- Subjects with QT interval corrected for heart rate by Fridericia's formula (QTcF) <450 milliseconds (msec) or QTcF<480 msec with branch block. Corrected QT interval duration (QTc) is QT interval corrected by Fridericia formula (QTcF), machine or manual over read. QTcF is based on single or averaged QTc value of triplicate electrocardiogram (ECG).