Tricyclic antidepressant poisoning with QRS prolongation — MRCEM SBA MCQ

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EasyToxicology and environmental emergenciesTricyclic antidepressant poisoning with QRS prolongationMRCEM SBA

A 29-year-old woman attends the emergency department two hours after taking an unknown quantity of amitriptyline. She is drowsy but maintains her airway. Her heart rate is 118 beats/min and blood pressure is 126/78 mmHg. Cardiac monitoring shows sinus tachycardia without a ventricular arrhythmia. Her ECG shows a QRS duration of 142 ms, compared with 84 ms on a previous ECG; the QTc is 430 ms. Venous pH is 7.39 and potassium is 4.1 mmol/L. She has not had a seizure. Which drug should be administered now?

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Correct answer: B — Administer intravenous sodium bicarbonate

The recent amitriptyline ingestion and newly widened QRS indicate clinically significant tricyclic-antidepressant cardiotoxicity. Intravenous sodium bicarbonate is the immediate treatment for this conduction abnormality, even though the initial venous pH is normal. The patient needs continued cardiac monitoring, repeat ECG and blood-gas assessment after treatment, airway reassessment and monitored admission with poisons-service advice. The treatment should not wait for a seizure, hypotension or ventricular arrhythmia to develop. Magnesium sulphate (A) is attractive because poisoning can produce dangerous ECG changes, but her QTc is not prolonged and she has no torsade de pointes; it does not address the widened QRS. Sodium chloride 0.9% (C) would be appropriate for clinically important hypotension, which she does not have. Diazepam (D) is appropriate for a seizure, but none has occurred and it will not correct the conduction abnormality. Lidocaine (E) may be considered with specialist advice for a ventricular arrhythmia that persists despite toxin-directed treatment; she has no ventricular arrhythmia. Her QRS change, rather than the normal pH or currently preserved blood pressure, determines the immediate drug choice.

Reference: Guideline for the Management of Tricyclic Antidepressant Overdose (2009) — https://heeoe.hee.nhs.uk/sites/default/files/tricyclic_antidepressant_od_cem5075-gemnet-2009_0.pdf Management of patients with suspected but unidentified poisoning in the emergency department (Royal College of Emergency Medicine Best Practice Guideline 2025) (January 2026) — https://ep.bmj.com/content/edpract/early/2026/01/05/archdischild-2025-329469.full.pdf