QT interval
Last edited 10/2020 and last reviewed 03/2023
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:
- Barr CS, Naas A, Freeman M, et al. QT dispersion and sudden unexpected death in chronic heart failure. Lancet 1994;343:327-329
- NHS Specialist Pharmacy Service (January 2020). What issues should be considered regarding drug-induced QT prolongation?