title: The effect of repeated altitude exposures on the incidence of decompression sickness.
abstract: BACKGROUND
  Repeated altitude exposures in a single day occur during special operations parachute training, hypobaric chamber training, unpressurized flight, and extravehicular space activity. Inconsistent and contradictory information exists regarding the risk of decompression sickness (DCS) during such hypobaric exposures.
  
  
  OBJECTIVE
  We hypothesized that four short exposures to altitude with and without ground intervals would result in a lower incidence of DCS than a single exposure of equal duration.
  
  
  METHODS
  The 32 subjects were exposed to 3 different hypobaric exposures--condition A: 2 h continuous exposure (control); condition B: four 30-min exposures with descent/ascent but no ground interval between the exposures; condition C: four 30-min exposures with descent/ascent and 60 min of ground interval breathing air between exposures. All exposures were to 25,000 ft with 100% oxygen breathing. Subjects were observed for symptoms of DCS, and precordial monitoring of venous gas emboli (VGE) was accomplished with a SONOS 1000 echo-imaging system.
  
  
  RESULTS
  DCS occurred in 19 subjects during A (mean onset 70+/-29 min), 7 subjects in B (60+/-34 min), and 2 subjects in C (40+/-18 min). There was a significant difference in DCS incidence between B and A (p = 0.0015) and C and A (p = 0.0002), but no significant difference between B and C. There were 28 cases of VGE in A (mean onset 30+/-23 min), 21 in B (41+/-35 min), and 21 in C (41+/-32 min) with a significant onset curve difference between B and A and between C and A, but not between B and C. Exposure A resulted in four cases of serious respiratory/neurological symptoms, while B had one and C had none. All symptoms resolved during recompression to ground level.
  
  
  CONCLUSIONS
  Data indicate that repeated simulated altitude exposures to 25,000 ft significantly reduce DCS and VGE incidence compared with a single continuous altitude exposure.
authors.affiliation: AFRL/HEPR, High Altitute Protection Research, Brooks AFB, TX 78535-5104, USA. andrew.pilmanis@brooks.af.mil
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Pilmanis
authors.forename: James T
authors.initials: JT
authors.lastname: Webb
authors.forename: Nandini
authors.initials: N
authors.lastname: Kannan
authors.forename: Ulf
authors.initials: U
authors.lastname: Balldin
mesh_terms: D000293:Adolescent; D000328:Adult; D000337:Aerospace Medicine; D000531:Altitude; D001274:Atmospheric Pressure; D003665:Decompression Sickness / Q000175:diagnosis / Q000209:etiology / Q000517:prevention & control*; D004618:Embolism, Air / Q000175:diagnosis / Q000517:prevention & control; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008889:Military Personnel; D012307:Risk Factors; D013997:Time Factors