title: Beat-to-beat QT dynamics in healthy subjects.
abstract: BACKGROUND
  Measures of QT dynamics express repolarization abnormalities that carry prognostic information, but the reproducibility of beat-to-beat QT dynamics has never been established. The QT interval is prolonged at night, but how the circadian rhythm and heart rate influence the dynamic QT measurements is still unsettled. The aims of the present study were: (1) to describe the reproducibility of beat-to-beat QT dynamics with respect to intrasubject, between-subject, and between-observer variability and (2) to describe the normal range, circadian variation, and heart rate dependence of QT dynamics.
  
  
  METHODS
  Ambulatory Holter recordings were performed three times on 20 healthy volunteers and were analyzed by two experienced cardiologists. Slope and intercept of the QT/RR regression, the variability of QT and R-R intervals expressed as the standard deviation, and the relation between QT and RR variability expressed as a variability ratio were measured among other QT dynamics.
  
  
  RESULTS
  The reproducibility of all QT dynamics was good. All QT dynamics showed circadian variation when calculated on an hourly basis. The day/night variation in slope could be explained by the differences in heart rate, whereas the day/night variation in intercept was heart rate independent.
  
  
  CONCLUSIONS
  The present study shows that reliable automatic QT measurements could be performed, encouraging further evaluation of the clinical value of QT dynamics in risk stratification of cardiac patients.
authors.affiliation: Laboratory of Experimental Cardiology, Department of Medical Physiology, University of Copenhagen, Denmark.
authors.forename: Berit T
authors.initials: BT
authors.lastname: Jensen
authors.forename: Charlotte E
authors.initials: CE
authors.lastname: Larroude
authors.forename: Lars P
authors.initials: LP
authors.lastname: Rasmussen
authors.forename: Niels-Henrik
authors.initials: NH
authors.lastname: Holstein-Rathlou
authors.forename: Michael V
authors.initials: MV
authors.lastname: Hojgaard
authors.forename: Erik
authors.initials: E
authors.lastname: Agner
authors.forename: Jørgen K
authors.initials: JK
authors.lastname: Kanters
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D002940:Circadian Rhythm / Q000502:physiology; D015716:Electrocardiography, Ambulatory; D005260:Female; D006329:Heart Conduction System / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D015203:Reproducibility of Results; D018709:Statistics, Nonparametric