Determination of the QT Interval in Left Bundle Branch Block: Development of a Novel Formula

Background: Determination of the prolonged QT interval in left bundle branch block (LBBB), which should be of special concern to identify individuals at high risk of potentially fatal cardiac arrhythmia risk, has been problematic. Methods: Electrocardiograms (ECGs) (n = 17) in intermittent LBBB were used to develop a new formula for the calculation of QTc in LBBB. This formula and 5 others were compared in a population with LBBB (n = 2610). The QT was corrected for heart rate (HR) using the Bazett formula (QTcBZT) and the spline QT formula (QTcRBK), which is relatively independent of HR. The JT interval was significantly related to HR. Results: The new approach (QTc(LBBB)NEW = 0.945*QTcRBK(LBBB) - 26) in LBBB showed the highest correlation with intrinsic QTc (without LBBB) and had minimal HR dependency. Previous formulae to determine the QT interval in LBBB showed significant HR dependency except for one proposed by Rautaharju et al. Inclusion of an HR correction factor in existing formulae blunted HR dependency but not if the QT interval was adjusted by the QTcBZT. In men and women, use of the QTcBZT markedly increases the proportion of individuals with prolonged QTc, which was more evident with increasing HR. The 99th and 97.5th percentiles for QTc(LBBB)NEW for men and women identified abnormal QT prolongation in LBBB. Conclusions: A new formula that modifies the QT and JT intervals in LBBB predicts the QT interval in the absence of LBBB. Abnormal QT intervals in the 99th and 97.5th percentiles can identify patients with LBBB who have QT interval prolongation.