TCA Overdose (Tricyclics)

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TCA Overdose

• Tricyclic antidepressants (TCAs) are a class of drugs used to treat depression, chronic pain, and other neuropsychiatric disorders. 
• TCA overdose can lead to severe toxicity and even death. 
• In this lecture, we will discuss the clinical and ECG features of TCA overdose, as well as its management.

Clinical Features of TCA Overdose

• Symptoms and signs of TCA overdose usually occur within 1-2 hours, which include:

1. Sedation and coma
2. Seizures
3. Hypotension
4. Tachycardia
5. Wide complex dysrhythmias
6. Anticholinergic syndrome (tachycardia, mydriasis, dry mouth)

• TCAs are rapidly absorbed in the gastrointestinal tract.
• Toxic effects are produced by blockade at muscarinic (M1), histamine (H1), and α1-adrenergic receptors.

ECG Features of TCA Overdose

• An ECG is a vital tool in the prompt diagnosis of poisoning with sodium-channel blocking medications such as TCA, which cause CNS and cardiovascular toxicity in overdose in the form of seizures and ventricular dysrhythmias.
  
• The most common ECG features of TCA overdose include:

1. Intraventricular conduction delay: QRS > 100 ms in lead II
2. Terminal R wave > 3 mm in aVR or R/S ratio > 0.7 in aVR
3. Sinus tachycardia secondary to muscarinic (M1) receptor blockade
4. Terminal right axis deviation of the QRS complex, which manifests as a dominant R’ wave in aVR due to the right-sided intraventricular conducting system being more susceptible to the effects of sodium channel blockade.

• The degree of QRS widening on ECG is correlated with adverse events:

1. QRS > 100 ms is predictive of seizures.
2. QRS > 160 ms is predictive of ventricular arrhythmias (e.g. VT). 

Management of Tricyclic Overdose
Overdose >10mg/kg with signs of cardiotoxicity (ECG changes) requires resuscitative management:

• Patients need to be managed in a monitored area equipped for airway management.
• Secure IV access, administer oxygen, and attach monitoring equipment.
• Administer IV sodium bicarbonate.
• Intubate as soon as possible and hyperventilate to maintain a pH of 7.50 to 7.55.
• Once the airway is secure, place a nasogastric tube and give 50g (1g/kg) of activated charcoal
• Treat seizures with IV benzodiazepines (e.g. midazolam 1-2mg)
• Treat hypotension with a crystalloid bolus (10-20 mL/kg). If this is unsuccessful in restoring BP then consider starting vasopressors (e.g. noradrenaline infusion)
• If arrhythmias occur, the first step is to give more sodium bicarbonate. Lidocaine (1.5mg/kg) IV is a third-line agent (after bicarbonate and hyperventilation) once pH is > 7.5
• Avoid Ia (procainamide) and Ic (flecainide) antiarrhythmics, beta-blockers, and amiodarone as they may worsen hypotension and conduction abnormalities
• Admit the patient to the intensive care unit for ongoing management

Take home message:

• TCA overdose can lead to severe toxicity and even death.
• The best markers for suspected overdose are a history of depression, suicidality, and overdose with a sudden deterioration in mental status and vital signs.
• Clinical features of TCA overdose include sedation, seizures, hypotension, tachycardia, wide complex dysrhythmias, and anticholinergic syndrome.
• An ECG is a vital tool in the prompt diagnosis of TCA overdose.

• Management of significant TCA overdose requires resuscitative management, including IV sodium bicarbonate, intubation and hyperventilation, and treatment of seizures and hypotension.