A 58-year-old woman on sotalol for paroxysmal AF develops recurrent polymorphic ventricular tachycardia with twisting QRS complexes around the baseline. Her QT interval measures 560 ms. She is hemodynamically stable during episodes. What is the first-line intravenous therapy?
- A IV amiodarone loading
- B IV magnesium sulfate ✓
- C IV lidocaine bolus
- D IV adenosine
Correct answer: B. IV magnesium sulfate
Explanation
Torsades de pointes with a prolonged QT is treated first with IV magnesium sulfate 1 to 2 g, which suppresses early afterdepolarizations even when serum magnesium is normal. Amiodarone is contraindicated conceptually because it further prolongs repolarization and can worsen torsades. Lidocaine does not shorten the QT and is ineffective here. Overdrive pacing and correcting potassium and magnesium levels are adjunctive measures once magnesium has been given.
Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.
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