title: Heart rate nonlinear dynamics during sudden hypoxia at 8230 m simulated altitude.
abstract: OBJECTIVE
  Acute hypobaric hypoxia is associated with autonomic changes that bring a global reduction of linear heart rate variability (HRV). Although changes in nonlinear HRV can be associated with physiologic stress and are relevant predictors of fatal arrhythmias in ischemic heart disease, to what extent these components vary in sudden hypobaric hypoxia is not known.
  
  
  METHODS
  Twelve military pilots were supplemented with increasing concentrations of oxygen during decompression to 8230 m in a hypobaric chamber. Linear and nonlinear HRV was evaluated at 8230 m altitude before, during, and after oxygen flow deprivation. Linear HRV was assessed through traditional time-domain and frequency-domain analysis. Nonlinear HRV was quantified through the short-term fractal correlation exponent alpha (alphas) and the Sample Entropy index (SampEn).
  
  
  RESULTS
  Hypoxia was related to a decrease in linear HRV indexes at all frequency levels. A non-significant decrease in alphas (basal, 1.39 +/- 0.07; hypoxia, 1.11 +/- 0.13; recovery, 1.41 +/- 0.05; P = .054) and a significant increase in SampEn (basal, 1.07 +/- 0.11; hypoxia, 1.45 +/- 0.12; recovery, 1.43 +/- 0.09; P = .018) were detected.
  
  
  CONCLUSIONS
  The observed pattern of diminished linear HRV and increased nonlinear HRV is similar to that seen in subjects undergoing heavy exercise or in patients with ischemic heart disease at high risk for ventricular fibrillation.
authors.affiliation: Departamento de Docencia e Investigación, Facultad de Ciencias Médicas, Pontificia Universidad Católica Argentina, Buenos Aires, Argentina.
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Vigo
authors.forename: Santiago
authors.initials: S
authors.lastname: Pérez Lloret
authors.forename: Alejandro J
authors.initials: AJ
authors.lastname: Videla
authors.forename: Daniel
authors.initials: D
authors.lastname: Pérez Chada
authors.forename: Horacio M
authors.initials: HM
authors.lastname: Hünicken
authors.forename: Jorge
authors.initials: J
authors.lastname: Mercuri
authors.forename: Ramón
authors.initials: R
authors.lastname: Romero
authors.forename: Leonardo C
authors.initials: LC
authors.lastname: Nicola Siri
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Cardinali
mesh_terms: D000328:Adult; D000531:Altitude; D001273:Atmosphere Exposure Chambers; D001274:Atmospheric Pressure; D001341:Autonomic Nervous System / Q000503:physiopathology; D001359:Aviation; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D008889:Military Personnel; D010101:Oxygen Consumption