title: Modeling the diving bradycardia: Toward an "oxygen-conserving breaking point"?
abstract: OBJECTIVE
  Although it has been demonstrated that the exponential decay model fits the heart rate (HR) kinetics in short static breath holding (BH), this model might be inaccurate when BH is maintained for several minutes. The aim of this study was to build a new meaningful model to quantify HR kinetics during prolonged static BH.
  
  
  METHODS
  Nonlinear regression analysis was used to build a model able to quantify the beat-to-beat HR reduction kinetics observed in prolonged static BH performed both in air and in immersed condition by 11 trained breath-hold divers. Dynamic changes in cardiac autonomic regulation through heart rate variability indices [root mean square of successive difference of R-R intervals (RMSSD); short-term fractal scaling exponent: (DFAα1)] and peripheral oxygen saturation (SpO2) were also analyzed to strengthen the model.
  
  
  RESULTS
  The tri-phasic model showed a sharp exponential drop in HR immediately followed by a slight linear rise up until a breaking point preceding a linear drop in HR. The breaking points had similar level of SpO2 whether in air or in immersed condition (95.1 ± 2.1 vs. 95.2 ± 3.0 %, respectively; P = 0.49), and the subsequent linear drop in HR was concomitant with a shift in cardiac autonomic regulation in air (RMSSD: +109.0 ± 47.8 %; P < 0.001; DFAα1: -18.0 ± 17.4 %; P < 0.05) and in immersion (RMSSD: +112.6 ± 55.8 %; P < 0.001; DFAα1: -26.0 ± 12 %; P < 0.001).
  
  
  CONCLUSIONS
  In addition to accurately fitting the HR kinetics, the most striking finding is an "oxygen-conserving breaking point" highlighted by the model, which might be interpreted as unique adaptive feature against hypoxic damages in the human diving bradycardia.
authors.affiliation: CETAPS Laboratory, EA no 3832, Faculté des Sciences du Sport, Boulevard Siegfried, Normandie University, Mont Saint-Aignan, France, guillaume.costalat1@univ-rouen.fr.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Costalat
authors.forename: Aurélien
authors.initials: A
authors.lastname: Pichon
authors.forename: Fabrice
authors.initials: F
authors.lastname: Joulia
authors.forename: Frédéric
authors.initials: F
authors.lastname: Lemaître
mesh_terms: D000328:Adult; D001919:Bradycardia / Q000503:physiopathology*; D062485:Breath Holding; D004242:Diving / Q000502:physiology*; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010100:Oxygen / Q000502:physiology*; D010101:Oxygen Consumption / Q000502:physiology*; D011659:Pulmonary Gas Exchange / Q000502:physiology; D055815:Young Adult
chemical_list: D010100:Oxygen