Tricyclic Antidepressant Overdose

Introduction

• TCAs are an important cause of drug-related morbidity and mortality in the United States.¹
• Major agents in this class include amitriptyline, nortriptyline, desipramine, imipramine, and clomipramine.
• Although their use for primary depression has declined, TCAs continue to be prescribed for chronic pain syndromes, migraine prophylaxis, and several psychiatric conditions.²
• Overdose produces rapid, life-threatening toxicity characterized by central nervous system (CNS) depression, seizures, and prominent cardiovascular disturbances.³
• Prompt recognition and aggressive supportive management are essential as severe complications can develop within hours of ingestion.
• This summary reviews the pharmacokinetics, mechanisms of toxicity, clinical manifestations, diagnostic evaluation, and evidence-based management of TCA overdose.

Pharmacokinetics

Pharmacokinetics Under Therapeutic Conditions

• TCAs are rapidly absorbed from the gastrointestinal tract and undergo significant first-pass metabolism, reaching peak plasma concentrations within two to eight hours.
• They are highly lipophilic and extensively protein-bound, resulting in a large volume of distribution and elimination half-lives ranging from 7 to 58 hours.
• TCAs are metabolized primarily by CYP2D6 to active metabolites that may persist in plasma before undergoing glucuronidation and renal excretion, with additional elimination via the biliary system.
• Enterohepatic recirculation further prolongs clearance and contributes to variable elimination times.^3,4

Altered Kinetics in Overdose

• Gastrointestinal absorption is delayed by anticholinergic inhibition of gastric motility, while enterohepatic recirculation continues to prolong drug availability.
• When large quantities are ingested, hepatic metabolic pathways become saturated, increasing bioavailability and diminishing first-pass metabolism.⁵
• Acidemia increases the unbound fraction of TCAs by reducing protein binding, thereby enhancing tissue penetration and toxicity.³
• Genetic variations in CYP2D6 may further impair drug clearance when metabolic capacity is overwhelmed.⁶
• These combined effects result in delayed absorption, higher circulating concentrations of active drug, and prolonged elimination.

Mechanisms of Toxicity

Cardiovascular Toxicity

• TCAs block fast sodium channels in the His-Purkinje system and the ventricular myocardium, slowing depolarization, decreasing conduction velocity, prolonging repolarization, and lengthening refractory periods.
• This sodium channel blockade leads to QRS widening, ventricular arrhythmias, and potentially cardiovascular collapse.
• Peripheral alpha-1 blockade by TCAs causes vasodilation, and together with myocardial depression further contributes to hypotension.^2,7

Central Nervous System Toxicity

• TCAs inhibit GABA-A receptors, likely contributing to seizure activity.
• Metabolic acidosis increases the unbound fraction of TCAs and enhances their bioavailability, which may further promote seizures.³
• TCAs block H1 histamine receptors, contributing to sedation and central nervous system depression.⁴

Anticholinergic Effects

• TCAs antagonize central and peripheral muscarinic acetylcholine receptors, resulting in delirium, tachycardia, hyperthermia, mydriasis, urinary retention, and ileus.^1,3,4

Clinical Manifestations

Overview

• Patients with TCA poisoning often present with sedation, but confusion, delirium, hallucinations, cardiac conduction delays, arrhythmias, hypotension, and anticholinergic signs such as hyperthermia, flushing, and dilated pupils are also common manifestations.^1,3,4,6,7
• Due to variable absorption kinetics, patients may have unpredictable presentations and can initially appear well but deteriorate rapidly.
• Acute ingestions of 10 to 20 mg/kg frequently result in significant cardiovascular and central nervous system toxicity.¹

Cardiovascular Toxicity

• Cardiovascular toxicity is common and results from fast sodium channel blockade, producing QRS prolongation and ventricular arrhythmias.
• Sinus tachycardia is extremely common and due to both anticholinergic effects and reflex response to hypotension.
• Hypotension develops due to reduced myocardial contractility, impaired calcium influx, and peripheral vasodilation from alpha-1 adrenergic antagonism.
• The overall incidence of serious ventricular arrhythmias is relatively low, but hypotension is more frequent.
• Severe toxicity can progress to cardiovascular collapse and shock.^2,7

Central Nervous System Toxicity

• CNS toxicity includes abrupt-onset seizures due to reduced GABAergic inhibition.⁸
• Seizures can lead to cardiovascular deterioration, including worsening hypotension and ventricular arrhythmias.
• Sedation may progress from drowsiness to coma as toxicity worsens.
• Early agitation or delirium can occur before CNS depression predominates.
• Respiratory depression is common in severe TCA poisoning.⁴

