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

Describe and interpret this ECG


Main Abnormalities

This ECG displays the characteristic electrocardiographic features of quetiapine toxicity:

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


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.

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 |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

One comment

  1. 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

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