title: Respiratory muscle strength may explain hypoxia-induced decrease in vital capacity.
abstract: OBJECTIVE
  High altitude exposure has consistently been reported to decrease forced vital capacity (FVC), but the mechanisms accounting for this observation remain incompletely understood. We investigated the possible contribution of a hypoxia-related decrease in respiratory muscle strength.
  
  
  METHODS
  Maximal inspiratory and expiratory pressures (MIP and MEP), sniff nasal inspiratory pressure (SNIP), FVC, peak expiratory flow rate (PEF), and forced expiratory volume in 1 s (FEV1) were measured in 15 healthy subjects before and after 1, 6, and 12 h of exposure to an equivalent altitude of 4267 m in a hypobaric chamber.
  
  
  RESULTS
  Hypoxia was associated with a progressive decrease in FVC (5.59 +/- 0.24 to 5.24 +/- 0.26 L, mean +/- SEM, P < 0.001), MIP (130 +/- 10 to 114 +/- 8 cm H2O, P < 0.01), MEP (201 +/- 12 to 171 +/- 11 cm H2O, P < 0.001), and SNIP (125 +/- 7 to 98 +/- 7 cm H2O, P < 0.001). MIP, MEP, and SNIP were strongly correlated to FVC (r ranging from 0.77 to 0.92). FEV1 didn't change, and PEF increased less than predicted by the reduction in air density (11-20% of sea-level value compared with 32% predicted).
  
  
  CONCLUSIONS
  We conclude that a decrease in respiratory muscle strength may contribute to the decrease in FVC observed at high altitude.
authors.affiliation: Department of Physiology, Faculty of Medicine, Free University of Brussels, Belgium.
authors.forename: Gaël
authors.initials: G
authors.lastname: Deboeck
authors.forename: Jean Jacques
authors.initials: JJ
authors.lastname: Moraine
authors.forename: Robert
authors.initials: R
authors.lastname: Naeije
mesh_terms: D000328:Adult; D000531:Altitude; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D012129:Respiratory Function Tests; D012132:Respiratory Muscles / Q000502:physiology*; D014797:Vital Capacity / Q000502:physiology*