## Why Lidocaine (Class IB) is right Lidocaine is a Class IB antiarrhythmic that blocks voltage-gated sodium channels responsible for Phase 0 depolarization (marked **A**). Class IB agents have rapid on-and-off kinetics, making them ideal for acute ventricular arrhythmias in the immediate post-MI setting. Lidocaine shortens the action potential duration and is the first-line IV agent for PVCs and ventricular tachycardia in acute coronary syndromes. Per Guyton & Hall and KD Tripathi, Class IB drugs are preferred over Class IA and IC in acute ischemic settings because they do not prolong QT interval and carry lower proarrhythmic risk. ## Why each distractor is wrong - **Flecainide (Class IC)**: Although it blocks sodium channels (Phase 0), Class IC agents produce the most potent Na+ channel blockade and significantly prolong conduction. The CAST trial demonstrated increased mortality with flecainide in post-MI patients with structural heart disease, making it contraindicated in this clinical scenario. - **Amiodarone (Class III)**: While amiodarone is a broad-spectrum antiarrhythmic effective for ventricular arrhythmias, it primarily acts via potassium channel blockade (Phase 3), not sodium channel blockade (Phase 0). It is reserved for refractory cases and chronic management, not acute post-MI PVCs. - **Verapamil (Class IV)**: Verapamil blocks L-type calcium channels (Phase 2), not sodium channels (Phase 0). It is ineffective for ventricular arrhythmias and is contraindicated in acute MI due to negative inotropic effects and risk of cardiogenic shock. **High-Yield:** Class IB antiarrhythmics (lidocaine) are first-line for acute ventricular arrhythmias post-MI because they block Phase 0 sodium influx with rapid kinetics and minimal QT prolongation, unlike Class IA and IC agents. [cite:Guyton & Hall 14e Ch 9; KD Tripathi 9e Ch 38]
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