Torsades de Pointes in Long QT Syndrome MCQ — NEET PG Practice Question | NEETPGAI
Torsades de Pointes in Long QT Syndrome
medium
stethoscope Medicine
A 28-year-old woman with congenital long QT syndrome (LQT2) presents to the emergency department with palpitations and syncope during an emotional conversation. ECG shows a baseline QTc of 520 ms. During monitoring, she develops the arrhythmia marked **B** in the diagram — a polymorphic ventricular tachycardia with continuously changing QRS axis twisting around the isoelectric baseline. She is hemodynamically stable. Which of the following is the most appropriate immediate pharmacological intervention?
A. Intravenous magnesium sulfate 2 g bolus over 1-2 minutes
B. Intravenous amiodarone 300 mg bolus followed by infusion
C. Intravenous isoproterenol infusion at 2-10 mcg/min
D. Intravenous procainamide 15 mg/kg over 30 minutes
Explanation
Why intravenous magnesium sulfate 2 g bolus over 1-2 minutes is right
Torsades de Pointes (TdP) — the polymorphic VT marked B with characteristic twisting of the QRS axis around the isoelectric baseline — is a medical emergency in the setting of prolonged QT interval. In hemodynamically stable patients with recurrent TdP, intravenous magnesium sulfate is the FIRST-LINE pharmacological agent. Magnesium suppresses early afterdepolarizations (EADs) that trigger TdP, even in patients with normal serum magnesium levels. The standard dose is 2 g IV bolus over 1–2 minutes, which may be repeated. This is the cornerstone of acute TdP management per ACC/AHA/HRS 2017 VA Guidelines and HRS 2013 LQTS recommendations.
Why each distractor is wrong
Intravenous amiodarone 300 mg bolus followed by infusion: Amiodarone is a Class III antiarrhythmic that PROLONGS the QT interval and is explicitly CONTRAINDICATED in acute TdP management. Class III agents worsen repolarization abnormalities and increase the risk of recurrent TdP and degeneration to ventricular fibrillation.
Intravenous isoproterenol infusion at 2–10 mcg/min: While beta-agonists increase heart rate and shorten QT interval (useful in some settings), isoproterenol is AVOIDED in congenital LQTS, particularly LQT1 and LQT2 (as in this patient). It can paradoxically trigger arrhythmias in genotype-specific LQTS. Overdrive pacing is preferred if rate acceleration is needed.
Intravenous procainamide 15 mg/kg over 30 minutes: Procainamide is a Class IA antiarrhythmic that PROLONGS the QT interval and is CONTRAINDICATED in TdP. Class IA agents suppress phase 0 depolarization and delay repolarization, exacerbating the underlying electrophysiological substrate and increasing TdP risk.
High-YieldNEET PG
Magnesium is the FIRST-LINE drug for acute TdP regardless of serum magnesium level; AVOID all Class I and Class III antiarrhythmics in TdP.
