title: QT/RR and T-peak-to-end/RR curvatures and slopes in chronic heart failure: relation to sudden cardiac death.
abstract: BACKGROUND
  Previous studies investigated the QT/RR relationship by linear regressions of QT and RR intervals. However, the pattern of the QT/RR relationship is not necessarily linear. This study investigated the QT/RR and T-peak-to-end (Tpe)/RR curvatures and corresponding slopes in chronic heart failure (CHF) patients, and studied their differences between sudden cardiac death (SCD) victims and others.
  
  
  METHODS
  Holter ECG recordings of 650 CHF patients were analyzed. RR, QT and Tpe series were obtained and for each patient, the data of each subject were fitted with a non-linear regression function of the form: QT=χ+ϕ(1-RR(γ)), where γ is the QT/RR curvature. The same regression formula was applied to the Tpe interval series. The slopes (dimensionless units) were calculated at the averaged RR intervals and at RR of 1 second.
  
  
  RESULTS
  The median (difference between 75th and 25th percentile) of the curvature parameter was 0.226 (2.39) for QT/RR and -0.002 (3.64) for Tpe/RR in the overall sample. For the QT/RR slope, these values were 0.170 (0.12) and 0.190 (0.10) when evaluated at RR=1 and at the averaged RR, respectively, while for the Tpe/RR slope the values were 0.016 (0.04) and 0.020 (0.04), respectively. The Tpe/RR slope showed high statistical significance for separation of SCD victims and others, particularly when evaluated at the averaged RR (median values of 0.040 vs 0.020, p=0.002), but also when evaluated at RR=1 second (0.026 vs 0.015, p=0.023). Patients with values of Tpe/RR slope above 0.042 had double incidence of SCD, for the case of the slope being evaluated at RR=1 second, and triple incidence for the case of the slope being evaluated at the averaged RR. The QT/RR slope and curvature, as well as the Tpe/RR curvature, were not different in SCD victims and in others.
  
  
  CONCLUSIONS
  Non-linear regression models based on curvature and slope characteristics, individually obtained for each patient, were used to characterize the QT/RR and Tpe/RR relationships. Steeper Tpe/RR slopes, obtained after adjusting for the curvature parameter, were associated with higher incidence of SCD. The curvature parameter itself did not show SCD predictive value.
authors.affiliation: Centro de Investigación Biomédica en Red (CIBER), Spain; Aragón Institute of Engineering Research, IIS Aragón, Universidad de Zaragoza, Zaragoza, Spain. Electronic address: Julia.Ramirez@unizar.es.
authors.forename: Julia
authors.initials: J
authors.lastname: Ramírez
authors.affiliation: Centro de Investigación Biomédica en Red (CIBER), Spain; Aragón Institute of Engineering Research, IIS Aragón, Universidad de Zaragoza, Zaragoza, Spain.
authors.forename: Pablo
authors.initials: P
authors.lastname: Laguna
authors.affiliation: Institut Catalad'Ciencies Cardiovasculars, Hospital Santa Creu I Sant Pau, Barcelona, Spain.
authors.forename: Antonio
authors.initials: A
authors.lastname: Bayés de Luna
authors.affiliation: St. Paul's Cardiac Electrophysiology, University of London, and Imperial College, London, UK.
authors.forename: Marek
authors.initials: M
authors.lastname: Malik
authors.affiliation: Centro de Investigación Biomédica en Red (CIBER), Spain; Aragón Institute of Engineering Research, IIS Aragón, Universidad de Zaragoza, Zaragoza, Spain.
authors.forename: Esther
authors.initials: E
authors.lastname: Pueyo
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015984:Causality; D002908:Chronic Disease; D015897:Comorbidity; D016757:Death, Sudden, Cardiac / Q000453:epidemiology*; D003936:Diagnosis, Computer-Assisted / Q000379:methods*; D004562:Electrocardiography / Q000379:methods* / Q000706:statistics & numerical data*; D005260:Female; D006333:Heart Failure / Q000175:diagnosis* / Q000401:mortality*; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D011379:Prognosis; D015203:Reproducibility of Results; D012307:Risk Factors; D012680:Sensitivity and Specificity; D013030:Spain; D015996:Survival Rate; D055815:Young Adult
keywords: ECG; QT/RR; Regression curvature; SCD; Tpe/RR