All of the following statements about antiarrhythmic drug classification are correct EXCEPT:

## Vaughan-Williams Classification of Antiarrhythmics ### Overview of Classes I–IV **Key Point:** The Vaughan-Williams classification divides antiarrhythmics into four classes based on their primary electrophysiologic mechanism. Each class has distinct sites of action and clinical indications. ### Class I — Sodium Channel Blockers - **Mechanism:** Block fast sodium channels → slow phase 0 depolarization → slow conduction velocity - **Subclasses:** IA (quinidine, procainamide, disopyramide), IB (lidocaine, mexiletine), IC (flecainide, propafenone) - **Statement is CORRECT** ### Class II — Beta-Blockers - **Mechanism:** Block β-adrenergic receptors → slow AV nodal conduction → increase AV nodal refractoriness - **Examples:** Propranolol, metoprolol, esmolol - **Statement is CORRECT** ### Class III — Potassium Channel Blockers - **Mechanism:** Block potassium channels (especially delayed rectifier K⁺ channels) → prolong action potential duration (APD) and refractory period - **Examples:** Amiodarone, sotalol, dofetilide, ibutilide - **Statement is CORRECT** ### Class IV — Calcium Channel Blockers (INCORRECT STATEMENT) - **Mechanism:** Block L-type calcium channels → slow AV nodal conduction and increase AV nodal refractoriness - **Primary site of action:** **AV node** (not ventricular myocardium) - **Clinical use:** SVT termination, atrial fibrillation rate control, NOT ventricular arrhythmias - **Why the statement is wrong:** Class IV drugs are **NOT effective for ventricular arrhythmias** because they do not significantly block calcium channels in ventricular muscle; they act primarily on the AV node **High-Yield:** Verapamil and diltiazem (Class IV) are **contraindicated in ventricular tachycardia** because they can paradoxically worsen hemodynamics in VT and may precipitate cardiogenic shock. ### Mechanism of Action Comparison Table | Class | Primary Target | Mechanism | Effect on Conduction | Clinical Use | | --- | --- | --- | --- | --- | | I | Na⁺ channel | ↓ Phase 0 slope | ↓ Conduction velocity | Atrial & ventricular arrhythmias | | II | β-receptor | ↓ cAMP → ↓ Ca²⁺ entry | ↓ AV nodal conduction | SVT, AF rate control | | III | K⁺ channel | ↑ APD & refractoriness | ↑ Refractory period | Atrial & ventricular arrhythmias | | IV | Ca²⁺ channel (AV node) | ↓ AV nodal conduction | ↓ AV nodal conduction | SVT, AF rate control **NOT VT** | **Clinical Pearl:** Class IV drugs (verapamil, diltiazem) are used for **supraventricular** arrhythmias and rate control in atrial fibrillation, NOT for ventricular arrhythmias. In VT, they may cause hemodynamic collapse. **Warning:** A common exam trap is confusing the AV nodal selectivity of Class IV drugs with broader myocardial effects. They do NOT block ventricular sodium or potassium channels effectively.