs phase 3 repolarization, action-potential duration (APD), and the effective refractory period (ERP) in myocardial cells. * **Class I:** Blocks **fast sodium (Na+) channels** โ slows conduction velocity. * **Class II:** Non-competitively blocks **beta-adrenergic receptors** โ provides sympatholytic effects, reducing heart rate. * **Class IV:** Weakly blocks **calcium (Ca2+) channels** โ slows AV node conduction and suppresses automaticity. It is metabolized in the liver to its active metabolite, **desethylamiodarone**, which also contributes significantly to its antiarrhythmic effects.
Dosing by species
- For atrial fibrillation or ventricular arrhythmias primarily in ambulatory patients ยท 8-10 mg/kg PO every 12 hours ยท PO ยท q12h ยท Use caution if patient has bradycardia, AV blocks, or thyroid disorders.
- For ventricular arrhythmias secondary to occult cardiomyopathy in Doberman pinschers ยท 10 mg/kg PO twice daily for one week and then 8 mg/kg PO once daily. After 6 months reduce to 5 mg/kg PO once daily. ยท PO ยท q12h then q24h ยท Long-term ยท For severe V-Tach, mexiletine is added at 5-8 mg/kg three times daily for one week. Once efficacy confirmed, patient weaned off mexiletine.
- For ventricular tachycardia when other first line drugs (Class I antiarrhythmics ยฑ beta-blockers) are ineffective ยท 10 mg/kg PO q12h for one week and then 5 mg/kg PO q12h for maintenance. ยท PO ยท q12h ยท Long-term
- For SVTs or Vtach ยท 10-20 mg/kg PO once daily (q24h) loading for 5-7 days; 5-10 mg/kg PO once daily thereafter. ยท PO ยท q24h ยท Long-term
- Ventricular and supraventricular arrhythmias, atrial fibrillation, ventricular pre-excitation syndromes ยท Dose not specified in monograph ยท PO/IV ยท Not specified ยท Not specified ยท Use as an IV infusion for recent onset atrial fibrillation has variable efficacy for restoring sinus rhythm but a high frequency of severe adverse effects.
- For conversion of atrial fibrillation or ventricular tachycardia ยท 5 mg/kg/hr for one hour, followed by 0.83 mg/kg/hr for 23 hours and then 1.9 mg/kg/hour for the following 30 hours. ยท IV ยท CRI ยท Up to 54 hours ยท Infusion was discontinued when conversion occurred or when any side effects were noted. Regimen should be further adapted based upon PK/PD studies.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Hypersensitivity to amiodarone or iodine
- Severe sinus-node dysfunction
- Severe sinus bradycardia
- 2nd or 3rd degree AV block
- Bradycardial syncope
Adverse effects
- Vomiting
- Anorexia
- Hepatopathy (elevated liver enzymes, bilirubinemia)
- Bradycardia
- Neutropenia
- Thrombocytopenia
- Positive Coombs' test
- Corneal deposits
- Injection site pain (IV)
- Facial pruritus and hyperemia (IV)
- Hind limb weakness (horses)
- Diarrhea (horses)
Drug interactions
- Warfarin (Anticoagulants) ยท Significantly increases serum levels and/or pharmacologic/toxic effects of the anticoagulant.
- Digoxin ยท Significantly increases serum levels and/or toxic effects of digoxin. ยท major
- Cyclosporine ยท Increases cyclosporine levels; may increase creatinine.
- Lidocaine ยท Significantly increases serum levels and/or toxic effects of lidocaine. ยท moderate
- Methotrexate ยท Significantly increases serum levels and/or toxic effects with prolonged amiodarone administration. ยท major
- Phenytoin ยท Significantly increases serum levels and/or toxic effects of phenytoin.
- Procainamide ยท Significantly increases serum levels and/or toxic effects of procainamide.
- Quinidine ยท Significantly increases serum levels and/or toxic effects of quinidine.
- Azole Antifungals (ketoconazole, itraconazole) ยท Additive effects on QTc interval; possible serious arrhythmias.
- Cisapride ยท Additive effects on QTc interval; possible serious arrhythmias.
- Disopyramide ยท Additive effects on QTc interval; possible serious arrhythmias.
- Dolasetron ยท Additive effects on QTc interval; possible serious arrhythmias.
Monitoring
- ECG (for antiarrhythmic efficacy and proarrhythmic effects)
- Gastrointestinal signs (anorexia, vomiting)
- CBC (for neutropenia, thrombocytopenia)
- Serial liver enzymes and bilirubin (baseline and routine monitoring)
- Thyroid function tests
- Blood pressure
- Pulmonary radiographs (if clinical signs such as dyspnea or cough occur)
Overdose
Clinical overdosage experience is limited. **Expected Signs:** * Hypotension * Bradycardia * Cardiogenic shock * AV block * Hepatotoxicity **Treatment:** * Therapy is primarily **supportive**. * Bradycardia may be managed with a pacemaker or beta-1 agonists (e.g., isoproterenol). * Hypotension may be managed with positive inotropic agents or vasopressors. * *Note:* Neither amiodarone nor its active metabolite are dialyzable.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.