A 52-year-old man with a history of hypertension and type 2 diabetes presents to the emergency department with palpitations and syncope. His ECG shows a prolonged QT interval (QTc = 520 ms) with normal sinus rhythm. Serum electrolytes reveal K⁺ = 3.2 mEq/L, Mg²⁺ = 1.6 mg/dL, and Ca²⁺ = 8.2 mg/dL. He is currently on metoprolol and enalapril. What is the most appropriate immediate next step in management?

Explanation

## Pathophysiology of QT Prolongation and Torsades de Pointes Risk **Key Point:** QT prolongation reflects delayed repolarization during phase 3 of the cardiac action potential. Hypokalemia, hypomagnesemia, and hypocalcemia impair the rapid outward potassium current (I~K~) and slow the return to resting potential, creating a substrate for early afterdepolarizations (EADs) and torsades de Pointes. **High-Yield:** The triad of electrolyte abnormalities (K⁺ <3.5, Mg²⁺ <2.0, Ca²⁺ <8.5) in a patient with QT prolongation is a medical emergency because it increases the risk of life-threatening arrhythmias. ## Mechanism: Action Potential Phase 3 and Electrolyte Dependence During phase 3 (repolarization): - Inward L-type calcium current (I~Ca,L~) is inactivating - Outward delayed rectifier potassium currents (I~K~, I~Ks~, I~Kr~) dominate - **Hypokalemia** reduces the driving force for K⁺ efflux (smaller gradient from inside to outside), slowing repolarization - **Hypomagnesemia** impairs I~Kr~ channel function and prolongs action potential duration (APD) - **Hypocalcemia** may prolong the plateau phase by reducing inactivation of I~Ca,L~ EADs occur when repolarization is incomplete and a secondary depolarization (driven by reactivated L-type calcium channels) occurs during phase 3. This is the mechanism of torsades de Pointes in the setting of QT prolongation. ## Management Algorithm ```mermaid flowchart TD A[QT prolongation + syncope + electrolyte abnormalities]:::outcome --> B{Immediate risk of arrhythmia?}:::decision B -->|Yes| C[Correct electrolytes FIRST]:::action C --> D[IV Mg²⁺ 1–2 g bolus]:::action D --> E[IV K⁺ to target >4.0 mEq/L]:::action E --> F[Recheck QTc and monitor telemetry]:::action B -->|Stable, no symptoms| G[Oral supplementation + observation]:::action F --> H{QTc normalized?}:::decision H -->|Yes| I[Discontinue QT-prolonging drugs]:::action H -->|No| J[Consider temporary pacing if recurrent EADs]:::action ``` **Clinical Pearl:** Magnesium is the most critical electrolyte to correct first because it directly stabilizes the cardiac membrane and suppresses EADs, even before potassium normalization is complete. IV magnesium sulfate (1–2 g over 5–20 minutes) is the standard first-line intervention. **Tip:** Remember that **correcting the underlying electrolyte abnormality is more effective than antiarrhythmic drugs** in preventing torsades de Pointes. Amiodarone and other Class IA/III agents can paradoxically prolong QT and worsen the arrhythmia in this setting. ## Why Electrolyte Correction is Superior to Antiarrhythmics | Intervention | Mechanism | Efficacy in QT Prolongation | Risk | | --- | --- | --- | --- | | **IV Mg²⁺** | Stabilizes membrane, blocks EADs | ★★★★★ | Minimal (hypermagnesemia rare) | | **IV K⁺** | Restores driving force for repolarization | ★★★★ | Hyperkalemia if overcorrected | | **Amiodarone** | Blocks multiple ion channels | ★★ | **Prolongs QT further** | | **Temporary pacing** | Overdrive suppression of ectopy | ★★★ | Only if medical therapy fails | [cite:Harrison 21e Ch 242]