A New Formula for Estimating the True QT Interval in Left Bundle Branch Block.

Introduction: QT prolongation is an independent risk factor for cardiac mortality. Left bundle branch block (LBBB) is more common in patients as they age. Widening of the QRS in LBBB causes false QT prolongation and thus makes true QT assessment difficult. We aimed to develop a simple formula to achieve a good estimate of the QT interval in the presence of LBBB.

Methods And Results: To determine the effect of QRS duration on the QT interval, QRS and QT were measured in sinus rhythm and during right ventricular apical pacing in 62 patients (age 55 ± 11 years, 60% male) undergoing electrophysiology studies. A QT formula for LBBB (QT-LBBB) was derived based on the effect of increased QRS on QT . The predictive accuracy of the QT-LBBB formula was then tested in 22 patients (age 66 ± 13 years, 64% male) with intermittent LBBB with comparisons to prior QT formulae and JT index. On average, the net increase in QRS constituted 92% of the net increase in QT . A new formula, QT-LBBB = QT - (0.86 * QRS - 71), which takes the net increase in QRS into account, best predicted the QT interval with heart rate corrected QTc in the test set of LBBB ECGs when compared to the baseline value and prior formulae.

Conclusion: The QT-LBBB formula developed in this study best estimates the true QT interval in the presence of LBBB. It is simple and therefore can be easily utilized in clinical practice.