Tricyclic Antidepressant Overdose for MRCEM: ECG Changes and Sodium Bicarbonate

Quick answer: tricyclic antidepressant (TCA) overdose kills through sodium channel blockade in the heart. The key investigation is the ECG: a QRS over 100 ms predicts seizures and a QRS over 160 ms predicts ventricular arrhythmias. The specific treatment is IV sodium bicarbonate (50 mmol, which is 50 mL of 8.4%, repeated as needed), aiming for a pH of 7.45–7.55. Treat seizures with benzodiazepines, and avoid phenytoin, flumazenil and class Ia and Ic antiarrhythmics.

TCAs such as amitriptyline are still widely prescribed, often for pain, and they are among the most dangerous drugs in overdose. In the MRCEM SBA the question usually gives you an ECG finding or a deteriorating patient and asks for the next step. In practice, always check TOXBASE and your local protocol.

Why TCAs are so toxic

TCAs act on several receptors and channels at once, which explains the clinical picture:

Clinical features

• Early: anticholinergic features, with sinus tachycardia, dilated pupils, dry mouth, urinary retention and agitation.
• Severe: drowsiness progressing to coma, seizures, hypotension and broad-complex arrhythmias.

Patients can deteriorate quickly, typically within the first few hours. Acidosis makes toxicity worse, because it increases the amount of free drug and its effect on sodium channels. This is why seizures and hypoventilation are so dangerous, and why alkalinisation works.

The ECG

Every patient needs a 12-lead ECG on arrival and continuous cardiac monitoring. Look for:

• Sinus tachycardia, the most common finding.
• QRS over 100 ms: increased risk of seizures.
• QRS over 160 ms: increased risk of ventricular arrhythmias.
• Terminal R wave over 3 mm in aVR, or an R/S ratio over 0.7 in aVR.
• Right axis deviation of the terminal QRS and QT prolongation.

Initial management

• ABC approach. Have a low threshold for intubation if the conscious level is falling, and avoid hypoventilation, as a respiratory acidosis will worsen toxicity.
• Activated charcoal can be considered if the patient presents within 1 hour of a significant ingestion and the airway is safe or protected.
• Bloods: venous or arterial gas for pH, U&Es and glucose, plus a paracetamol level as in any deliberate overdose.
• Repeat the ECG regularly and after any change in the patient’s condition.

Sodium bicarbonate

Sodium bicarbonate is the specific treatment. It works in two ways: the sodium load helps to overcome sodium channel blockade, and the alkalosis reduces the binding of the drug to the channel.

Give it for:

• QRS widening over 100 ms,
• ventricular arrhythmias,
• hypotension that does not respond to fluids, or
• seizures or significant acidosis.

Dose: 50 mmol IV (50 mL of 8.4% sodium bicarbonate) in adults, repeated as needed, aiming for an arterial pH of 7.45–7.55. It can be given even if the patient is not acidotic.

Managing complications

Drugs to avoid

• Class Ia and Ic antiarrhythmics (such as procainamide and flecainide): they block sodium channels and make things worse.
• Amiodarone: avoid where possible, as it can worsen hypotension and conduction problems.
• Phenytoin: it is a sodium channel blocker.
• Flumazenil: in a mixed overdose with benzodiazepines it can precipitate seizures.

Observation and discharge

• Patients who remain asymptomatic with a normal ECG for 6 hours after ingestion are unlikely to develop serious toxicity and can be medically cleared.
• Patients with a wide QRS, arrhythmias, seizures, hypotension or reduced consciousness need critical care.
• Every patient needs a mental health assessment once medically fit.

Common exam traps

• Giving phenytoin for seizures: use benzodiazepines.
• Giving amiodarone or flecainide for a broad-complex tachycardia: give sodium bicarbonate first.
• Withholding bicarbonate because the pH is normal: a wide QRS is the indication, not the pH.
• Using flumazenil in a drowsy mixed overdose: it can trigger seizures.
• Stopping CPR early: prolonged resuscitation is justified in TCA overdose.
• Mixing up the thresholds: over 100 ms for seizures, over 160 ms for arrhythmias.

Summary

• TCA overdose causes anticholinergic features, seizures, hypotension and arrhythmias.
• ECG: QRS over 100 ms predicts seizures, over 160 ms predicts ventricular arrhythmias. Look at aVR.
• Sodium bicarbonate 50 mmol IV, repeated, to a pH of 7.45–7.55.
• Benzodiazepines for seizures. Avoid phenytoin, flumazenil and class Ia and Ic antiarrhythmics.
• Observe for at least 6 hours with ECG monitoring.

Frequently asked questions

What ECG changes are seen in tricyclic antidepressant overdose?

Sinus tachycardia, a widened QRS, a prominent terminal R wave in aVR, right axis deviation and QT prolongation.

When is sodium bicarbonate given in TCA overdose?

For a QRS over 100 ms, ventricular arrhythmias, hypotension not responding to fluids, seizures or significant acidosis. The target pH is 7.45–7.55.

Which drugs should be avoided in TCA overdose?

Phenytoin, flumazenil and class Ia and Ic antiarrhythmics. Amiodarone is also best avoided.

How long should a patient be observed after a TCA overdose?

At least 6 hours after ingestion. Patients who remain asymptomatic with a normal ECG can then be medically cleared.

Next steps

Try our 20 free MRCEM SBA sample questions to test yourself. For more, explore our Toxicology questions and the Pharmacology and Poisoning course, or see the full question bank and mock tests in our membership plans.

More MRCEM clinical topics: Paracetamol overdose, Refractory VF, Diabetic ketoacidosis, Sepsis in the ED.

This article is for exam preparation. In clinical practice, always use TOXBASE and your local protocol.