Anticholinergic Toxicity

• Anticholinergic effects are common and can support the diagnosis of TCA poisoning.
• Clinical features include hyperthermia, flushed skin, dilated pupils with poor light response, delirium, intestinal ileus, and urinary retention.
• Impaired sweating and disrupted thermoregulation can contribute to fever, especially during seizures.
• Severe cases may rarely progress to toxic megacolon or intestinal perforation.^3,4

Diagnostic Evaluation

General Diagnostic Approach

• Diagnostic evaluation focuses on confirming TCA ingestion, assessing severity, and identifying potential co-ingestants.
• Initial testing should include an ECG, fingerstick glucose, acetaminophen and salicylate levels, and a pregnancy test in women of childbearing age.⁴

ECG

• An ECG should be obtained immediately in all suspected TCA overdoses, and continuous cardiac monitoring with repeat ECGs is recommended.
• QRS widening is the characteristic ECG manifestation of TCA toxicity.
  • QRS <100 msec is generally not associated with seizures or ventricular arrhythmias.
  • QRS >100 msec increases seizure risk, and QRS >160 msec significantly increases the risk of ventricular arrhythmias.⁹
  • Although its reproducibility has been questioned, multiple studies support QRS duration as a predictor of seizures and dysrhythmias.⁴
• Additional ECG findings concerning for toxicity include deep or slurred S waves in leads I and aVL, an R wave >3 mm or an R/S ratio >0.7 in lead aVR, PR or QT prolongation, His-Purkinje system block, right bundle branch block, and Brugada-type patterns.
• ECG findings assist in diagnosis and risk stratification, but none are fully reliable and significant toxicity can occur despite reassuring tracings.¹⁰

Laboratory Values

• Serum and urine TCA levels correlate poorly with clinical severity and are rarely available in a useful timeframe, so they should not be used to guide treatment decisions.⁹
  • Significant toxicity may occur at concentrations not traditionally considered dangerous, and qualitative assays can yield false positives due to cross-reactivity with several medications.⁴
• Metabolic acidosis can occur in severe TCA overdose due to respiratory depression and myocardial impairment, which cause poor tissue perfusion and lactate accumulation.
  • This is clinically important because it increases the unbound fraction of TCAs, worsening cardiotoxicity.³

Management

Initial Resuscitation

• Initial management includes sodium bicarbonate for cardiotoxicity, isotonic saline for hypotension, and benzodiazepines for agitation or seizures.
• Continuous reassessment with pulse oximetry and cardiac monitoring is essential because patients can deteriorate rapidly.⁴

Reduced Absorption

• Gastric lavage may be considered within one to two hours of a life-threatening ingestion.
• Activated charcoal can reduce absorption but should only be given when the airway is protected.

Alkalinization

• Sodium bicarbonate is the primary treatment for TCA-induced cardiotoxicity because it narrows the QRS complex, treats ventricular arrhythmias, and improves blood pressure.
• This therapy remains effective even without acidosis, and hyperventilation should not be combined with bicarbonate due to risk of severe alkalosis.

Antiarrhythmic Therapy

• Most antiarrhythmics, including Class Ia, Ic, and III agents, should be avoided because they exacerbate sodium channel blockade or prolong the QT interval.
• Lidocaine, phenytoin, beta-blockers, glucagon, and magnesium have inconsistent or limited benefit, and physostigmine is contraindicated due to seizure and cardiac arrest risk.

Hypotension

• Initial treatment consists of isotonic fluid boluses and sodium bicarbonate when QRS widening is present.
• Persistent hypotension should be managed with direct-acting vasopressors such as norepinephrine or phenylephrine, with hypertonic saline reserved for refractory cases.

Seizure Management

• Benzodiazepines are the preferred therapy for seizures caused by TCA overdose due to GABA-A inhibition.
• Barbiturates or propofol may be used for refractory seizures, and phenytoin should be avoided because it may worsen sodium channel blockade.^3,4

Refractory Toxicity

• Lidocaine or magnesium can be considered when arrhythmias persist despite appropriate bicarbonate therapy.
• Lipid emulsion therapy may be used for refractory hemodynamic instability or cardiac arrest, and ECMO can be lifesaving in profound cardiovascular collapse.⁴