title: Slow QT interval adaptation to heart rate changes in normal ambulatory subjects.
abstract: BACKGROUND
  Clinical formulas for QT correction utilize instantaneous HR. We showed previously that longer-term HR affects QT duration. We extend these findings, identifying more accurate models of QT behavior.
  
  
  METHODS
  Multiple models of QT dependence on HR were tested in 2 independent populations. Holter recordings were analyzed in population A (healthy volunteers, n = 14, 6 males, age 26.9 ± 12.3 yr). The hypotheses generated in population A were tested in an independent group population B, healthy volunteers, n = 15, 9 males, age 52.9 ± 15.6 yr). Linear models of QT interval dependence on a weighted average of RR intervals in the preceding 3 minutes were compared to models based on the immediately preceding RR interval (instantaneous HR).
  
  
  RESULTS
  In population A, linear models based on RR intervals over the preceding minute performed better than the best nonlinear model based on the single RR interval immediately preceding the QT interval. Linear models including HR values preceding the QT interval by more than 60 s further improved model fit. This model hierarchy was confirmed in population B. Linear formula for QT correction based on exponential decay of HR effect with 60 s time constant outperformed Bazett and Fridericia formulas in both populations.
  
  
  CONCLUSIONS
  QT duration in normal ambulatory subjects is affected by noninstantaneous HR, including HR history dating back more than 60 s. Exponential decay of this "memory effect" with time constant of 1 minute provides an accurate description of QT adaptation. This may be of clinical importance when HR is not steady.
authors.affiliation: University of Pittsburgh, Pittsburgh, PA.
authors.forename: Eathar
authors.initials: E
authors.lastname: Razak
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
authors.forename: Theresa
authors.initials: T
authors.lastname: Donovan
authors.forename: Rachel
authors.initials: R
authors.lastname: Lampert
authors.forename: Jan
authors.initials: J
authors.lastname: Němec
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D015716:Electrocardiography, Ambulatory; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D012016:Reference Values