title: Effect of acute hypoxia on respiratory muscle fatigue in healthy humans.
abstract: BACKGROUND
  Greater diaphragm fatigue has been reported after hypoxic versus normoxic exercise, but whether this is due to increased ventilation and therefore work of breathing or reduced blood oxygenation per se remains unclear. Hence, we assessed the effect of different blood oxygenation level on isolated hyperpnoea-induced inspiratory and expiratory muscle fatigue.
  
  
  METHODS
  Twelve healthy males performed three 15-min isocapnic hyperpnoea tests (85% of maximum voluntary ventilation with controlled breathing pattern) in normoxic, hypoxic (SpO2 = 80%) and hyperoxic (FiO2 = 0.60) conditions, in a random order. Before, immediately after and 30 min after hyperpnoea, transdiaphragmatic pressure (P(di,tw)) was measured during cervical magnetic stimulation to assess diaphragm contractility, and gastric pressure (P(ga,tw)) was measured during thoracic magnetic stimulation to assess abdominal muscle contractility. Two-way analysis of variance (time x condition) was used to compare hyperpnoea-induced respiratory muscle fatigue between conditions.
  
  
  RESULTS
  Hypoxia enhanced hyperpnoea-induced P(di,tw) and P(ga,tw) reductions both immediately after hyperpnoea (P(di,tw) : normoxia -22 +/- 7% vs hypoxia -34 +/- 8% vs hyperoxia -21 +/- 8%; P(ga,tw) : normoxia -17 +/- 7% vs hypoxia -26 +/- 10% vs hyperoxia -16 +/- 11%; all P < 0.05) and after 30 min of recovery (P(di,tw) : normoxia -10 +/- 7% vs hypoxia -16 +/- 8% vs hyperoxia -8 +/- 7%; P(ga,tw) : normoxia -13 +/- 6% vs hypoxia -21 +/- 9% vs hyperoxia -12 +/- 12%; all P < 0.05). No significant difference in (di,tw) or P(ga,tw) reductions was observed between normoxic and hyperoxic conditions. Also, heart rate and blood lactate concentration during hyperpnoea were higher in hypoxia compared to normoxia and hyperoxia.
  
  
  CONCLUSIONS
  These results demonstrate that hypoxia exacerbates both diaphragm and abdominal muscle fatigability. These results emphasize the potential role of respiratory muscle fatigue in exercise performance limitation under conditions coupling increased work of breathing and reduced O2 transport as during exercise in altitude or in hypoxemic patients.
authors.affiliation: HP2 laboratory (INSERM ERI17), Joseph Fourier University, Grenoble University Hospital, Grenoble, France. sverges@chu-grenoble.fr
authors.forename: Samuel
authors.initials: S
authors.lastname: Verges
authors.forename: Damien
authors.initials: D
authors.lastname: Bachasson
authors.forename: Bernard
authors.initials: B
authors.lastname: Wuyam
mesh_terms: D000009:Abdominal Muscles / Q000378:metabolism / Q000502:physiology* / Q000503:physiopathology; D000208:Acute Disease; D000328:Adult; D006801:Humans; D000860:Hypoxia / Q000378:metabolism / Q000503:physiopathology*; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology*; D012129:Respiratory Function Tests / Q000379:methods; D012132:Respiratory Muscles / Q000378:metabolism / Q000502:physiology* / Q000503:physiopathology; D013997:Time Factors; D055815:Young Adult