Tricyclic Antidepressant AND Sodium Channel Blocker Toxicity

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Tricyclic antidepressant (TCA) and sodium-channel-blocker toxicity is one of the most acutely dangerous toxidromes in internal medicine and emergency care, because these drugs combine three cardiotoxic and neurotoxic effects — fast sodium-channel blockade, alpha-1 adrenergic antagonism, and anticholinergic action — to produce a characteristic clinical picture of altered consciousness, seizures, hypotension, and a widened QRS that can progress to ventricular tachycardia, ventricular fibrillation, and asystole. The physician's task is to recognise the toxidrome early (within minutes), to give intravenous sodium bicarbonate to overcome the sodium-channel blockade, to control seizures with benzodiazepines, to support the circulation, and to avoid the drugs that worsen the picture (class Ia and Ic antiarrhythmics, flumazenil, and most anticonvulsants other than benzodiazepines). ^{[3] [6]}

The FRACP candidate must be able to defend three positions without hedging: (1) a QRS duration of 100 milliseconds or more in a suspected TCA or sodium-channel-blocker overdose is the trigger for intravenous sodium bicarbonate ^[4] — given as a bolus of 1 to 2 mmol/kg, repeated to achieve a serum pH of 7.45 to 7.55 and a QRS of less than 100 ms; (2) hypotension is treated with intravenous fluids, sodium bicarbonate, and direct-acting vasopressors (noradrenaline or adrenaline — alpha-1 agonists); and (3) class Ia antiarrhythmics (quinidine, procainamide, disopyramide) and class Ic antiarrhythmics (flecainide, propafenone) are contraindicated because they worsen sodium-channel blockade, and flumazenil is contraindicated because it precipitates seizures in the context of TCA overdose. Lead with the decision, then the evidence, then the trap. ^{[2] [3]}

References7ShowHide

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3. [3]Braitberg G, Oakley E Small dose... big poison Aust Fam Physician, 2010.PMID 21301654
4. [4]Boehnert MT, Lovejoy FH Jr Value of the QRS duration versus the serum drug level in predicting seizures and ventricular arrhythmias after an acute overdose of tricyclic antidepressants. N Engl J Med, 1985.PMID 4022081
5. [5]Liebelt EL, Francis PD, Woolf AD ECG lead aVR versus QRS interval in predicting seizures and arrhythmias in acute tricyclic antidepressant toxicity. Ann Emerg Med, 1995.PMID 7618783
6. [6]Carroll S, McGee M, Audette M, Tuttle M, et al. Lidocaine for diphenhydramine-induced ventricular tachycardia: A case report Am J Emerg Med, 2026.PMID 41610479
7. [7]Zaccara G, Franco V Pharmacokinetic Interactions Between Antiseizure and Psychiatric Medications Curr Neuropharmacol, 2023.PMID 35611779