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Corrected QT interval (QTc interval)

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

The QT interval is measured from the start of the QRS complex to the end of the T wave - the end of the T wave was defined as the point of return to the isoelectric line. When a U wave is present, the QT interval is measured to the nadir of the curve between the T and U wave (1)

  • represents the duration of activation and recovery of the ventricular myocardium
  • in the normal adult it should be between 0.33 and 0.44 seconds
  • QT interval in excess of 0.44 sec is a marker of myocardial electrical instability - prolongation of QT interval is associated with possible development of ventricular arrhythmia, syncope and sudden death
  • a review stated (2):
    • QT interval varies with heart rate
    • females have a longer QT interval than males
    • definitions vary in the literature but as a guide, normal QTc intervals are <450 milliseconds (ms) for men and <460 ms for women
    • a QTc between these values and 500 ms is considered prolonged
      • a QTc >500 ms is considered clinically significant and is likely to confer an increased risk of arrhythmia

Notes:

  • the corrected QT interval (QTc) is calculated by dividing the QT interval by the square root of the preceeding R - R interval. Normal = 0.42 s (1)

Reference:

  1. Barr CS, Naas A, Freeman M, et al. QT dispersion and sudden unexpected death in chronic heart failure. Lancet 1994;343:327-329
  2. NHS Specialist Pharmacy Service (January 2020). What issues should be considered regarding drug-induced QT prolongation?

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