Toxicology — Tricyclic Antidepressant Overdose

• Norepinephrine and serotonin reuptake inhibition (therapeutic and toxic mechanism): TCAs block presynaptic reuptake pumps for norepinephrine and serotonin in a dose-dependent manner. At therapeutic doses this produces antidepressant effects; in overdose, accumulation of catecholamines at synapses produces sympathomimetic toxidrome including tachycardia, hypertension, and agitation. The sympathomimetic phase often precedes the depressant phase as direct membrane effects dominate in severe overdose.

• Anticholinergic effects via muscarinic receptor antagonism: TCAs possess potent antimuscarinic properties independent of their monoamine activity. This produces the characteristic "atropine-like" toxidrome: mydriasis, dry mouth, decreased sweating, urinary retention, tachycardia, and altered mental status. The anticholinergic effects often persist longer than sympathomimetic effects and represent a key diagnostic feature recognized in the emergency department.

• Myocardial membrane stabilization (quinidine-like effect): This is the most critical mechanism driving mortality. TCAs block fast sodium channels in myocardial cells (sodium channel blockade) in a concentration-dependent manner, slowing both depolarization and repolarization. This produces a characteristic widened QRS complex, prolonged PR interval, and prolonged QT interval on electrocardiography. The mechanism parallels Class IA antiarrhythmic agents (quinidine, procainamide), explaining why sodium bicarbonate (which restores the sodium gradient and accelerates channel recovery) is the specific antidote. At higher concentrations, this effect causes severe bradycardia, conduction block, and refractory dysrhythmias including ventricular tachycardia, ventricular fibrillation, and asystole.

• Cardiac depression and negative inotropy: Beyond sodium channel blockade, TCAs exert direct depressant effects on myocardial contractility through calcium channel antagonism and direct depression of contractile proteins. This contributes to hypotension, particularly when combined with vasodilation from alpha-adrenergic blockade. Hypotension develops late but represents a poor prognostic sign indicating severe toxicity.

• Seizure threshold lowering and direct CNS effects: TCAs lower the seizure threshold through multiple mechanisms including GABA antagonism, membrane stabilization effects in the CNS, and altered neurotransmitter balance. Seizures occur in 10-20% of moderate-to-severe overdoses and often precede or accompany dysrhythmias. The seizure risk correlates with overdose severity and serves as a marker of critical toxicity.

• Gamma-aminobutyric acid (GABA) antagonism: TCAs competitively antagonize GABA-A receptors, reducing inhibitory neurotransmission. This mechanism contributes to seizure susceptibility, agitation, and altered mental status beyond anticholinergic effects.

• Altered drug metabolism and prolonged toxicity: In overdose, saturation of hepatic metabolism and enterohepatic recirculation prolong the elimination half-life significantly (normal half-life 12-24 hours; overdose may extend to 48+ hours). Additionally, anticholinergic effects reduce gastric motility and increase absorption time, creating a prolonged toxicity window.

• Intentional overdose for suicide: Represents the predominant etiology in adult cases, accounting for approximately 70-80% of significant TCA overdoses. TCAs are specifically chosen by individuals with psychiatric illness or suicidal ideation because of their known high toxicity and lethality. This underscores the importance of suicide risk assessment in all patients presenting with TCA overdose.

• Accidental ingestion by children: Young children (<6 years) represent a significant at-risk population due to accidental ingestion of family members' medications. Even a single tablet can produce toxicity in toddlers due to reduced body weight and immature metabolism. Accidental overdoses generally involve smaller amounts but can still produce severe effects.

• Medication errors and prescription mismanagement: Miscommunication between patients and providers, confusion with similar medication names, or inadvertent dosing errors can lead to unintentional overdose in therapeutic settings, particularly in elderly patients with polypharmacy.

• Chronic therapeutic overdose: Accumulation of TCAs with renal dysfunction, severe hepatic disease, or drug interactions (particularly with CYP450 inhibitors) can produce chronic toxicity with slower symptom onset. This typically presents with CNS effects and dysrhythmias without the acute sympathomimetic phase seen in acute overdose.

