title: Alterations in autonomic cardiac modulation in response to normobaric hypoxia.
abstract: OBJECTIVE
  The present study aimed to determine if autonomic cardiac modulation was influenced by acute exposure to normobaric hypoxia.
  
  
  METHODS
  Ten healthy male lowland dwellers completed five block-randomised single-blinded, crossed-over acute exposures to a normobaric hypoxic environment, each separated by 24 hours' recovery (20.3%, 17.4%, 14.5%, 12.0% and 9.8% FIO2). Supine recordings were made of arterial oxygen saturation and electrocardiogram (ECG). RR intervals from the ECG trace were analysed for time (SDNN, lnrMSSD), frequency (lnVLF, lnLF, lnHF, lnTP, LFnu, and HFnu), and nonlinear (DFA-α1 and SampEn) heart rate variability components.
  
  
  RESULTS
  A significant reduction in arterial SaO2 occurred with reduced FIO2, along with a rise in heart rate (Cohen's d = 1.16, 95% Confidence Interval [2.64-6.46]), significant at 9.8% FIO2. A decrease in autonomic cardiac modulation was also found as shown by a statistically significant (at 9.8% FIO2) decrease in lnTP (d = 1.84 [1.74-1.94]), and SampEn (d = 0.98 [0.83-1.12]) and an increase in DFA-α1 (d = 0.72 [0.60-0.84]) from normoxia at 9.8% FIO2.
  
  
  CONCLUSIONS
  The decrease in variability indicated a reduction in autonomic cardiac modulation. There appears to be a threshold ∼9.8% FIO2 (∼6000 m equiv.), below which significant alterations in autonomic control occur.
authors.affiliation: a Department of Life Sciences, College of Life and Natural Sciences , University of Derby , Buxton , UK.
authors.forename: David
authors.initials: D
authors.lastname: Giles
authors.affiliation: b School of Sport and Exercise Science , University of Chichester , Chichester , UK.
authors.forename: John
authors.initials: J
authors.lastname: Kelly
authors.affiliation: a Department of Life Sciences, College of Life and Natural Sciences , University of Derby , Buxton , UK.|c School of Sport and Physical Education , University of Canterbury , Christchurch , New Zealand.
authors.forename: Nick
authors.initials: N
authors.lastname: Draper
mesh_terms: D000328:Adult; D001341:Autonomic Nervous System / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology*; D018592:Cross-Over Studies; D006321:Heart / Q000502:physiology; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D055815:Young Adult
keywords: Normobaric hypoxia; autonomic nervous system; cardiac function; heart rate variability; sample entropy