TCA overdose - sodium channel blockade and ECG predictors (QRS widening)
Core concept
- TCAs block cardiac fast Na channels (open-state, use-dependent) -> slowed phase 0 -> QRS widening
- Also block K channels (QT), muscarinic (antimuscarinic toxidrome), alpha-1 (hypotension), H1 (sedation), GABA-A (seizures) and monoamine reuptake
- QRS duration is the single best predictor of major toxicity - not the dose, not the level, not the tachycardia
- Right-axis shift of the terminal 40 ms -> terminal R wave in aVR
- Acidosis and hyperkalaemia worsen Na channel blockade; alkalosis and Na loading reverse it
3 more sections, plus exam facts
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