title: Beat-to-beat QT dynamics in paroxysmal atrial fibrillation.
abstract: BACKGROUND
  QT dynamics parameters are used only in sinus rhythm. However, because many patients with paroxysmal atrial fibrillation undergo antiarrhythmic treatment that changes QT, developing methods for measuring QT dynamics during atrial fibrillation is important.
  
  
  OBJECTIVE
  The purpose of this study was to evaluate whether QT dynamics in atrial fibrillation can be measured more reliably if additional RR intervals are included in the QT calculation.
  
  
  METHODS
  QT and RR intervals were measured in 15 patients with atrial fibrillation and sinus rhythm on the same 24-hour Holter recording. Full QT adaptation is not instantaneous but lags behind over several beats. To correct for this lag, we adapted a weighted average method using five successive RR intervals. Linear regression was performed on (QT, RR) and (QT, RR(modified)) pairs. Variability ratio (standard deviation of all QT intervals/standard deviation of all RR intervals) and modified variability ratio (standard deviation of all QT intervals/modified standard deviation of all RR intervals) were calculated.
  
  
  RESULTS
  QT-RR slope was reduced in atrial fibrillation compared with sinus rhythm (0.076 +/- 0.009 vs 0.113 +/- 0.0013, P = .0005). When correcting for lag, using the QT-RR(modified) slope, the slope in atrial fibrillation became similar to the slope in sinus rhythm (0.126 +/- 0.013 vs 0.126 +/- 0.013, P = .9547). The variability ratio was reduced in atrial fibrillation compared with sinus rhythm (0.175 +/- 0.017 vs 0.240 +/- 0.031, P = .009), but when correcting for the lag, the modified variability ratio was similar in atrial fibrillation and sinus rhythm (0.262 +/- 0.029 vs 0.267 +/- 0.038, P = .80).
  
  
  CONCLUSIONS
  The results of this study demonstrate that QT dynamics can be measured reliably in atrial fibrillation using 24-hour Holter recordings.
authors.affiliation: Laboratory of Experimental Cardiology, Department of Medical Physiology, University of Copenhagen, Denmark.
authors.forename: Charlotte E
authors.initials: CE
authors.lastname: Larroude
authors.forename: Berit T
authors.initials: BT
authors.lastname: Jensen
authors.forename: Erik
authors.initials: E
authors.lastname: Agner
authors.forename: Egon
authors.initials: E
authors.lastname: Toft
authors.forename: Christian
authors.initials: C
authors.lastname: Torp-Pedersen
authors.forename: Kristian
authors.initials: K
authors.lastname: Wachtell
authors.forename: Jørgen K
authors.initials: JK
authors.lastname: Kanters
mesh_terms: D000200:Action Potentials; D000368:Aged; D001281:Atrial Fibrillation / Q000503:physiopathology*; D002940:Circadian Rhythm; D015716:Electrocardiography, Ambulatory; D005260:Female; D006329:Heart Conduction System / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008955:Models, Cardiovascular; D012815:Signal Processing, Computer-Assisted