The rate dependency of the QT interval has led to the development of formulae to correct or normalize the QT interval and therefore allow determination of whether a QT interval recorded on an individual patient at any given rate was within the normal range. This is termed āthe corrected QT intervalā or QTc. Of the various formulae, that developed by Bazett in 1920: QTc = QT/ sq.root RR interval, is the most widely used. It is included with the other automated measurements, including the actual QT interval, by most automated ECG machines. Also frequently included is the value using the formula developed by Fredirecia also in 1920,. This employs the cube root of the RR interval.
Shown here are lead II ECGs from 3 patients with different heart rates (67, 110. and 136 beats/min). The measured and corrected (using the Bazett formula) QT intervals are shown below each ECG. Note the shortening in the actual QT interval (QT) associated with the more rapid rates, reflecting the rate dependency of the QT interval, but their similarity after correction using the Bazett formula (QTc)