ECG Case 022
Post-intubation ECG of a young adult presenting with coma following a 6g quetiapine overdose. Describe the ECG.

Describe and interpret this ECG
ECG ANSWER and INTERPRETATION
Main Abnormalities
This ECG displays the characteristic electrocardiographic features of quetiapine toxicity:
- Sinus tachycardia due to anticholinergic effects
- Prolonged QT interval (QT interval > half the RR interval; QTc = 560ms)
A similar pattern would be seen with other atypical antipsychotic agents such as olanzapine or clozapine.
Significance of QT prolongation
- QT prolongation is a common source of concern in patients with antipsychotic toxicity, because of the theoretical risk of Torsades de Pointes
- A QTc interval > 500 ms is commonly cited as a marker of increased risk of TdP
- However, tachycardia (which is almost ubiquitous in significant poisoning with quetiapine, olanzapine or clozapine) is actually protective against TdP
- For this reason, TdP rarely occurs with quetiapine toxicity
CLINICAL PEARLS
QT interval nomogram
- Many Australian toxicologists use the QT interval nomogram to assess risk of TdP
- The absolute QT interval is measured manually in multiple leads and the median QT interval plotted on the nomogram (read how to do this here)
- Plots above the line indicate significant QT prolongation and consequent risk of TdP
By my measurements, our patient has an absolute QT of ~320 ms with HR 120 so plots below the line — i.e. not at significant risk of TdP.
References
Further Reading
- Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
- Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
- Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
- Rawshani A. Clinical ECG Interpretation ECG Waves
- Smith SW. Dr Smith’s ECG blog.
- Wiesbauer F. Little Black Book of ECG Secrets. Medmastery PDF
TOP 100 ECG Series
Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |
Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Co-creator of the LITFL ECG Library. Twitter: @rob_buttner
Hi. It would be very helpful if you to marked the point where it is easier to determine the qt interval of in this example
Thanks
What about the transition state (R:S) being V2-V3 is that physiological?