• Age >40 years and cardiac risk factors: Older adults demonstrate reduced cardiac reserve and tolerance for arrhythmias. Pre-existing cardiac disease, electrolyte abnormalities (hypokalemia, hypomagnesemia), baseline QT prolongation, or medications that prolong QT interval substantially increase mortality risk.

• Specific TCA agents: Amitriptyline and imipramine carry the highest toxicity profiles, while nortriptyline is considered relatively safer due to lower cardiotoxicity at equivalent serum concentrations. Doxepin carries moderate toxicity. This distinction rarely affects emergency management but guides risk stratification.

• Concurrent substance use or polysubstance ingestion: Co-ingestion with other QT-prolonging drugs, sodium channel blockers, anticholinergics, or CNS depressants markedly increases both mortality and complication rates. Alcohol co-ingestion impairs gastric motility and increases absorption.

Early phase (0-6 hours) — Anticholinergic and sympathomimetic toxidrome

• Altered mental status and agitation: Represents one of the earliest CNS manifestations, progressing from mild confusion and anxiety to severe delirium, hallucinations, and combativeness. The mechanism involves CNS anticholinergic effects combined with sympathomimetic stimulation. Patients may be extremely agitated and difficult to manage in the emergency department.

• Tachycardia (often marked, >120 bpm): Among the earliest vital sign abnormalities, resulting primarily from sympathomimetic effects of catecholamine accumulation combined with anticholinergic-mediated vagal blockade. May be accompanied by palpitations.

• Hypertension: Early hypertension results from sympathomimetic effects; this is a characteristic early finding that distinguishes TCA toxicity from many other poisonings. May reach systolic pressures of 160-180 mmHg or higher.

• Hyperthermia: Occurs due to anticholinergic suppression of sweating combined with agitation and hypermetabolism. Can be life-threatening, particularly if seizures occur concurrently (drug-induced fever + seizure-related hyperthermia).

• Mydriasis (widely dilated pupils): Pathognomonic anticholinergic finding; pupils may be 6-8 mm and non-reactive to light. This finding present in the context of a toxic patient strongly suggests TCA or other anticholinergic poisoning.

• Dry mucous membranes, decreased skin turgor, urinary retention: Components of anticholinergic toxidrome; these findings reflect parasympathetic blockade and help establish the diagnosis.

Progressive phase (6-24 hours) — Cardiac and CNS complications

• QRS widening and electrocardiographic abnormalities: The most characteristic and prognostically important finding in TCA overdose. QRS widening begins early but becomes more pronounced with increasing severity. A QRS ≥100 ms indicates moderate toxicity; ≥160 ms indicates severe toxicity with substantial dysrhythmia risk. Concurrently, the PR interval prolongs (AV block risk) and the QT interval lengthens. These changes reflect sodium and potassium channel blockade in cardiac myocytes.

• Dysrhythmias: Range from sinus tachycardia with conduction delays to life-threatening ventricular arrhythmias. Ventricular tachycardia (VT) and ventricular fibrillation (VF) represent the most catastrophic complications. The "Brugada-like" pattern (prominent R wave in V1, ST elevation) may be observed. Dysrhythmias can occur suddenly even after apparent clinical improvement, and some patients develop refractory dysrhythmias unresponsive to standard ACLS protocols.

• Seizures: Occur in 10-20% of moderate-to-severe overdoses, typically within the first 6-24 hours. Usually brief, generalized tonic-clonic seizures, but can be recurrent or progress to status epilepticus if untreated. Seizure activity substantially increases mortality risk and necessitates immediate aggressive management.

• Hypotension and cardiovascular collapse: Develops later as direct myocardial depression and vasodilation dominate. Represents a sign of severe toxicity; hypotensive patients have dramatically higher mortality rates. Initially compensatory tachycardia may maintain blood pressure, but as severity increases, bradycardia and hypotension develop together — an ominous sign.

• Altered mental status progression to coma: Initially agitation and delirium progress to lethargy and ultimately coma in severe cases. Coma represents severe toxicity and increases aspiration risk.

