Regarding the Vaughan-Williams classification of antiarrhythmic drugs, all of the following statements are correct EXCEPT:

Explanation

## Vaughan-Williams Classification of Antiarrhythmics ### Overview The Vaughan-Williams classification divides antiarrhythmic drugs into four classes based on their electrophysiological effects on the cardiac action potential. ### Class-by-Class Mechanisms | Class | Mechanism | Primary Effect | Examples | |-------|-----------|----------------|----------| | I | Na⁺ channel blockade | ↓ Phase 0 slope, ↑ PR interval | Quinidine, Procainamide, Flecainide | | II | β-adrenergic antagonism | ↑ AV nodal refractoriness, ↓ HR | Metoprolol, Propranolol, Esmolol | | III | K⁺ channel blockade | ↑ Action potential duration, ↑ QT | Amiodarone, Sotalol, Dofetilide | | IV | Ca²⁺ channel blockade | ↑ AV nodal refractoriness, ↓ conduction | Verapamil, Diltiazem | ### Analysis of Each Option **Option 1 (Class I) — CORRECT** - Sodium channel blockers slow phase 0 depolarization - Decrease conduction velocity in atria, ventricles, and His-Purkinje system - Increase PR and QRS intervals on ECG **Option 2 (Class II) — CORRECT** - Beta-blockers slow AV nodal conduction and increase refractoriness - Reduce heart rate and contractility - Slow SA nodal automaticity **Option 3 (Class III) — CORRECT** - Potassium channel antagonists prolong repolarization - Extend the action potential duration (APD) and effective refractory period (ERP) - Lengthen QT interval **Option 4 (Class IV) — INCORRECT** - Calcium channel blockers (verapamil, diltiazem) DECREASE the rate of spontaneous depolarization in the SA node - They slow SA nodal automaticity and AV nodal conduction - They do NOT increase the rate of spontaneous depolarization **Key Point:** Class IV drugs (calcium channel blockers) are negative chronotropes — they SLOW the SA node, not accelerate it. This is the opposite of what the option states. **High-Yield:** Remember: Class II and IV drugs both slow AV nodal conduction and are useful for SVT. Class I and III drugs primarily affect ventricular arrhythmias (though amiodarone is a pan-antiarrhythmic). **Clinical Pearl:** Verapamil and diltiazem are the only calcium channel blockers with antiarrhythmic properties; dihydropyridines (nifedipine, amlodipine) lack significant AV nodal effects and are not antiarrhythmics. **Mnemonic: "ABCD" for Class IV effects** - **A** — AV nodal slowing - **B** — Beta-like (negative inotrope) - **C** — Calcium channel block - **D** — Decreased automaticity