ECG (toxicology)

#complete #carded

Related FACEM curriculum (2022) learning objectives:

• ME 3.8.1.4(f) Principles of assessment of toxicological presentations in the ED, including: Investigations (ECG: sodium channel blockade, QT prolongation)

• Non-invasive, inexpensive, and identifies lethal conduction abnormalities.
• Recommended as screening in deliberate self-poisoning.
• Provides real-time prognostic information.
• Serial ECGs monitor progression or regression of toxicity.

Electropathophysiology

• ECG changes reflect toxic effects on cardiac conduction.
• Fast sodium channel blockade:
  • Slows sodium influx during phase 0.
  • Widened QRS, right axis deviation, bradycardia, ventricular tachycardia, fibrillation.
• Potassium efflux blockade:
  • Prolonged QT interval.
  • Torsades de pointes.
• Na+-K+-ATPase pump blockade:
  • Increased automaticity, AV node conduction block (1st-3rd degree).
• Calcium channel blockade:
  • Bradycardia, AV node block, intraventricular conduction defects.
• Beta-blockade:
  • Bradycardia, AV node conduction block.
• Myocardial ischaemia: ST-segment changes, conduction abnormalities.
• Electrolyte abnormalities:
  • Hyperkalaemia: Peaked T waves, conduction defects.
  • Hypocalcaemia, hypokalaemia, hypomagnesaemia: QT prolongation.

Normal ECG Parameters

• Rate: 60–100 bpm.
• PR interval: <200 ms (<5 small squares).
• QRS duration: <100 ms (<2.5 small squares).
• QTc interval: <450 ms.
• QTc = QT/√R–R.

Agents Causing ECG Changes

• Fast sodium channel blockade:
  • Tricyclic antidepressants, Class 1A/1C antidysrhythmics, local anaesthetics, phenothiazines, carbamazepine, propranolol, chloroquine.
• Potassium efflux blockade:
  • Antipsychotics, Class 1A/1C/III antidysrhythmics, antidepressants, antihistamines, chloroquine, macrolides, methadone.

Tricyclic Antidepressants

• QRS >100 ms: Associated with seizures.
• QRS >160 ms: Associated with ventricular dysrhythmias.
• Right axis deviation of terminal QRS:
  • R wave >3 mm or R/S ratio >0.7 in aVR.

Systematic 12-lead ECG Analysis

1. Determine rate and rhythm.
2. Assess PR interval for heart block.
3. Measure QRS duration in lead II manually.
4. Evaluate for right axis deviation of terminal QRS.
5. Check QT interval (prolongation risks torsades de pointes).
6. Assess for cardiac ectopy or increased automaticity.
7. Look for hyperkalaemia.
8. Check for myocardial ischaemia.