Physical examination findings across presentation

• Anticholinergic stigmata constellation: "Hot as a hare, dry as a bone, red as a beet, mad as a hatter" — hyperthermia, dry mouth and skin, flushed appearance, and mental status changes represent the classic anticholinergic syndrome.

• Cardiovascular exam abnormalities: Tachycardia may be severe (>140 bpm) with regular rhythm; early hypertension (SBP 140-180 mmHg); later development of bradycardia and hypotension in severe cases. Murmurs are absent unless pre-existing cardiac disease. Poor peripheral perfusion indicates decompensation.

• Neurological findings: Beyond agitation and altered mental status, patients demonstrate hyperreflexia, muscle rigidity (from CNS excitation), tremor, and clonus. Pupils are characteristically dilated and non-responsive to light. Focal neurological findings should prompt consideration of other etiologies (CVA, metabolic derangement).

Important clinical variants

• Mild-to-moderate overdose: Presents with anticholinergic findings and sinus tachycardia without QRS widening (QRS <100 ms). Prognosis is generally excellent with supportive care; dysrhythmia risk minimal.

• Severe overdose: Presents with profound agitation/altered mental status, severe hypertension or hypotension, marked tachycardia, and QRS widening ≥160 ms. Seizures and dysrhythmias are likely without aggressive intervention. Mortality risk substantial.

• Delayed presentation: Some patients present hours after ingestion with minimal initial symptoms but progressive deterioration. This reflects the slowly accumulating cardiotoxic effects and emphasizes the need for cardiac monitoring for extended periods.

Clinical history and context

• Establish timing of ingestion (critical for prognosis and intervention planning; earlier presentation allows for gastric decontamination)
• Confirm medication history and identify specific TCA agent if possible (amitriptyline/imipramine carry highest mortality)
• Assess for co-ingestions (other QT-prolonging drugs, CNS depressants, alcohol)
• Determine intent (intentional vs accidental) and assess psychiatric status
• Inquire about previous TCA overdoses or suicide attempts

Physical examination pearls

• Measure core temperature (hyperthermia indicates severe toxicity and seizure/dysrhythmia risk)
• Assess mental status severity and track changes (worsening mental status correlates with increasing toxicity)
• Evaluate for anticholinergic stigmata (presence in appropriate clinical context strongly supports diagnosis)
• Perform detailed neurological exam looking for focal deficits (should prompt imaging for other etiologies)

Electrocardiography (most critical single diagnostic test)

• Normal ECG (QRS <100 ms): Argues against significant TCA toxicity; low risk of dysrhythmias
• Mild widening (QRS 100-120 ms): Indicates moderate toxicity; some dysrhythmia risk; requires cardiac monitoring
• Moderate widening (QRS 120-160 ms): Indicates severe toxicity; substantial dysrhythmia and seizure risk; requires ICU admission and aggressive management
• Severe widening (QRS >160 ms): Indicates life-threatening toxicity; high risk of spontaneous dysrhythmias and refractory VT/VF; demands intensive intervention including sodium bicarbonate

• PR prolongation: Indicates AV nodal conduction delay
• QT prolongation: Indicates repolarization abnormality
• Right axis deviation with prominent R wave in aVR and S wave in I and aVL: "Terminal R wave in aVR" represents a classic (though non-specific) finding
• ST-segment changes or T-wave inversions: May occur but are non-specific

Laboratory studies

• Serum TCA concentration: Most commonly measured for amitriptyline, imipramine, and nortriptyline. Therapeutic levels: amitriptyline/imipramine 100-250 ng/mL; nortriptyline 50-150 ng/mL. Toxic levels: >500 ng/mL associated with marked toxicity; >1000 ng/mL indicates severe toxicity with very high mortality without treatment. However, clinical presentation trumps serum level — a patient with severe symptoms and a QRS >160 ms requires aggressive treatment regardless of reported level, and levels may not be available acutely. Levels are most useful for confirmation and prognostic stratification after clinical decisions are made.

