title: The effect of partial acclimatization to high altitude on loop gain and central sleep apnoea severity.
abstract: OBJECTIVE
  Loop gain is an engineering term that predicts the stability of a feedback control system, such as the control of breathing. Based on earlier studies at lower altitudes, it was hypothesized that acclimatization to high altitude would lead to a reduction in loop gain and thus central sleep apnoea (CSA) severity.
  
  
  METHODS
  This study used exposure to very high altitude to induce CSA in healthy subjects to investigate the effect of partial acclimatization on loop gain and CSA severity. Measurements were made on 12 subjects (age 30 ± 10 years, body mass index 22.8 ± 1.9, eight males, four females) at an altitude of 5050 m over a 2-week period upon initial arrival (days 2-4) and following partial acclimatization (days 12-14). Sleep was studied by full polysomnography, and resting arterial blood gases were measured. Loop gain was measured by the 'duty cycle' method (duration of hyperpnoea/cycle length).
  
  
  RESULTS
  Partial acclimatization to high-altitude exposure was associated with both an increase in loop gain (duty cycle fell from 0.60 ± 0.05 to 0.55 ± 0.06 (P = 0.03)) and severity of CSA (apnoea-hypopnoea index increased from 76.8 ± 48.8 to 115.9 ± 20.2 (P = 0.01)), while partial arterial carbon dioxide concentration fell from 29 ± 3 to 26 ± 2 (P = 0.01).
  
  
  CONCLUSIONS
  Contrary to the results at lower altitudes, at high-altitude loop gain and severity of CSA increased.
authors.affiliation: Department of Medicine, University of Sydney, Sydney, New South Wales, Australia.
authors.forename: Gareth
authors.initials: G
authors.lastname: Andrews
authors.forename: Philip N
authors.initials: PN
authors.lastname: Ainslie
authors.forename: Kelly
authors.initials: K
authors.lastname: Shepherd
authors.forename: Andrew
authors.initials: A
authors.lastname: Dawson
authors.forename: Marianne
authors.initials: M
authors.lastname: Swart
authors.forename: Samuel
authors.initials: S
authors.lastname: Lucas
authors.forename: Keith R
authors.initials: KR
authors.lastname: Burgess
mesh_terms: D000064:Acclimatization / Q000502:physiology*; D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D000531:Altitude; D002245:Carbon Dioxide / Q000097:blood; D025461:Feedback, Physiological / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D017286:Polysomnography; D012720:Severity of Illness Index; D020182:Sleep Apnea, Central / Q000503:physiopathology*; D013997:Time Factors
chemical_list: D002245:Carbon Dioxide