title: Differential index, a novel graphical method for measurements of heart rate variability.
abstract: BACKGROUND
  Commonly used methods to evaluate heart rate variability require extensive filtering of the registrations in order to exclude artefacts and ectopic beats. We developed and validated a novel graphical method for time-domain measurements of heart rate variability, the differential index, which does not require filtering and is simple to use.
  
  
  METHODS
  The 24-h ambulatory long-term electrocardiogram recordings from 120 patients with angina pectoris and 49 control subjects were computerised without any filtering process. Sample density histograms of differences in the RR interval for successive beats were constructed and the widths of the histograms were used to obtain the differential index. For comparison, the same registrations were analysed by conventional methods.
  
  
  RESULTS
  The differential index was most closely related (P<0.001) to conventional short-term time domain (e.g. percent of differences between adjacent normal RR intervals >50 ms, pNN50, r=0.81) and frequency-domain (e.g. high frequency power, r=0.84) components, but also to long-term time domain (e.g. standard deviation of all normal-to-normal RR intervals for all 5-min segments of the entire registration, SDNNIDX, r=0.72) and frequency-domain (e.g. low frequency power, r=0.64) components.
  
  
  CONCLUSIONS
  The differential index method shows good agreement with established indices of heart rate variability. The insensitivity to recording artefacts and short-lasting disturbances of sinus rhythm make the differential index method particularly suited when data quality is imperfect. The simplicity of the method is valuable when large numbers of registrations are to be evaluated.
authors.affiliation: Division of Internal Medicine, Karolinska Institute Danderyd Hospital, S-182 88 Stockholm, Sweden.
authors.forename: Inge
authors.initials: I
authors.lastname: Björkander
authors.forename: Thomas
authors.initials: T
authors.lastname: Kahan
authors.forename: Mats
authors.initials: M
authors.lastname: Ericson
authors.forename: Claes
authors.initials: C
authors.lastname: Held
authors.forename: Lennart
authors.initials: L
authors.lastname: Forslund
authors.forename: Nina
authors.initials: N
authors.lastname: Rehnqvist
authors.forename: Paul
authors.initials: P
authors.lastname: Hjemdahl
mesh_terms: D000787:Angina Pectoris / Q000503:physiopathology*; D001341:Autonomic Nervous System / Q000503:physiopathology; D015716:Electrocardiography, Ambulatory; D005260:Female; D006339:Heart Rate; D006801:Humans; D008297:Male; D008875:Middle Aged