• Electrolytes with particular attention to potassium and magnesium: Hypokalemia and hypomagnesemia substantially increase dysrhythmia risk and should be aggressively corrected. Both reduce the threshold for dysrhythmias in the setting of sodium channel blockade. Check serum calcium as well (hypocalcemia worsens conduction abnormalities).

• Arterial or venous blood gas: Assess for metabolic acidosis (worsens QRS widening and dysrhythmia risk) and respiratory status. Acidosis pushes TCA molecules from ionized to non-ionized state, increasing myocardial penetration and toxicity.

• Glucose: Hypoglycemia can contribute to seizures and altered mental status; hyperglycemia common in stress response. Check glucose immediately, particularly in altered patients.

• Liver and renal function: Important for prognosis and duration of monitoring; renal dysfunction prolongs elimination and increases toxicity.

• Troponin and myoglobin (CK-MB): Check if concerned for rhabdomyolysis (particularly if prolonged seizures or hyperthermia) or myocardial infarction. Myocardial injury can be directly TCA-induced.

• Toxicology screen: Useful for identifying co-ingested substances (cocaine, amphetamines would intensify sympathomimetic effects; benzodiazepines or opioids would worsen CNS depression). GC-MS confirmation may be needed for legal documentation but does not change acute management.

Imaging studies

• Chest X-ray: Obtain in ICU-bound patients to assess for pulmonary edema (can develop from acute myocardial dysfunction), aspiration (particularly in altered patients), or other findings. Not necessary in mild

Immediate stabilisation

• Airway and ventilation: intubate for coma, refractory seizures, or impending cardiovascular collapse. Avoid hypoventilation-induced respiratory acidosis — acidemia increases the non-ionized fraction of drug and worsens myocardial sodium-channel blockade. Continuous cardiac monitoring, IV access, and immediate ECG in all suspected ingestions; consult a regional poison center (1-800-222-1222).
• Activated charcoal: reasonable for the patient presenting early after ingestion with a protected airway; anticholinergic ileus prolongs the absorption window. Do not administer to the obtunded, unintubated patient because of aspiration risk. Gastric lavage and multiple-dose charcoal are not routine.

First-line therapy

• Hypertonic sodium bicarbonate (1-2 mEq/kg IV bolus, repeated to effect, then a bicarbonate infusion) is the antidote, recommended by the American Heart Association 2023 focused update on management of poisoning-related cardiac arrest and by American College of Medical Toxicology guidance. Indications are QRS widening (conventionally ≥100 ms), ventricular dysrhythmia, or hypotension. Mechanism is dual: the sodium load overcomes channel blockade by raising the transmembrane sodium gradient, and alkalemia (target arterial pH roughly 7.45-7.55) reduces drug binding to the channel. Monitor sodium, potassium, and pH; hyperventilation is an adjunct when bicarbonate alone is insufficient.
• Benzodiazepines (e.g., lorazepam or diazepam) are first-line for seizures — seizure-induced acidosis rapidly worsens cardiotoxicity, so termination is urgent. Escalate to a barbiturate or propofol with intubation for refractory seizures.

Escalation and refractory toxicity

• Vasopressors: crystalloid first, then a direct-acting alpha agonist such as norepinephrine for persistent hypotension; indirect agents are less reliable because reuptake blockade depletes releasable catecholamine stores.
• Antiarrhythmic: a class IB agent (lidocaine) is the accepted second-line option for ventricular dysrhythmia unresponsive to bicarbonate.
• Rescue measures: intravenous lipid emulsion and extracorporeal life support (VA-ECMO/ECPR) are reasonable in refractory shock or arrest per AHA guidance on poisoning; prolonged CPR is justified because toxicity is reversible.

Contraindicated / avoid

• Class IA, IC, and III antiarrhythmics (procainamide, amiodarone) — additive sodium-channel and repolarization blockade.
• Phenytoin for seizures, physostigmine (bradyasystolic arrest), flumazenil (seizure precipitation), and beta blockers. Hemodialysis is ineffective given high protein binding and large volume of distribution.
• After medical stabilisation, psychiatric evaluation for suicide risk is mandatory.

