title: New formula for evaluation of the QT interval in patients with left bundle branch block.
abstract: BACKGROUND
  Left bundle branch block (LBBB) and QT prolongation both are associated with a worse prognosis. LBBB lengthens the QT interval. To date it is not known whether QT prolongation during LBBB differs in repolarization from QT prolongation during narrow QRS.
  
  
  OBJECTIVE
  The purpose of the present proof-of-concept-study was to develop a formula that allows comparison of the adjusted QT interval during LBBB with reference values and thereby allows interpretation of the QT interval irrespective of QRS widening.
  
  
  METHODS
  Sixty consecutive patients with sinus rhythm (SR) and narrow QRS underwent electrophysiologic study for ablation. In all patients, the intrinsic QRS ,QT, and JT times were measured during SR, and ventricular pacing from both the right ventricular apex (RVA) and the right ventricular outflow tract (RVOT) caused LBBB. We determined prolongation of the QT during as compared to SR (ΔQT). ΔQT was then divided by the QRS length during pacing QRS (QRSb). This describes the percentage of the QRS duration at LBBB, which must be subtracted from the measured QT (QTb) to determine the modified QT interval (QTm).
  
  
  RESULTS
  The ratio of ΔQT to paced QRS was calculated as 48.3% (RVA) and 48.8% (RVOT) (mean 48.5%). The ratio intrinsic of JTi to paced JT was 1.0055 (RVA) and 1.0087 (RVOT). There was no significant difference in intrinsic JT vs paced JT (P = .2).
  
  
  CONCLUSIONS
  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).
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany; Department of Cardiology, Witten/Herdecke University, Witten, Germany. Electronic address: Harilaos.bogossian@klinikum-luedenscheid.de.
authors.forename: Harilaos
authors.initials: H
authors.lastname: Bogossian
authors.affiliation: Division of Electrophysiology, Department of Cardiovascular Medicine, University of Münster, Münster, Germany.
authors.forename: Gerrit
authors.initials: G
authors.lastname: Frommeyer
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Ilias
authors.initials: I
authors.lastname: Ninios
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Fuad
authors.initials: F
authors.lastname: Hasan
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Quy Suu
authors.initials: QS
authors.lastname: Nguyen
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Zana
authors.initials: Z
authors.lastname: Karosiene
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Dejan
authors.initials: D
authors.lastname: Mijic
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Axel
authors.initials: A
authors.lastname: Kloppe
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Hawal
authors.initials: H
authors.lastname: Suleiman
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Bandorski
authors.affiliation: Department of Cardiology, Witten/Herdecke University, Witten, Germany; Department of Cardiology, Helios Klinikum Wuppertal, Wuppertal, Germany.
authors.forename: Melchior
authors.initials: M
authors.lastname: Seyfarth
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Lemke
authors.affiliation: Division of Electrophysiology, Department of Cardiovascular Medicine, University of Münster, Münster, Germany.
authors.forename: Lars
authors.initials: L
authors.lastname: Eckardt
authors.affiliation: Märkische Kliniken GmbH, Department of Cardiology and Angiology, Klinikum Lüdenscheid, Lüdenscheid, Germany; Department of Cardiology, Witten/Herdecke University, Witten, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Zarse
mesh_terms: D000328:Adult; D000368:Aged; D002037:Bundle-Branch Block / Q000175:diagnosis* / Q000453:epidemiology* / Q000628:therapy; D015331:Cohort Studies; D015897:Comorbidity; D004562:Electrocardiography / Q000379:methods / Q000639:trends*; D005069:Evaluation Studies as Topic; D005260:Female; D005500:Follow-Up Studies; D005544:Forecasting; D005858:Germany; D006801:Humans; D008133:Long QT Syndrome / Q000175:diagnosis* / Q000453:epidemiology* / Q000628:therapy; D008297:Male; D008875:Middle Aged; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D015996:Survival Rate; D013997:Time Factors
keywords: JT interval; Left bundle branch block; Long QT; QT formula; QT interval; QT prolongation