title: Altitude decompression sickness between 6858 and 9144 m following a 1-h prebreathe.
abstract: BACKGROUND
  The zero prebreathe altitude threshold for developing 5% decompression sickness (DCS) symptoms in men has been reported to be 6248 m (20,500 ft). However, such an altitude threshold when 1 h of oxygen prebreathe is used has not been well documented and was the primary purpose of this study.
  
  
  METHODS
  The 51 male human subjects were exposed to 9144 m (30,000 ft), 8382 m (27,500 ft), 7620 m (25,000 ft), and/or 6858 m (22,500 ft) for 8 h. They were monitored for symptoms of DCS and venous gas emboli (VGE).
  
  
  RESULTS
  DCS symptom incidence after 4 h of exposure decreased with exposure altitude from 87% at 9144 m to 26% at 6858 m. VGE were lower during the 4-h 6858-m exposures (32%) than at the higher altitudes (76-85%). The symptom incidences during the first 4 h of exposure were lower at 6858 m and 7620 m following a 1-h prebreathe as compared with analogous zero-prebreathe exposures. There were no differences between incidences of VGE or DCS at any of the four altitudes after 8 vs. 4 h of exposure.
  
  
  CONCLUSIONS
  The altitude threshold for 5% DCS symptoms is below 6858 m after 1 h of prebreathe. However, during 6858-m and 7620-m exposures, a 1-h prebreathe is highly beneficial in reducing DCS incidence and delaying the onset of DCS, keeping the incidence to less than 6% during the first 90 min of exposure. Use of 4-h vs. 8-h exposures does not appear to underestimate DCS risk at or above 7620 m.
authors.affiliation: Biosciences and Protection Division, Air Force Research Laboratory, Brooks City-Base, TX, USA. james.webb@brooks.af.mil
authors.forename: James T
authors.initials: JT
authors.lastname: Webb
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Pilmanis
mesh_terms: D000328:Adult; D000337:Aerospace Medicine; D000531:Altitude; D015308:Anaerobic Threshold / Q000502:physiology*; D001273:Atmosphere Exposure Chambers; D003665:Decompression Sickness / Q000503:physiopathology* / Q000517:prevention & control*; D004618:Embolism, Air / Q000097:blood / Q000000981:diagnostic imaging; D006801:Humans; D008297:Male; D008875:Middle Aged; D008889:Military Personnel; D010100:Oxygen / Q000008:administration & dosage*; D014463:Ultrasonography
chemical_list: D010100:Oxygen