Mechanism of action (Pharmacodynamics)
Amiodarone has a polyvalent (mixed) mechanism of action, uniquely combining the electrophysiological properties of all four classes of antiarrhythmic agents (I, II, III, and IV).
- Main effect (Class III): Blockade of potassium ($K^+$) channels. This leads to slowing (prolongation) of repolarization, an increase in the total duration of the action potential (AP), and the effective refractory period (ERP).
- Additional effects: The drug blocks sodium ($Na^+$) channels (preventing cell membrane depolarization), calcium ($Ca^{2+}$) channels, and also $\beta$-adrenoreceptors.
Due to this broad spectrum of effects on ion currents, the drug affects all parts of the heart.
Pharmacokinetics and metabolism
The drug is characterized by complex, prolonged pharmacokinetics and the ability to accumulate in the body.
- Absorption: When administered orally, it occurs slowly and incompletely. Bioavailability varies widely, ranging from 20–50%.
- Distribution: Characterized by high plasma protein binding (96%). Amiodarone is typical for pronounced material accumulation — it actively accumulates in tissues. Because of this, there is a prolonged latent period: several weeks are required for the maximum therapeutic effect to develop.
- Metabolism: Occurs in the liver with the formation of an active metabolite — N-desethylamiodarone. The drug itself is a substrate for CYP3A4 and CYP3A5 isoenzymes.
- Elimination: Occurs predominantly through the intestines and very slowly. The elimination pattern is biphasic: the initial phase lasts about 10 days, the terminal phase — up to 100 days. The half-life ($T_{1/2}$) ranges from 10 to 50 days, ensuring prolonged circulation in the blood.
Drug interactions (cytochrome P450 system)
Amiodarone is a clinically significant inhibitor of several hepatic isoenzymes, which requires dose adjustment during combination therapy:
- CYP2D6 inhibition: Slows down the metabolism of psychotropic drugs (haloperidol, clozapine, tricyclic antidepressants), cardiological agents (metoprolol, timolol), opioids (codeine), and proton pump inhibitors (omeprazole).
- CYP3A4/CYP3A5 inhibition: Affects the metabolism of a vast number of drugs, including statins (lovastatin), macrolides, immunosuppressants (cyclosporine), and other antiarrhythmics.
Indications for use
In clinical practice, amiodarone is positioned as a universal antiarrhythmic.
- Rhythm disorders: Highly effective in both ventricular and supraventricular tachyarrhythmias and extrasystoles of any localization.
- Coronary artery disease: Used in stable exertional angina and vasospastic angina. In this case, it works as a mixed-action agent: simultaneously reducing myocardial oxygen ($O_2$) demand and increasing its delivery (similar to nicorandil).
Side effects
The specific toxicity profile is largely due to tissue accumulation of the drug:
- Cardiac effects: Development of bradycardia, impaired atrioventricular (AV) conduction.
- Extracardiac effects:
- Thyroid dysfunction (may manifest as either hypothyroidism or hyperthyroidism).
- Specific localization of deposits: deposition of drug microcrystals in the cornea of the eye (so-called "corneal deposits").
Dosage forms and administration guidelines
According to the prescription reference book, the drug is used in the following dosages:
- Oral (0.2 g tablets): The initial dose is 0.6–0.8 g per day. This dose must be divided into 3–4 intakes and taken after meals.
- Parenteral (5% — 3 ml ampoule solution): Administered intravenously slowly. Before administration, the contents of the ampoule must be pre-dissolved in 40 ml of a 5% glucose solution.
Briefly: answers to topical questions
What cardiac side effects are characteristic of amiodarone?
Amiodarone is characterized by the following cardiac side effects:
- Bradycardia (bradycardia) — excessive slowing of the heart rate, sinus bradycardia ≤ 50 bpm.
- AV conduction disturbance (disturbances of AV conduction) — suppression of atrioventricular conduction up to II–III degree block.
- Arterial hypotension (arterial hypotension) — decrease in blood pressure levels.
- Proarrhythmic risk (proarrhythmic risk) — rare occurrence of torsades de pointes ventricular tachycardia.
What extracardiac side effects does amiodarone cause?
Amiodarone causes the following extracardiac side effects affecting various organ systems:
- Endocrine system (systema endocrinum) — thyroid dysfunction leading to hypothyroidism or hyperthyroidism.
- Visual organs (organa visus) — deposition of drug microcrystals in the cornea of the eye ("corneal deposits", keratopathy).
- Respiratory system (systema respiratorium) — fibrous pneumonitis ("amiodarone lung", pulmonary fibrosis).
- Dermatology (dermatologia) — photosensitization and pigment metabolism disorders with lipofuscin deposition, giving the skin a bluish-gray tint.
- Nervous system (systema nervosum) — tremor, ataxia, peripheral neuropathy, dizziness.
- Digestive system (systema digestorium) — nausea, vomiting, constipation, drug-induced hepatitis, liver cirrhosis.
What are the absolute contraindications to prescribing amiodarone?
The provided sources lack a complete list of absolute contraindications for prescribing amiodarone, but they mention one specific restriction: thyroid hyperfunction.
- Thyroid hyperfunction (hyperthyreosis) — a condition in which amiodarone therapy is contraindicated.
Amiodarone is a "universal soldier" (blocks all channels: K+, Na+, Ca2+, and β-receptors), but "slow" ($T_{1/2}$ up to 50 days, accumulates in tissues, effect takes weeks).
Class and exam questions
Which class of antiarrhythmics does amiodarone belong to?
Formally, it belongs to Class III (potassium channel blockers), but it has a polyvalent action and exhibits the properties of all four classes (I, II, III, IV).
Why does the therapeutic effect not occur immediately upon taking tablets?
The drug has pronounced material accumulation (accumulates in tissues) and a bioavailability of 20–50%. A latent period of several weeks is required to saturate the depots and develop the maximum effect.
What specific extracardiac side effects need to be monitored?
It is necessary to monitor thyroid function (risk of hypo- or hyperthyroidism) and vision status (possible deposition of microcrystals in the cornea of the eye).
How does amiodarone affect the metabolism of other drugs?
It is a potent inhibitor of hepatic isoenzymes CYP2D6 and CYP3A4, thereby slowing down elimination and potentially increasing the toxicity of co-administered drugs (e.g., macrolides, statins, haloperidol).
What else to study on this topic
- Classification of antiarrhythmic drugs (Classes I–IV according to Vaughan Williams)
- Myocardial electrophysiology: action potential phases and ion currents
- Cytochrome P450 system: clinical significance of isoenzyme inhibitors and inducers
- Antianginal agents: drugs that reduce $O_2$ demand and increase its delivery
- Comparison of amiodarone and sotalol (features of class III agents)
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