title: QT interval variability and adaptation to heart rate changes in patients with long QT syndrome.
abstract: BACKGROUND
  Increased QT variability (QTV) has been reported in conditions associated with ventricular arrhythmias. Data on QTV in patients with congenital long QT syndrome (LQTS) are limited.
  
  
  METHODS
  Ambulatory electrocardiogram recordings were analyzed in 23 genotyped LQTS patients and in 16 healthy subjects (C). Short-term QTV was compared between C and LQTS. The dependence of QT duration on heart rate was evaluated with three different linear models, based either on the RR interval preceding the QT interval (RR(0)), the RR interval preceding RR(0) (RR(-1)), or the average RR interval in the 60-second period before QT interval (mRR).
  
  
  RESULTS
  Short-term QTV was significantly higher in LQTS than in C subjects (14.94 +/- 9.33 vs 7.31 +/- 1.29 ms; P < 0.001). It was also higher in the non-LQT1 than in LQT1 patients (23.00 +/- 9.05 vs 8.74 +/- 1.56 ms; P < 0.001) and correlated positively with QTc in LQTS (r = 0.623, P < 0.002). In the C subjects, the linear model based on mRR predicted QT duration significantly better than models based on RR(0) and RR(-1). It also provided better fit than any nonlinear model based on RR(0). This was also true for LQT1 patients. For non-LQT1 patients, all models provided poor prediction of QT interval.
  
  
  CONCLUSIONS
  QTV is elevated in LQTS patients and is correlated with QTc in LQTS. Significant differences with respect to QTV exist among different genotypes. QT interval duration is strongly affected by noninstantaneous heart rate in both C and LQT1 subjects. These findings could improve formulas for QT interval correction and provide insight on cellular mechanisms of QT adaptation.
authors.affiliation: Cardiovascular Institute, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA. nemecj@upmc.edu
authors.forename: Jan
authors.initials: J
authors.lastname: Nemec
authors.forename: Marie
authors.initials: M
authors.lastname: Buncová
authors.forename: Vladimir
authors.initials: V
authors.lastname: Shusterman
authors.forename: Bruce
authors.initials: B
authors.lastname: Winter
authors.forename: Win-Kuang
authors.initials: WK
authors.lastname: Shen
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Ackerman
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D004562:Electrocardiography / Q000379:methods*; D005260:Female; D006339:Heart Rate; D006801:Humans; D008133:Long QT Syndrome / Q000175:diagnosis* / Q000503:physiopathology*; D008297:Male; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity