title: Incidence of decompression sickness in hypoxia training with and without 30-min O2 prebreathe.
abstract: BACKGROUND
  All naval aviators, navigators, and aircrewmen are required to participate in hypoxia familiarization training. This training is performed in a hypobaric chamber and is considered high risk due to the potential for barotrauma and/or decompression sickness (DCS). Prior analysis of the DCS in U.S. Navy hypobaric chambers revealed a significantly higher incidence among inside observers (IOs) compared with students. In response to these reports, all IOs are required to denitrogenate by breathing 100% oxygen for 30 min prior to altitude exposure (prebreathing). Although the Army, Navy, and Air Force prebreathe for 30 min prior to most hypobaric training exposures, there have been no reports validating the efficacy of this measure. This study examined the incidence of altitude DCS during training exposures to simulated altitudes of 25,000 ft (25k) and 35,000 ft (35k) in IOs and students, some of whom prebreathed and some of whom did not.
  
  
  METHODS
  Exposures and DCS cases for a period of 9 yr were tabulated from training reports maintained at the Naval Operational Medicine Institute in Pensacola, FL. Chi-square or Fisher's Exact test was used to compare the data sets and p < or = 0.05 was considered significant.
  
  
  RESULTS
  The overall DCS incidence for students and IOs for all chamber profiles was 0.25%. The incidence for 25k was 0.29% for students who did not prebreathe and 0.15% for IOs who did (p = 0.10). Within the student group there was a 0.44% DCS incidence for 25k with no prebreathe and a 0.17% DCS incidence for 35k with prebreathe (p = 0.004).
  
  
  CONCLUSIONS
  A 30-min prebreathe prior to altitude exposure appears to contribute to a reduction in the risk of DCS during hypobaric chamber training.
authors.affiliation: Department of Biomedical Sciences, Naval Aerospace Medical Research Laboratory, Pensacola, FL 32508, USA. gmrice@namrl.navy.mil
authors.forename: G Merrill
authors.initials: GM
authors.lastname: Rice
authors.forename: Charles A
authors.initials: CA
authors.lastname: Vacchiano
authors.forename: Jeffrey L
authors.initials: JL
authors.lastname: Moore
authors.forename: David W
authors.initials: DW
authors.lastname: Anderson
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000337:Aerospace Medicine; D000531:Altitude; D001273:Atmosphere Exposure Chambers; D016009:Chi-Square Distribution; D003665:Decompression Sickness / Q000453:epidemiology* / Q000517:prevention & control*; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D015994:Incidence; D008887:Military Medicine; D010102:Oxygen Inhalation Therapy / Q000379:methods*