title: Rate-independent QT shortening during exercise in healthy subjects: terminal repolarization does not shorten with exercise.
abstract: BACKGROUND
  QT interval for a given heart rate differs between exercise and recovery (QT hysteresis) due to slow QT adaptation to changes in heart rate. We hypothesized that QT hysteresis is evident within stages of exercise and investigated which component of the QT contributes to hysteresis.
  
  
  RESULTS
  Nineteen healthy volunteers performed a staged exercise test (four stages, 3 min each). Continuous telemetry was analyzed with software to compare QT intervals in a rate-independent fashion. QRST complexes during each minute were sorted by RR interval, and complexes in bins of 20 ms width were signal-averaged. QT and QTp (onset of QRS to peak T wave) were measured, and terminal QT calculated (peak to end of T wave, Tpe = QT - QTp). QT, QTp, and Tpe at the same heart rate were compared between the first and last minute of each stage. QT shortened from the first to last minute of exercise in each stage (Stage I: 358 +/- 30 to 346 +/- 25 ms, P < 0.001; Stage II: 342 +/- 27 to 331 +/- 24 ms, P = 0.003; Stage III: 329 +/- 21 to 322 +/- 18 ms, P = 0.03; Stage IV: 313 +/- 22 to 303 +/- 23 ms, P = 0.005). QTp also shortened in each stage, while Tpe was unchanged.
  
  
  CONCLUSIONS
  QT hysteresis occurs during exercise in normals, and the major determinant is shortening of the first component of the T wave. Terminal repolarization (peak to end of T wave), a surrogate for transmural dispersion of repolarization, does not shorten significantly with exercise.
authors.affiliation: Oates Institute for Experimental Therapeutics, Clinical Research Center, Department of Biomedical Informatics, Vanderbilt University Medical Center, Nashville, Tennessee 37232-9119, USA. prince.kannankeril@vanderbilt.edu
authors.forename: Prince J
authors.initials: PJ
authors.lastname: Kannankeril
authors.forename: Paul A
authors.initials: PA
authors.lastname: Harris
authors.forename: Kris J
authors.initials: KJ
authors.lastname: Norris
authors.forename: Irfan
authors.initials: I
authors.lastname: Warsy
authors.forename: Phillip D
authors.initials: PD
authors.lastname: Smith
authors.forename: Dan M
authors.initials: DM
authors.lastname: Roden
mesh_terms: D000293:Adolescent; D000328:Adult; D004562:Electrocardiography / Q000379:methods*; D005260:Female; D006329:Heart Conduction System / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D005082:Physical Exertion / Q000502:physiology*; D012016:Reference Values; D055815:Young Adult