Tricyclic antidepressant overdose pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Raviteja Guddeti, M.B.B.S. [2]

Most of the toxic effects of TCAs are caused by four major pharmacological effects. TCAs have anticholinergic effects, cause excessive blockade of norepinephrine reuptake at the preganglionic synapse, direct alpha adrenergic blockade, and importantly they block sodium membrane channels with slowing of membrane depolarization, thus having quinidine like effects on the myocardium.^[1]

Tricyclics have a narrow therapeutic index, i.e., the therapeutic dose is close to the toxic dose.^[2] In the medical literature the lowest reported toxic dose is 6.7 mg per kg body weight. Although there are differences in toxicity with the drug class, ingestions of 10 to 20 mg per kilogram of body weight are a risk for moderate to severe poisoning, however, doses ranging from 1.5 to 5 mg/kg may even present a risk. Most poison control centers refer any case of TCA poisoning (especially in children) to a hospital for monitoring.^[3] Factors that increase the risk of toxicity include advancing age, cardiac status, and concomitant use of other drugs.^[4] However, serum drug levels are not useful for evaluating risk of arrhythmia or seizure in tricyclic overdose.^[5] TCA levels >1000 ng/ml were associated with QT and QRS prolongation and convulsions.

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