Quetiapine Toxicity Overview

• Brisk sinus tachycardia (HR = 120 bpm)
• Prolonged QTc interval (QTc = 560ms; the absolute QT interval is more than half the R-R interval)

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

Related Topics

• Beta-blocker toxicity
• Calcium-channel blocker toxicity
• Carbamazepine cardiotoxicity
• Tricyclic overdose (sodium-channel blocker toxicity)

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest

Advanced Reading

• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
• Wagner GS. Marriott’s Practical Electrocardiography 12e
• Chan TC. ECG in Emergency Medicine and Acute Care
• Rawshani A. Clinical ECG Interpretation
• Mattu A. ECG’s for the Emergency Physician
• Hampton JR. The ECG In Practice, 6e