The QT interval is affected by the patient’s heart rate. As the heart rate increases, the QT interval decreases; as the heart rate decreases, the QT interval increases. For this reason, evaluating the QT interval based on a stand ard heart rate of 60 is recommended. This corrected QT interval is known as QTc.

The following formula is used to determine the QTc

The normal QTc for women is less than 0.45 second and for men is less than 0.43 second. When the QTc is longer than 0.50 second in men or women, torsades de pointes is more likely to develop.