Amiodarone prolongs the QT interval and can cause torsades de pointes, yet paradoxically it is LESS arrhythmogenic than other class III drugs. This is because amiodarone also blocks:

• A L-type Ca2+ channels and beta-adrenoceptors, reducing triggered activity from afterdepolarisations ✓
• B Sodium channels only in depolarised tissue, preventing re-entry without affecting normal tissue
• C Acetylcholine-regulated K+ channels (IKAch), preventing vagally-induced arrhythmias
• D Alpha-1 adrenoceptors, preventing catecholamine-induced triggered activity

Explanation

Amiodarone is a class III drug but has multi-channel blocking properties (classes I, II, III, and IV). Its blockade of beta-adrenoceptors (class II) and L-type Ca2+ channels (class IV) suppresses early and delayed afterdepolarisations (the triggers for torsades de pointes), making amiodarone 'paradoxically safe' despite QT prolongation. Pure class III drugs (sotalol, dofetilide) lack these additional properties, leading to higher torsades risk.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.