The ECG is a vital tool in the prompt diagnosis of poisoning with sodium-channel blocking medications such as:
- Tricyclic antidepressants (= most common)
- Type Ia antiarrhythmics (quinidine, procainamide)
- Type Ic antiarrhythmics (flecainide, encainide)
- Local anaesthetics (bupivacaine, ropivacaine)
- Antimalarials (chloroquine, hydroxychloroquine)
The two main adverse effects of sodium-channel blocker poisoning are seizures and ventricular dysrhythmias (due to blockade of sodium channels in the CNS and myocardium)
Handy tip: An ECG should be taken in all patients who present with a deliberate self-poisoning (or altered GCS of unknown aetiology) to screen for TCA overdose.
Electrocardiographic Features of Sodium-Channel Blockade
Features consistent with sodium-channel blockade are:
- Interventricular conduction delay – QRS > 100 ms in lead II
- Right axis deviation of the terminal QRS:
- Terminal R wave > 3 mm in aVR
- R/S ratio > 0.7 in aVR
Patients with tricyclic overdose will also usually demonstrate sinus tachycardia secondary to muscarinic (M1) receptor blockade.
Clinical Features of Tricyclic Overdose
In overdose, the tricyclics produce rapid onset (within 1-2 hours) of:
- Sedation and coma
- 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)
Management of Significant Tricyclic Overdose (> 10mg/kg) with Signs of Cardiotoxicity (ECG changes)
- Patients need to be managed in a monitored area equipped for airway management and resuscitation.
- Secure IV access, adminster high flow oxygen and attach monitoring equipment.
- Administer IV sodium bicarbonate 100 mEq (1-2 mEq / kg); repeat every few minutes until BP improves and QRS complexes begin to narrow.
- Intubate as soon as possible.
- Hyperventilate to maintain a pH of 7.50 – 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. diazepam 5-10mg).
- 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.
Example 1a – TCA overdose
Typical ECG of TCA toxicity demonstrating:
- Sinus tachycardia with first-degree AV block (P waves hidden in the T waves, best seen in V1-2).
- Broad QRS complexes.
- Positive R’ wave in aVR.
Example 1b – Worsening TCA toxicity
- A second ECG of the same patient showing worsening TCA cardiotoxicity with marked QRS broadening producing a sine wave appearance reminiscent of hyperkalaemia.
Example 1c – Resolution of TCA toxicity with treatment (bicarbonate and hyperventilation)
- Third ECG of the same patient after serum alkalinisation with sodium bicarbonate, intubation and hyperventilation.
- The QRS duration has narrowed back to normal and the R’ wave in aVR has disappeared.
Example 2 – Massive TCA overdose
- Another example of severe TCA cardiotoxicity after ingestion of 35 mg/kg doxepin.
- There is marked QRS widening with tachycardia and a positive R’ wave in aVR.
Example 3 – Flecainide overdose
- Similar ECG changes are seen with other sodium-channel blocking agents.
- This ECG demonstrates QRS widening and positive R’ wave in aVR consistent with sodium-channel blockade in a patient with flecainide poisoning.
Example 4 – Flecainide overdose
- Another example of flecainide cardiotoxicity.
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for the FACEM part 2 exam.
- ECG Reference Sites on the WEB — the best of the rest
- Cameron P, Jelinek G, Kelly AM, Murray L, Brown AFT. Textbook of Adult Emergency Medicine (3rd edition), Churchill Livingstone Elsevier 2009.
- Hutchinson MD, Traub SJ. Tricyclic Antidepressant Poisoning. Up To Date, 2008.http://www.uptodate.com
- Life in the Fast Lane. Toxicology Conundrum 022 — Tricyclic antidepressant toxicity.
- Murray L, Daly F, Little M, Cadogan M. Toxicology Handbook (second edition). Elsevier, 2011.