A 58-year-old man with a history of myocardial infarction 6 months ago presents to the emergency department with palpitations and syncope. ECG shows a prolonged QT interval (520 ms) and polymorphic ventricular tachycardia. Serum electrolytes reveal K⁺ 3.2 mmol/L and Mg²⁺ 1.6 mg/dL. He is on amiodarone for arrhythmia prophylaxis. What is the most appropriate immediate next step in management?

Explanation

## Clinical Context: Drug-Induced Long QT Syndrome with Torsades de Pointes This patient has **acquired long QT syndrome** secondary to amiodarone use, compounded by **hypokalemia and hypomagnesemia**. The polymorphic VT (torsades de pointes) is a life-threatening arrhythmia that requires immediate electrolyte correction. ## Pathophysiology of the Cardiac Action Potential in Long QT **Key Point:** The QT interval represents the duration of ventricular depolarization and repolarization (phases 0–3 of the cardiac action potential). Prolongation occurs when repolarization is delayed, typically due to: - Blockade of potassium channels (phase 3 repolarization) - Increased inward calcium current (phase 2 plateau prolongation) - Reduced outward potassium current Amiodarone blocks multiple ion channels (K⁺, Na⁺, Ca²⁺), extending the action potential duration and QT interval. Hypokalemia and hypomagnesemia further impair repolarization by reducing the driving force for K⁺ efflux and impairing K⁺ channel function. ## Mechanism of Torsades de Pointes Polymorphic VT in the setting of prolonged QT is triggered by **early afterdepolarizations (EADs)** — abnormal depolarizations occurring during phase 2–3 of the action potential. These occur when: 1. The action potential is abnormally prolonged 2. Intracellular calcium is elevated 3. Electrolyte imbalances (↓K⁺, ↓Mg²⁺) destabilize the membrane potential ## Management Algorithm ```mermaid flowchart TD A[Polymorphic VT + Prolonged QT]:::outcome --> B{Hemodynamically stable?}:::decision B -->|No| C[Synchronized cardioversion]:::urgent B -->|Yes| D[Check K⁺ and Mg²⁺]:::action D --> E{Electrolyte deficit?}:::decision E -->|Yes| F[IV Mg²⁺ 2g over 5-10 min]:::action E -->|No| G[Supportive care, discontinue QT-prolonging drugs]:::action F --> H[Repeat ECG, monitor QT]:::action H --> I[Correct K⁺ to >4.0 mEq/L]:::action ``` ## Why IV Magnesium Is the Correct Answer **High-Yield:** Magnesium is the **first-line acute therapy** for torsades de pointes, regardless of serum magnesium level. Mg²⁺ acts by: - Blocking L-type calcium channels → reduces EADs - Stabilizing the cardiac membrane potential - Improving potassium channel function - Reducing triggered activity The standard dose is **2 g IV bolus over 5–10 minutes**, followed by infusion if needed. Efficacy is independent of baseline serum Mg²⁺ level (even "normal" levels may be functionally insufficient in the setting of arrhythmia). ## Concurrent Measures **Clinical Pearl:** After magnesium administration: 1. **Correct hypokalemia** — target K⁺ >4.0 mmol/L (preferably 4.5–5.0 mmol/L in acute arrhythmia) 2. **Discontinue amiodarone** and other QT-prolonging agents 3. **Avoid class IA antiarrhythmics** (quinidine, procainamide, disopyramide) — they further prolong QT 4. **Repeat ECG** to assess QT interval shortening ## Why Observation Alone Is Insufficient Passive observation without electrolyte correction allows continued EAD generation and recurrent torsades. The arrhythmia is **life-threatening** and requires active intervention. [cite:Harrison 21e Ch 240]