title: A New Formula for Estimating the True QT Interval in Left Bundle Branch Block.
abstract: BACKGROUND
  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.
  
  
  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 QRSLBBBon QTLBBB. 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 QRSLBBBconstituted 92% of the net increase in QTLBBB. A new formula, QT-LBBB = QTLBBB- (0.86 * QRSLBBB- 71), which takes the net increase in QRSLBBBinto 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Cardiology, First Affiliated Hospital of Dalian Medical University, Dalian, Liaoning, China.|Arrhythmia Center, Ningbo First Hospital, Ningbo, Zhejiang, China.
authors.forename: Binhao
authors.initials: B
authors.lastname: Wang
authors.affiliation: Department of Cardiology, First Affiliated Hospital of Dalian Medical University, Dalian, Liaoning, China.|Lankenau Medical Center and Lankenau Institute for Medical Research, Jefferson Medical College, Philadelphia, Pennsylvania, USA.
authors.forename: L I
authors.initials: LI
authors.lastname: Zhang
authors.affiliation: Department of Cardiology, First Affiliated Hospital of Dalian Medical University, Dalian, Liaoning, China.
authors.forename: Peixin
authors.initials: P
authors.lastname: Cong
authors.affiliation: Arrhythmia Center, Ningbo First Hospital, Ningbo, Zhejiang, China.
authors.forename: Huimin
authors.initials: H
authors.lastname: Chu
authors.affiliation: Department of Cardiology, First Affiliated Hospital of Dalian Medical University, Dalian, Liaoning, China.
authors.forename: Ying
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Department of Cardiology, First Affiliated Hospital of Dalian Medical University, Dalian, Liaoning, China.
authors.forename: Jinqiu
authors.initials: J
authors.lastname: Liu
authors.affiliation: Lankenau Medical Center and Lankenau Institute for Medical Research, Jefferson Medical College, Philadelphia, Pennsylvania, USA.
authors.forename: William
authors.initials: W
authors.lastname: Surkis
authors.affiliation: Department of Cardiology, First Affiliated Hospital of Dalian Medical University, Dalian, Liaoning, China.
authors.forename: Yunlong
authors.initials: Y
authors.lastname: Xia
mesh_terms: D000200:Action Potentials; D000328:Adult; D000368:Aged; D002036:Bundle of His / Q000503:physiopathology*; D002037:Bundle-Branch Block / Q000175:diagnosis* / Q000503:physiopathology; D002304:Cardiac Pacing, Artificial; D003661:Decision Support Techniques; D004562:Electrocardiography; D022062:Electrophysiologic Techniques, Cardiac; D005260:Female; D006339:Heart Rate; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012815:Signal Processing, Computer-Assisted; D013997:Time Factors
keywords: JT index; QT formula; QT interval; acquired long-QT syndrome; left bundle branch block