An abnormal finding on ECG.

QT interval changes with heart rate, so usually calculated as corrected QT (QTc), where average QT is divided by square root of RR interval (ie 1 second, if heart rate 60).

Associated with dysrhythmia, especially torsades de pointes (polymorphic ventricular tachycardia).

Seen with:

• Genetic predisposition – Long QT syndrome
• Certain drugs – antipsychotics (eg chlorpromazine, quetiapine), antiarrhythmics (!?), tricyclic antidepressants (eg amitriptyline), other antidepressants (eg citalopram, venlafaxine), antihistamines (terfenadine, but also loratadine, diphenhydramine, astemizole), macrolides, quinine.