CRIT BIT CORNER

Mechanism of Action of TCAs

TCAs block the reuptake of norepinephrine and serotonin at presynaptic terminals, increasing the availability of these neurotransmitters in the CNS. In overdose, however, their effects go far beyond neurotransmitter modulation.

Key mechanisms contributing to toxicity include:

• Sodium channel blockade → slowed cardiac depolarization → QRS prolongation, ventricular arrhythmias
• Inhibition of muscarinic receptors → anticholinergic effects
• Alpha-1 adrenergic receptor antagonism → hypotension
• Histamine H1 receptor antagonism → sedation
• GABA-A antagonism (less commonly) → seizures

Pharmacokinetics

When Do OD Symptoms Present?

TCAs are rapidly absorbed from the GI tract with peak plasma levels typically occurring within 2–6 hours after ingestion, but delayed toxicity can still occur.

• Onset of symptoms: usually within 1–2 hours
• Peak effects: around 6 hours
• Duration of toxicity: may last 24 hours or longer, especially in sustained-release formulations

TCAs are highly lipophilic and bind extensively to tissue proteins, contributing to:

• Large volume of distribution
• Poor dialyzability
• Long half-life (up to 30 hours or more)
• Delayed clearance in liver disease or the elderly [4,6]

Treatment and Management

Immediate goals:

• Stabilize ABCs
• Prevent seizures and arrhythmias
• Correct acidosis

Main interventions:

☑ Sodium Bicarbonate (NaHCO₃)

• First-line for cardiotoxicity
• Dose: 1–2 mEq/kg IV bolus; repeat or infuse continuously
• Mechanism: Alkalinizes plasma to reduce ionized TCA, increases sodium gradient across membranes → decreases binding to Na+ channels [1,4]
• Aim for serum pH of 7.45–7.55

☑ Benzodiazepines

• For seizures
• Avoid phenytoin; can worsen cardiac conduction

☑ Activated Charcoal

• Consider if within 1–2 hours of ingestion and airway protected
• More likely recommended by tox for extended- or controlled-release TCAs [1,4]

☑ Intubation & Sedation

• For patients with seizures or AMS

☑ Vasopressors

• Norepinephrine is preferred for hypotension

❌ Avoid

• Type IA/IC antiarrhythmics (Quinidine, Procainamide, Flecainide), physostigmine, flumazenil

Monitoring Essentials in TCA Overdose

• Continuous ECG (watch QRS width, QTc)
• Mental status checks
• Seizure activity
• Blood pressure trends
• Electrolytes: K, Mg, Ca

Prognosis

With prompt recognition and aggressive supportive care, outcomes are generally favorable. ICU admission is indicated for:

• QRS >100 ms
• Seizures
• Hypotension
• Altered mental status

Most patients recover within 24–48 hours. Mortality is low with early intervention [4].

Toxidrome Correlation

This case is a textbook presentation of TCA toxicity:

Anticholinergic features: "Dry as a bone" (dry mucosa), "Blind as a bat" (mydriasis), "Red as a beet" (flushed), "Mad as a hatter" (AMS/seizures), and "Stuffed as a pipe" (absent bowel sounds). These symptoms directly correlate with muscarinic blockade [4,5].

Cardiotoxicity: Widened QRS, prominent terminal R wave in aVR, hypotension, and tachycardia confirm sodium channel blockade – a hallmark of severe TCA overdose [4]. QRS >160 ms indicates high risk for ventricular dysrhythmias [4].

CNS involvement: Seizure activity and profound encephalopathy were observed early. This is consistent with TCA-induced GABA antagonism and cerebral hypoperfusion secondary to hypotension [4,6].

Monitoring Focus in ICU

ECG (continuous): QRS, QTc, terminal R in aVR

Electrolytes: Hourly initially, especially K, Mg, Ca

Neuro status: Sedation vacations daily post-stabilization

Hemodynamics: Titration of Levophed/Vasopressin for MAP goals

Ventilation: Propofol sedative goals and neuroprotection

Clinical Pearls

TCA overdoses may require massive bicarbonate doses: 20+ amps is not uncommon [4].

Polytherapy electrolyte repletion is essential. This case required >30 doses of KCl and >10 g of Ca gluconate.

The combination of lidocaine and bicarb is often synergistic for ventricular dysrhythmias [1,4].

Anticholinergic toxidrome alone may mask severity – always correlate with ECG and perfusion.