QT interval

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?