ABSTRACT
The QT interval varies slightly between leads, and so in general the lead with the longest QT interval is taken for the measurement. Unfortunately, deciding whether or not the QT interval is normal is not entirely straightforward, because the duration varies according to the patient’s heart rate: the faster the heart rate, the shorter the QT interval. Bazett’s formula has limitations, and tends to overcorrect or undercorrect the QT interval at extremes of heart rate. Other formulae are also used, and so-called linear formulae tend to be more uniform over a wide range of heart rates. Correction of the QT interval is necessary because the normal QT interval varies with heart rate: the faster the heart rate, the shorter the normal QT interval. An alternative way to assess a QT interval is to correct it to what it would be if the patient’s heart rate was 60 beats/min.
