title: Heart rate-corrected QT interval in men increases during winter months.
abstract: BACKGROUND
  Sudden cardiac death increases during winter months in both men and women. The heart rate-corrected QT (QTc) interval exhibits circadian variation. However, little is known about QTc interval variation with month of year.
  
  
  OBJECTIVE
  We sought to determine whether the QTc interval varies with month of year.
  
  
  METHODS
  We retrospectively analyzed a database of 24,370 electrocardiograms (ECGs) to determine seasonal variation in QTc intervals. The analysis data set included 7,976 baseline ECGs, one each for 3,700 men and 4,276 women. ECGs selected for analysis were normal, recorded in regions north of the equator, and taken on subjects >or=18 years old. The QT correction for heart rate (HR) was performed using QTc = QT*(HR/60)(0.4). The monthly mean QTc intervals were compared, for men and women separately, using a one-way analysis of variance with the Bonferroni correction for multiple comparisons.
  
  
  RESULTS
  Subject ages ranged from 18 to 95 years. The monthly mean QTc intervals were consistently greater for women than for men by 5.2 +/- 2.3 ms. After correction for multiple comparisons, the difference between the greatest and least monthly mean QTc interval was 6.1 +/- 1.5 ms (P <.01) for men and 3.5 ms (nonsignificant) for women. The maximum monthly mean QTc interval of 413 +/- 18 ms (n = 560; P <.05) occurred in October for men and of 417 +/- 16 ms (n = 350) in March for women, but it was not significant.
  
  
  CONCLUSIONS
  Significant seasonal variation in QTc interval exists among male subjects >or=18 years of age with normal baseline ECGs, with the QTc interval being longest in October. No significant variation was seen for women.
authors.affiliation: Krannert Institute of Cardiology, Department of Medicine, Indiana University, Indianapolis, Indiana 46202, USA.
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Beyerbach
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Kovacs
authors.forename: Alexei A
authors.initials: AA
authors.lastname: Dmitrienko
authors.forename: Dawn M
authors.initials: DM
authors.lastname: Rebhun
authors.forename: Douglas P
authors.initials: DP
authors.lastname: Zipes
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D002940:Circadian Rhythm / Q000502:physiology; D004562:Electrocardiography; D005260:Female; D006329:Heart Conduction System / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012107:Research Design; D012189:Retrospective Studies; D012621:Seasons; D012737:Sex Factors