Emergency Medicine · Toxicology
The facts most likely to be tested
The classic triad of TCA overdose consists of anticholinergic toxicity, cardiovascular conduction abnormalities, and central nervous system depression.
Sodium bicarbonate is the first-line treatment for patients with a QRS complex duration >100 ms to narrow the complex and prevent ventricular arrhythmias.
TCA toxicity is primarily driven by the blockade of fast sodium channels in the myocardium, leading to prolonged QRS and right axis deviation.
Anticholinergic toxidrome presents as hyperthermia, flushed skin, dry mucous membranes, mydriasis, and urinary retention.
Seizures are a common and dangerous complication of TCA overdose and should be treated with benzodiazepines.
Hypotension in TCA overdose is caused by alpha-1 adrenergic receptor blockade and is initially managed with isotonic saline boluses.
Physostigmine is strictly contraindicated in TCA overdose because it can precipitate asystole and seizures due to increased cholinergic tone.
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A 24-year-old female is brought to the emergency department after being found unresponsive by her roommate. On physical exam, the patient is tachycardic, hyperthermic, and has dilated, non-reactive pupils with dry, flushed skin. An ECG reveals a sinus tachycardia with a QRS duration of 130 ms and a terminal R wave in lead aVR. The patient is currently hypotensive and has had one episode of a generalized tonic-clonic seizure.
What is the most appropriate next step in the management of this patient's cardiac conduction abnormality?
Intravenous sodium bicarbonate
The patient exhibits classic signs of TCA overdose, including anticholinergic toxidrome and cardiac conduction delay; sodium bicarbonate is indicated to treat the QRS prolongation by increasing serum pH and extracellular sodium concentration.
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Etiology / Epidemiology
Common in intentional overdose due to narrow therapeutic index. High lethality in suicidal patients.
Clinical Manifestations
Triad of anticholinergic effects, cardiotoxicity, and CNS depression. Wide complex tachycardia is the hallmark.
Diagnosis
Gold standard is ECG. QRS duration > 100 ms predicts seizures; > 160 ms predicts arrhythmias.
Treatment
First-line is Sodium Bicarbonate. Avoid Class 1A/1C antiarrhythmics.
Prognosis
High mortality if untreated. Cardiac arrest is the primary cause of death.
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Epidemiology & Etiology
Tricyclic antidepressants (TCAs) are frequently prescribed for depression and neuropathic pain. Overdose is highly dangerous due to narrow therapeutic index and rapid absorption. Intentional ingestion remains a leading cause of drug-related mortality in the emergency department.
Pertinent Anatomy
TCAs act on the myocardium and the central nervous system. The primary cardiac target is the fast sodium channel in the His-Purkinje system.
Pathophysiology
TCAs block fast sodium channels, leading to slowed conduction and wide complex tachycardia. Antagonism of muscarinic receptors causes anticholinergic toxidrome. Alpha-adrenergic blockade results in peripheral vasodilation and hypotension.
Clinical Manifestations
Patients present with anticholinergic toxidrome: dry skin, hyperthermia, mydriasis, and urinary retention. Seizures and hypotension are critical red flags. Cardiac exam reveals tachycardia and potential ventricular arrhythmias.
Diagnosis
The ECG is the gold standard for risk stratification. A QRS duration > 100 ms is highly sensitive for predicting seizures. A QRS duration > 160 ms is a specific predictor for ventricular arrhythmias.
Treatment
Administer Sodium Bicarbonate to narrow the QRS complex and treat metabolic acidosis. Avoid Class 1A and 1C antiarrhythmics as they worsen sodium channel blockade. Use Benzodiazepines for seizures. Physostigmine is strictly contraindicated due to risk of asystole.
Prognosis
Patients require continuous cardiac monitoring until asymptomatic for 6-12 hours. Cardiac arrest and refractory hypotension are the primary causes of death. Early intervention with alkalinization significantly improves survival.
Differential Diagnosis
SSRI Overdose: typically lacks QRS prolongation
Salicylate Toxicity: presents with respiratory alkalosis and metabolic acidosis
Diphenhydramine Overdose: similar anticholinergic signs but less cardiotoxicity
Cocaine Toxicity: presents with hypertension and tachycardia without anticholinergic signs
Beta-blocker Overdose: presents with bradycardia and hypoglycemia