Cardiovascular drugs: torsades de pointes: practice question

A Pharmacology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Pharmacology · Cardiovascular drugs: torsades de pointes

Sample item

A 58-year-old woman on sotalol has serum potassium 2.8 mmol/L. She has episodes of polymorphic ventricular tachycardia with a twisting axis, QTc of 580 ms, and between episodes she is alert with a pulse. Which drug should be given first?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Intravenous magnesium sulfate

Torsades de pointes arises from early afterdepolarisations when repolarisation is prolonged. Intravenous magnesium suppresses them even when serum magnesium is normal, and potassium is corrected with removal of the offending drug. Further QT-prolonging drugs worsen the arrhythmia, and overdrive pacing or isoprenaline may be needed for pause-dependent episodes.

Option by option

  • A. Intravenous amiodarone

    Amiodarone prolongs the QT interval and can worsen torsades.

  • B. Intravenous procainamide

    Procainamide is a class IA drug that prolongs the QT interval.

  • C. Intravenous ibutilide

    Ibutilide is a class III drug that prolongs repolarisation and can itself cause torsades.

  • D. Intravenous magnesium sulfate (correct)

    Correct. Magnesium is first-line and stabilises the membrane against afterdepolarisations.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is D, Intravenous magnesium sulfate. Torsades de pointes arises from early afterdepolarisations when repolarisation is prolonged.

Why is "Intravenous amiodarone" not the answer?

Amiodarone prolongs the QT interval and can worsen torsades.

Why is "Intravenous procainamide" not the answer?

Procainamide is a class IA drug that prolongs the QT interval.

Why is "Intravenous ibutilide" not the answer?

Ibutilide is a class III drug that prolongs repolarisation and can itself cause torsades.

What concept does this question test?

It tests Cardiovascular drugs: torsades de pointes in Pharmacology. Intravenous magnesium suppresses them even when serum magnesium is normal, and potassium is corrected with removal of the offending drug. Further QT-prolonging drugs worsen the arrhythmia, and overdrive pacing or isoprenaline may be needed for pause-dependent episodes.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Cardiovascular drugs: torsades de pointes?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.