Polymorphic VT — Long QT Torsades MCQ — NEET PG Practice Question | NEETPGAI
Polymorphic VT — Long QT Torsades
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stethoscope Medicine
A 62-year-old woman with depression (citalopram 40 mg daily) and recent respiratory infection (azithromycin) is admitted with acute gastroenteritis. She is given intravenous ondansetron for nausea. Twelve hours later, she collapses with sudden syncope. Telemetry captures the rhythm shown as **C** in the diagram — polymorphic ventricular tachycardia at 220 bpm with QRS complexes twisting around the baseline. Post-conversion ECG shows sinus rhythm with QTc 580 ms, prominent U waves, and T-wave alternans. Labs: K+ 2.9 mEq/L, Mg2+ 1.2 mg/dL. Which of the following is the FIRST-LINE pharmacologic therapy for the arrhythmia marked **C**?
A. Intravenous magnesium sulfate 2 g bolus over 10–15 minutes
B. Intravenous amiodarone 300 mg bolus followed by infusion
C. Intravenous sotalol 1.5 mg/kg bolus
D. Intravenous lidocaine 1–1.5 mg/kg bolus
Explanation
Why intravenous magnesium sulfate 2 g bolus over 10–15 minutes is right
Torsades de pointes (the polymorphic VT marked C with twisting QRS axis on prolonged QT background) is uniquely responsive to intravenous magnesium sulfate as first-line pharmacologic therapy, even when serum magnesium is normal. Magnesium stabilizes the cardiac membrane, shortens the QT interval, and suppresses early afterdepolarizations that trigger Torsades. This patient has acquired LQTS from multiple QT-prolonging drugs (citalopram, ondansetron, azithromycin) and electrolyte derangements (hypokalemia, hypomagnesemia), making magnesium replacement both diagnostic and therapeutic. Per ACC/AHA Ventricular Arrhythmia Guidelines, magnesium is the cornerstone of acute Torsades management.
Why each distractor is wrong
Intravenous amiodarone 300 mg bolus followed by infusion: Amiodarone is a class III antiarrhythmic that PROLONGS the QT interval. It is explicitly contraindicated in Torsades de pointes and acquired LQTS because it worsens the underlying substrate and increases recurrence risk.
Intravenous sotalol 1.5 mg/kg bolus: Sotalol is a class III antiarrhythmic with QT-prolonging properties. Like amiodarone, it is contraindicated in Torsades and would exacerbate the arrhythmia in this patient with already markedly prolonged QTc.
Intravenous lidocaine 1–1.5 mg/kg bolus: Lidocaine is a class IB antiarrhythmic and is reserved as a SECOND-LINE agent if magnesium fails to suppress Torsades. It does not prolong QT and is safe, but it is not the first-line choice for acute Torsades management.
High-YieldNEET PG
Torsades de pointes = magnesium first; avoid all QT-prolonging antiarrhythmics (class IA, IC, III). Correct electrolytes, discontinue offending drugs, and consider overdrive pacing.
ACC/AHA Ventricular Arrhythmia Guidelines; acquired long QT syndrome and Torsades de pointes management
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