ECG Case 022

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

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.