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QT prolongation — MRCPsych Paper A MCQ

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ModeratePsychopharmacologyQT prolongationMRCPsych Paper A

A 46-year-old man treated with pimozide develops sudden syncope. An ECG obtained immediately afterwards shows a QTc interval of 540 ms and recurrent bursts of a rapid ventricular rhythm in which the QRS complexes vary in amplitude and appear to twist around the isoelectric line. Which arrhythmia best explains this presentation?

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Correct answer: C — Torsade de pointes

The arrhythmia is torsade de pointes, a polymorphic ventricular tachycardia associated with delayed ventricular repolarisation and a prolonged QT interval. The characteristic ECG appearance is variation in QRS amplitude and axis, producing an apparent twisting around the isoelectric line. Pimozide is a QT-prolonging antipsychotic specifically associated with torsade de pointes, which can present with syncope or sudden cardiac arrest. Complete atrioventricular block produces marked bradycardia rather than rapid polymorphic ventricular activity. Monomorphic ventricular tachycardia has a consistent QRS morphology. Supraventricular tachycardia with aberrancy generally remains regular with a relatively uniform morphology, while atrial fibrillation produces an irregularly irregular rhythm without the characteristic twisting pattern.

Reference: Electronic Medicines Compendium, Orap 4 mg tablets: Summary of Product Characteristics, sections 4.4 and 4.8, updated 20 March 2018. https://www.medicines.org.uk/emc/product/6911/smpc