Cardiovascular — the leading cause of death

• Refractory ventricular tachycardia / ventricular fibrillation (emergency): progressive sodium-channel blockade slows phase 0 depolarization, creating conduction heterogeneity and reentry. Signalled by escalating QRS width and a terminal R wave in aVR before the rhythm degenerates. Standard ACLS alone often fails — bicarbonate must accompany defibrillation.
• Bradyasystolic arrest and high-grade AV block (emergency): profound His-Purkinje conduction failure at high drug concentrations. A slowing heart rate in a previously tachycardic poisoned patient is ominous, not reassuring.
• Cardiogenic shock (emergency): negative inotropy plus alpha-blockade-mediated vasodilation; signalled by hypotension unresponsive to fluids, rising lactate, and cool extremities.

Neurologic and metabolic

• Status epilepticus (emergency): GABA-A antagonism; each seizure generates lactic acidosis that acutely worsens sodium-channel binding, so seizures and dysrhythmias potentiate one another.
• Hyperthermia and rhabdomyolysis: impaired sweating plus muscle activity; signalled by rising core temperature, elevated creatine kinase, and pigmented granular casts with acute kidney injury.
• Anoxic brain injury: follows arrest or prolonged hypotension.

Respiratory

• Aspiration pneumonitis / ARDS (emergency if hypoxemic): depressed airway reflexes in coma, compounded by charcoal administration to an unprotected airway; signalled by new infiltrates and hypoxemia.

Complications of therapy

• Sodium bicarbonate: hypernatremia, hyperosmolality, volume overload with pulmonary edema, and hypokalemia from intracellular potassium shift during alkalinization — hypokalemia in turn prolongs repolarization and predisposes to torsades de pointes. Overshoot alkalemia (pH above the target range) must be avoided.
• Physostigmine: cholinergic excess with bradycardia and asystole — the reason it is avoided in TCA poisoning.
• Phenytoin: adds sodium-channel blockade and can precipitate the very dysrhythmia it is given to prevent.
• Lipid emulsion: hypertriglyceridemia, pancreatitis, and laboratory interference from lipemic serum.
• ECMO/ECPR: bleeding, limb ischemia, and thrombosis.
• Anticholinergic sequelae: urinary retention requiring catheterization and adynamic ileus, which prolongs drug absorption.

• QRS duration is the vital sign: the classic teaching is that a QRS above roughly 100 ms predicts seizures and above roughly 160 ms predicts ventricular dysrhythmia. Serial ECGs, not serum drug levels, drive management.
• Terminal R wave in aVR with right axis deviation is the buzzword ECG finding — unopposed conduction delay in the right-sided/terminal ventricular vector from sodium-channel blockade.
• Single best next step for a wide QRS or hypotension: IV sodium bicarbonate bolus, per the American Heart Association 2023 focused update on poisoning-related resuscitation. It is given before, not after, sending a drug level. Do not choose "obtain serum TCA concentration" as the intervention.
• The association examiners test: acidosis worsens toxicity. Hypoventilation, seizure-related lactic acidosis, or arrest all increase the non-ionized drug fraction binding myocardial sodium channels — which is why alkalinization works and why seizures must be stopped immediately.
• Seizure drug: a benzodiazepine. Phenytoin is the classic distractor — it is itself a sodium-channel blocker and is avoided.
• Physostigmine is the other classic distractor: it may be considered in pure antimuscarinic poisoning (e.g., antihistamine, jimsonweed), but in TCA overdose it risks bradyasystolic arrest and is contraindicated.
• Pressor choice: norepinephrine, a direct alpha agonist, is preferred over indirect agents whose effect depends on releasable norepinephrine stores already depleted by reuptake blockade.
• Hemodialysis does not work — high protein binding and a very large volume of distribution. Choose supportive care, bicarbonate, and ECMO in refractory cases instead.
• Do not be reassured by an improving heart rate: transition from tachycardia to bradycardia with hypotension marks decompensation, not recovery.
• The asymptomatic patient with a normal ECG and normal mental status still requires a period of cardiac monitoring, since anticholinergic ileus delays absorption; only after that observation window with no findings is discharge to psychiatric evaluation appropriate.