A 68-year-old man with a history of dilated cardiomyopathy presents with sustained ventricular tachycardia. He is initiated on intravenous procainamide. Which of the following best explains the antiarrhythmic mechanism of procainamide in this patient?

Explanation

## Procainamide Mechanism of Action **Key Point:** Procainamide is a Class IA antiarrhythmic agent that exerts its primary antiarrhythmic effect through blockade of fast sodium channels in the cardiac myocyte membrane. ### Sodium Channel Blockade (Primary Mechanism) Procainamide inhibits the inward fast sodium current (I~Na~) during phase 0 of the cardiac action potential. This results in: 1. **Slowing of phase 0 depolarization** — the rate of rise of the action potential is decreased 2. **Increased conduction time** — particularly in tissues dependent on fast sodium channels (ventricular myocardium, Purkinje fibers, atrial muscle) 3. **Widening of the QRS complex** on the ECG — a hallmark of Class I activity ### Secondary Mechanism: Potassium Channel Blockade Procainamide also possesses **Class III properties** (potassium channel blockade), which: - Prolongs the action potential duration (APD) - Extends the effective refractory period (ERP) - Increases the QT interval on ECG This dual action (Class IA + III) makes procainamide effective for both atrial and ventricular arrhythmias. ### Clinical Application in Ventricular Tachycardia In sustained ventricular tachycardia (as in this patient with dilated cardiomyopathy), the sodium channel blockade: - Slows conduction velocity in the reentrant circuit - Increases the refractory period - Breaks the reentrant loop or prevents its perpetuation **Clinical Pearl:** Procainamide is particularly useful in VT because it combines sodium channel blockade (slows conduction) with potassium channel blockade (prolongs refractoriness) — a synergistic effect for terminating reentrant arrhythmias. ### Why Class IA, Not Class IB or IC? | Class | Primary Effect | Secondary Effect | Example | |-------|---|---|---| | IA | Na^+^ channel block | K^+^ channel block (prolongs APD) | Procainamide, Quinidine | | IB | Na^+^ channel block | K^+^ channel activation (shortens APD) | Lidocaine, Mexiletine | | IC | Na^+^ channel block (marked) | Minimal effect on APD | Flecainide, Propafenone | Procainamide's **prolongation of APD** (via K^+^ blockade) is the distinguishing feature that makes it Class IA. **High-Yield:** On the NEET PG exam, remember: **Class IA = Na^+^ block + K^+^ block (APD prolongation)**. This dual action is why procainamide and quinidine are effective for both atrial and ventricular arrhythmias, unlike Class IB agents. **Warning:** Do not confuse procainamide's mechanism with: - ~~Flecainide (Class IC)~~ — marked Na^+^ block with minimal APD change - ~~Amiodarone~~ — has all four class properties but is primarily Class III - ~~Lidocaine~~ — Class IB with APD *shortening*, not prolongation [cite:Harrison 21e Ch 226]