Tricyclic Antidepressant Overdose
Tricyclic antidepressants are weak bases typically used for depression and as an adjunct for analgesia. They have a complex mechanism of action, competitively inhibiting noradrenaline and serotonin reuptake, and also blocking muscarinic receptors, histaminergic receptors, α-adrenoreceptors, GABA-a receptors, and fast sodium channels.
In overdose, toxicity is predominantly due to cardiac and central effects, though there are effects on most of the major organ systems.
Cardiac toxicity is due to antagonism of α-adrenoreceptors use-dependent blockade of fast sodium channels.
α-antagonism results in vasodilatation and subsequent hypotension. Hypotension may also be due to myocardial depression from sodium channel blockade.
Blockade of fast sodium channels occurs in the His-Purkinje system, as well as the atrial and ventricular myocardium. This results in decreased myocardial impulse conduction. They block channels in the inactivated state, resulting in a use-dependent blockade such that the effect is greater at faster heart rates. This results in an increased depolarisation and repolarisation time. ECG findings are consistent with this and are essentially pathognomonic:
- Widened QRS
- Right axis deviation of the terminal QRS
⩾3mm terminal R wave in aVR.
Additional ECG findings include:
- Any degree of heart block
- Ventricular arrhythmias
Central toxicity is predominantly due to anticholinergic effects, though antihistaminic effects contribute.
Anti-cholinergic effects tend to occur prior to cardiac effects, and include:
- Pupillary dilatation and blurred vision
Antihistaminic effects include obtundation.
Standard management of poisoning applies. TCAs are not dialysable and as they are weak bases are not amenable to urinary alkalinisation.
NaHCO3 and hyperventilation to a pH >7.5 is used to manage cardiac toxicity. There are a number of proposed mechanisms of action for the benefit of alkalinisation:
- Plasma alkalosis results in less ionised drug and increases distribution into tissues
- Plasma alkalosis increases protein binding of drug
- Intracellular alkalosis results in less bound intracellular drug, favouring its movement out of cells
Extracellular alkalosis results in reduced H+/K+ exchange, increasing intracellular potassium and hypopolarising the cell.
In addition to the alkalinising effects, sodium load from the NaHCO3 improves the sodium concentration gradient into cells
α-adrenoreceptor antagonism can be countered with use of an α-agonist such as noradrenaline.
Arrhythmias should be managed with drugs that do not prolong the action potential - so amiodarone and beta-blockers are contraindicated. Initial management should be using NaHCO3, though MgSO4 and lignocaine can be considered in refractory cases.
Seizures should be managed with benzodiazepines, phenytoin, propofol, and phenobarbital. Avoid agents which result in QRS prolongation.
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