Quetiapine Toxicity Overview

Clinical Features
  • Quetiapine (a second-generation atypical antipsychotic) is a leading cause of toxic coma in Australia.
  • Main toxic effects in overdose include coma, anticholinergic delirium, prolonged QTc and a brisk sinus tachycardia.
  • Doses > 3g are associated with coma.
  • Despite the prolonged QTc, Torsades de Pointes does not occur (see below).
  • A similar pattern of clinical and ECG features is seen with other atypical antipsychotics, such as olanzapine or clozapine.

Drug-Induced QT-Prolongation And Torsades
  • In the context of acute poisoning with QT-prolonging agents, the risk of TdP is better described by the absolute rather than corrected QT.
  • More precisely, the risk of TdP is determined by considering both the absolute QT interval and the simultaneous heart rate (i.e. on the same ECG tracing).
  • These values are then plotted on the QT nomogram (below) to determine whether the patient is at risk of TdP.
  • A QT interval-heart rate pair that plots above the line indicates that the patient is at risk of TdP.
  • From the nomogram, you can see that QTc-prolonging drugs that are associated with a relative tachycardia (e.g. quetiapine) are much less likely to cause TdP than those that are associated with a relative bradycardia (e.g. amisulpride).

ECG Example

This ECG (taken following a 6g quetiapine overdose) with characteristic features of quetiapine toxicity

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Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |