The combination of tachycardia, QRS and QTc prolongation, right axis deviation and terminal R wave in aVR > 3mm is highly specific for poisoning with sodium-channel blocking drugs, in particular the tricyclic antidepressants.
This patient had attempted suicide by deliberate self-poisoning with around 35mg/kg of Doxepin (a tricyclic antidepressant) an hour prior to presentation.In overdose, the tricyclics produce rapid onset (within 1-2 hours) of:
- Sedation and coma
- Seizures
- Hypotension
- Tachycardia
- Broad complex dysrhythmias
- Anticholinergic syndrome
Tricyclics mediate their cardiotoxic effects via blockade of myocardial fast sodium channels (QRS prolongation, tall R wave in aVR), inhibition of potassium channels (QTc prolongation) and direct myocardial depression. Other toxic effects are produced by blockade at muscarinic (M1), histamine (H1) and α1-adenergic receptors.
The degree of QRS broadening on the ECG is correlated with adverse events:
- QRS > 100 ms is predictive of seizures
- QRS > 160 ms is predictive of ventricular arrhythmias (e.g. VT)
The risk assessment for Doxepin ingestion is as follows:
- < 5 mg/kg — Minimal symptoms
- 5-10 mg / kg — Drowsiness and mild anticholinergic effects; major toxicity not expected
- > 10 mg / kg — Potential for all major toxic effects to occur within 1-2 h of ingestion
- > 30 mg / kg — Severe toxicity with pH-dependent cardiotoxicity and coma > 24 h
An overdose of this magnitude (> 30 mg/kg) is associated with profound TCA toxicity and likely to be rapidly fatal without intervention.