New formula for evaluation of the QT interval in patients with left bundle branch block

Right ventricular pacing causes prolongation of the QT due to a paced LBBB without prolongation of the JT time. In our study, we showed that QT prolongation caused by LBBB constitutes 48.5% of the QRS width. This is the value that must be subtracted from the measured QT in LBBB in order to estimate the modified QT. Thus, the resulting formula for “modified QT” estimation in LBBB is QTm = QTb - 48.5% * (QRSb).