title: Circadian variation in QT dispersion determined from a 12-lead Holter recording: a methodological study of an age- and sex-stratified group of healthy subjects.
abstract: BACKGROUND
  QT dispersion is considered to reflect inhomogeneity of myocardial repolarization.
  
  
  METHODS
  The circadian variation of QT interval dispersion was examined in 95 healthy subjects using 24-hour Holter monitoring. Three different methods of lead selection were applied: all 12 leads (QTdisp 12), only precordial leads (QTdisp 6), and the pair of leads selected at 3 a.m. in which the longest and shortest QT intervals were found in each individual subject (QTdisp 2).
  
  
  RESULTS
  A preliminary methodological study including measurements from every minute in 10 subjects revealed no significant circadian variation using mean values of QTdisp 12, QTdisp 6, or QTdisp 2 obtained every hour, every 2, or every 4 hours, except in QTdisp 6, which demonstrated a significant circadian variation (P < 0.01) in 1-hour measurements. Analysis of all 95 subjects using measurements obtained every 4 hours revealed a significant circadian variation in QTdisp 12 and QTdisp 6 (P < 0.0001), whereas no circadian variation was seen in QTdisp 2. A subdivision into 10-year age groups revealed that subjects at age >50 years had a significant circadian variation in QTdisp 12 and QTdisp 6, but not in QTdisp 2. Only in males a significant circadian variation was seen in QTdisp 12 (P < 0.0001), whereas QTdisp 6 demonstrated a circadian variation both in females (P < 0.001) and in males (P < 0.0001).
  
  
  CONCLUSIONS
  Selection of leads is of crucial importance for repetitive measurements of QT dispersion. Circadian variation was detected in subjects over 50 years of age, when all 12 or only the 6 precordial leads were taken into account.
authors.affiliation: Department of Cardiology, Copenhagen University Hospital, Hvidovre, Denmark. hanssh@dadlnet.dk
authors.forename: Stig
authors.initials: S
authors.lastname: Hansen
authors.forename: Verner
authors.initials: V
authors.lastname: Rasmussen
authors.forename: Klaus
authors.initials: K
authors.lastname: Larsen
authors.forename: Christian
authors.initials: C
authors.lastname: Torp-Pedersen
authors.forename: Gorm Boje
authors.initials: GB
authors.lastname: Jensen
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology; D002940:Circadian Rhythm / Q000502:physiology*; D015716:Electrocardiography, Ambulatory / Q000295:instrumentation*; D005260:Female; D006329:Heart Conduction System / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D015203:Reproducibility of Results; D012737:Sex Factors