Cardiotoxic and psychotropic medication poisoning — formative SAQs

1. Diagnosis and ECG predictor

The diagnosis is tricyclic antidepressant cardiotoxicity from sodium-channel blockade, with an anticholinergic toxidrome. The QRS duration of 140 milliseconds is the predictor of the dominant threat of ventricular arrhythmia and shock: a QRS over 100 milliseconds in the poisoned child predicts sodium-channel blockade and the risk of ventricular arrhythmia. The tall R wave in lead aVR supports the diagnosis. ^[2]

2. Antidote, dose and endpoint

The antidote is hypertonic sodium bicarbonate, 1 to 2 mL per kilogram of 8.4 per cent intravenously (1 to 2 mmol per kg), repeated every few minutes to narrow the QRS and raise the serum pH to 7.45 to 7.55. The endpoints are a narrowed QRS, an improved blood pressure and the target pH. Hyperventilation in the intubated child adds an alkalinising effect. ^[2]

3. Contraindicated antiarrhythmics

The class Ia antiarrhythmics quinidine and procainamide and the class Ic antiarrhythmic flecainide must NOT be given, and amiodarone is avoided for the same reason. Each blocks the cardiac sodium channel and worsens the conduction blockade, widening the QRS further and promoting re-entrant ventricular arrhythmia. The ventricular arrhythmia of tricyclic poisoning is treated with sodium bicarbonate first, not with a sodium-channel-blocking antiarrhythmic. ^[2]

4. Escalation plan

If the arrhythmia or the shock does not respond to bicarbonate, I escalate to intravenous lipid emulsion (the lipid sink for this lipophilic drug), continue prolonged cardiopulmonary resuscitation if arrest occurs, and activate a veno-arterial extracorporeal membrane oxygenation service early because the window for cannulation is narrow. I called the senior team, intensive care, retrieval and the Poisons Information Centre in parallel, and I arrange a mental-health assessment once she is safe. ^[2]