QT prolongation


  • Prolonged ventricular repolarisation → increased risk of ventricular arrythmias
    • Males >440-450 ms
    • Females >500 ms
    • Rule of thumb: Normal QT interveal is less than half of preceding RR interval
  • QT interval is from the beginning of the Q wave to the end of the T wave
    • Rate dependent and should become proportionately shorter with increasing heart rate

Clinical Features

  • Most are asymptomatic
  • History may include:
    • Syncope
    • Cardiac arrest
    • Family history of long QT or sudden death
  • Medication history may include QT prolonging medications

Differential Diagnosis

Drug List



  • ECG
  • CBC
  • Chem 10


  • ECG
    • On visual inspection, QT takes up more than half the R-R distance
    • Measure QT interval in lead II or V5-6
    • QTc = QT /√R-R


Pause Dependent (precipitated by bradycardia)

Adrenergic Dependent (precipited by tachycardia)


  • Consider admission, especially for QT >500 or if symptomatic
  • May require consultation for discontinuation of QT prolonging medications
  • Avoid prescribing medications that may contribute to prolonged QT

See Also


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