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QTc Prolongation — MRCPsych Paper B MCQ

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HardSchizophrenia & PsychosisQTc ProlongationMRCPsych Paper B

A 30-year-old man with schizophrenia takes olanzapine 20 mg daily. His baseline QTcF was 430 ms and a routine ECG now shows a QTcF of 480 ms. He has no syncope or presyncope, and serum potassium, calcium and magnesium are normal. Which absolute QTc threshold would escalate management from medication review, repeat ECG and consideration of dose reduction or switching to stopping the suspected causative antipsychotic and obtaining immediate cardiology review?

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Correct answer: E — QTc above 500 ms

The correct answer is E. A QTc above 500 ms is the high-risk threshold at which the suspected causative antipsychotic should generally be stopped or changed and immediate cardiology review obtained. This patient’s QTcF of 480 ms is nevertheless abnormal for a man: it warrants review of reversible causes and interacting medicines, repeat ECG, and consideration of reducing the dose or switching to an antipsychotic with less QT effect. Option D therefore identifies a lower threshold for clinical review and repeat measurement, not the threshold for immediate cessation. A is below accepted sex-specific action thresholds, while waiting until 550 ms would expose the patient to avoidable arrhythmic risk. Syncope or presyncope would require more urgent assessment even at a lower QTc.

Reference: National Institute for Health and Care Excellence. KardiaMobile 6L for measuring QT interval in people having antipsychotic medication, final scope, section 3.3.2, February 2022. https://www.nice.org.uk/guidance/htg694/documents/final-scope