title: Assessing autonomic response to repeated bouts of exercise below and above respiratory threshold: insight from dynamic analysis of RR variability.
abstract: OBJECTIVE
  The dynamics of vagal withdrawal and reactivation during pulses of exercise are described by indices computed from heart period (RR) variations, which may be sensitive to duration and load. We sought to assess the consistency over time of these indices, which is not well established.
  
  
  METHODS
  We recorded continuous electrocardiogram during series of five successive bouts (2 min) of submaximal exercise (at 40 and 70% of VO(2peak), different days). Autonomic responsiveness was inferred from quantification of onset and offset of RR dynamics of each individual bout. Consistency of results was assessed with intraclass correlation (ICC).
  
  
  RESULTS
  During exercise bouts, indices from tachycardic and bradycardic transients reach lower levels in response to higher exercise loads and progression of exercise. There is a significant effect of load and time (i.e., bout repetition) for all examined variables, with a clear interaction. However, no interaction is observed with the 60 s change in heart rate. ICC analysis demonstrates that various indices are characterized by large differences in stability, which is generally greater within the same day (e.g., tachyspeed ICC at 40% = 0.751, at 70% = 0.704, both days = 0.633; bradyspeed, respectively, = 0.545, 0.666, 0.516).
  
  
  CONCLUSIONS
  Intensity and duration of exercise modulate vagal withdrawal and reactivation. Analysis of RR variations, during successive brief exercise bouts at lower and higher intensity, ensures a consistency similar to that reported for autonomic cardiac regulation at rest and might guide the choice among multiple indices that are obtained from the tachogram.
authors.affiliation: Sezione Medicina dell'Esercizio e Sindromi Funzionali, Dipartimento di Riabilitazione e Recupero Funzionale, IRCCS Istituto Clinico Humanitas, Universita' degli Studi di Milano, Via Alessandro Manzoni, 56, 20089, Rozzano (Milan), Italy, daniela.lucini@unimi.it.
authors.forename: Daniela
authors.initials: D
authors.lastname: Lucini
authors.forename: Chiara
authors.initials: C
authors.lastname: Vigo
authors.forename: Fabio
authors.initials: F
authors.lastname: Tosi
authors.forename: Gianfranco
authors.initials: G
authors.lastname: Toninelli
authors.forename: Fabio
authors.initials: F
authors.lastname: Badilini
authors.forename: Massimo
authors.initials: M
authors.lastname: Pagani
mesh_terms: D000704:Analysis of Variance; D015444:Exercise; D005260:Female; D006321:Heart / Q000294:innervation / Q000502:physiology*; D006339:Heart Rate; D006801:Humans; D008297:Male; D010101:Oxygen Consumption; D012119:Respiration; D014630:Vagus Nerve / Q000502:physiology*; D055815:Young Adult