- 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).
- Hampton, JR. The ECG In Practice, 6e
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
- Wagner, GS. Marriott’s Practical Electrocardiography 12e
- Chan, TC. ECG in Emergency Medicine and Acute Care
- Mattu, A. ECG’s for the Emergency Physician
LITFL Further Reading
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis –alphabetical diagnostic approach to the ECG
- ECG CLINICAL CASES — ECG’s placed in clinical context with a challenging Q&A approach
- 100 ECG Quiz — Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS — the best of the rest
- LITFL ECG IMAGE Database — Searchable database of LITFL ECG’s
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for answering ECG exam questions.