Prolonged QT interval

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.

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