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Clozapine Sinus Tachycardia — MRCPsych Paper B MCQ

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ModerateSchizophrenia & PsychosisClozapine Sinus TachycardiaMRCPsych Paper B

A 32-year-old man is in the third week of gradual clozapine titration for treatment-resistant schizophrenia. On repeated observations his resting heart rate is 108–114 beats/min, and an ECG shows sinus tachycardia. He is afebrile and has normal blood pressure and oxygen saturation, with no chest pain, dyspnoea, flu-like symptoms or clinical anxiety. Serial troponin and C-reactive protein measurements, thyroid function and full blood count are normal. He denies excessive caffeine or other stimulant use. What is the most likely explanation for the tachycardia?

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Correct answer: B — Clozapine-induced sinus tachycardia

The most likely diagnosis is **clozapine-induced sinus tachycardia**, a very common adverse effect that is particularly frequent during initial titration. Clozapine alters autonomic cardiovascular regulation through reduced vagal activity and other cholinergic and adrenergic effects. However, resting tachycardia during the first two months is also a warning feature of clozapine-associated myocarditis. Here, the absence of fever, chest pain, dyspnoea or flu-like symptoms, together with normal serial troponin, CRP and ECG findings apart from sinus tachycardia, makes myocarditis less likely, although continued monitoring and safety-netting remain appropriate. Anxiety and caffeine are unsupported by the history. Normal thyroid function argues against thyrotoxicosis, while normal oxygen saturation and the absence of respiratory symptoms or other suggestive features make pulmonary embolism less likely.

Reference: Clozapine Viatris 100 mg Tablets, Summary of Product Characteristics, sections 4.4 and 4.8, revised February 2026. https://www.medicines.org.uk/emc/product/15498/smpc