title: Activity-responsive pacing produces long-term heart rate variability.
abstract: BACKGROUND
  Long-term heart rate variability (HRV) measures, including the standard deviation of means of successive 5-minute epochs of R-R interval intervals (SDANN) and the power law slope (beta), are important prognostic measures, yet their physiologic basis is unknown. We tested the hypothesis that long-term HRV arises from physical activity in a randomized cross-over study in patients with rate-responsive pacemakers.
  
  
  RESULTS
  Ten patients with complete heart block and dual-chamber pacemakers underwent 24-hour periods of ambulatory ECG in each of three pacing modes: atrially tracked, fixed-rate, and rate-responsive pacing. SDANN, ultra low frequency (ULF; frequencies <0.0033 Hz), and beta slope were calculated; and high-frequency power and root mean square of consecutive normal R-R intervals (rMSSD) were calculated as measures of short-term HRV, which have autonomic origins. Long-term HRV measures were similar with atrially tracked and rate-responsive pacing and were much greater than in fixed-rate pacing (SDANN P = 0.0001; ULF P = 0.0001; beta slope P = 0.0002). Short-term HRV measures were similarly low in fixed-rate and rate-responsive pacing (P = NS) and were significantly lower than with atrially tracked pacing (P = 0.0034).
  
  
  CONCLUSIONS
  Rate-responsive pacing reproduces long-term, but not short-term, measures of HRV, suggesting that they may be markers of heart rate responses to patient activity.
authors.affiliation: Cardiovascular Research Group, University of Calgary, Calgary, Alberta, Canada.
authors.forename: Satish R
authors.initials: SR
authors.lastname: Raj
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Roach
authors.forename: Mary-Lou
authors.initials: ML
authors.lastname: Koshman
authors.forename: Robert S
authors.initials: RS
authors.lastname: Sheldon
mesh_terms: D000328:Adult; D000368:Aged; D002304:Cardiac Pacing, Artificial; D018592:Cross-Over Studies; D015716:Electrocardiography, Ambulatory; D005260:Female; D006327:Heart Block / Q000503:physiopathology / Q000628:therapy; D006329:Heart Conduction System / Q000473:pathology / Q000601:surgery; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D010138:Pacemaker, Artificial; D011237:Predictive Value of Tests; D013223:Statistics as Topic; D013995:Time; D016896:Treatment Outcome