A patient is in the CCU with acute MI and develops ventricular fibrillation. After defibrillation, persistent ventricular ectopics continue. IV amiodarone is administered. Amiodarone primarily belongs to Vaughan Williams class III, but it also exhibits properties of classes I, II, and IV. Which unique pharmacological property makes IV amiodarone the preferred choice over lidocaine in shock-refractory VF/VT?

• A Amiodarone has faster onset of action than lidocaine in acidotic tissue due to its lipophilicity
• B Amiodarone does not reduce cardiac contractility unlike lidocaine, making it safer in the post-MI setting
• C Amiodarone has zero effect on the QT interval acutely, unlike lidocaine which causes marked QT prolongation in acute MI
• D IV amiodarone's acute effects are primarily mediated by sodium channel blockade (Class I) and adrenergic blockade (Class II) rather than potassium channel blockade, providing broader spectrum antiarrhythmic coverage ✓

Explanation

Amiodarone is a class III antiarrhythmic (potassium channel blocker, prolongs APD/QT) in chronic use, but its acute IV effects are predominantly mediated by sodium channel blockade (class I — reducing automaticity and conduction velocity) and non-competitive alpha/beta adrenergic blockade (class II — reducing sympathetic stimulation). The class III potassium channel effects develop with chronic oral therapy and accumulation in tissues. This acute pharmacological profile gives IV amiodarone a broad antiarrhythmic effect across multiple mechanisms. ACLS guidelines prefer amiodarone over lidocaine for shock-refractory VF/VT based on the ARREST and ALIVE trials showing improved survival to hospital admission.

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

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Written and medically reviewed by the StethoPrep medical team.