title: Heat stress effects for USAF anti-G suits with and without a counter-pressure vest.
abstract: BACKGROUND
  Aircrew have reported increased heat stress when wearing the USAF Combined Advanced Technology Enhanced Design G-Ensemble or COMBAT EDGE (CE). The perceived thermal burden has been attributed to the fact that CE includes an inflatable counter-pressure vest to ease the work of positive pressure breathing during G (PBG). This study compared the heat load of CE with that of the standard USAF anti-G system (STD) without the vest, and measured heat stress effects on G-tolerance in both suits.
  
  
  METHODS
  This study had 12 subjects (6 of them aircrew) who participated. Simulated preflight thermal stress (20 min walking at 35 degrees C with 85% relative humidity and radiant heat) was followed by return to a cooler environment (21 degrees C). G-tolerance and subjective stress levels were determined on the human centrifuge before and after the heat stress. Body weight, rectal and skin temperatures, and blood parameters were also assessed.
  
  
  RESULTS
  Baseline relaxed tolerance for +Gz gradual onset runs (GORs) were (mean +/- SD) 7.6 +/- 1.3 G for CE and 7.1 +/- 0.8 G for STD (p < 0.05). Maximal rectal temperature following heat stress peaked at 38.1 +/- 0.4 degrees C for both CE and STD, and mean nude weight loss was 1.10 +/- 0.24 kg for both. Relaxed GOR tolerances after heat stress were 7.1 +/- 1.3 for CE and 6.3 +/- 0.9 for STD (p < 0.01). The heat stress significantly reduced G tolerance for both CE and STD (p < 0.01).
  
  
  CONCLUSIONS
  Simulated preflight activity in hot conditions revealed no significant difference between CE and STD with regard to maximal core and skin temperature elevations or dehydration levels. CE supported a significantly higher baseline relaxed G-tolerance than STD, an advantage that persisted after heat stress and dehydration.
authors.affiliation: Air Force Research Laboratory, Biodynamics and Protection Division, Brooks Air Force Base, Texas 78235-5104, USA. ulf.balldin@brooks.af.mil
authors.forename: Ulf I
authors.initials: UI
authors.lastname: Balldin
authors.forename: Robert R
authors.initials: RR
authors.lastname: O'Connor
authors.forename: Paul M
authors.initials: PM
authors.lastname: Werchan
authors.forename: Wayne M
authors.initials: WM
authors.lastname: Isdahl
authors.forename: Peter F
authors.initials: PF
authors.lastname: Demitry
authors.forename: Roger L
authors.initials: RL
authors.lastname: Stork
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Morgan
mesh_terms: D000328:Adult; D000337:Aerospace Medicine; D001831:Body Temperature; D002498:Centrifugation; D003681:Dehydration; D005260:Female; D005676:Gravity Suits / Q000009:adverse effects*; D018882:Heat Stress Disorders / Q000209:etiology / Q000503:physiopathology*; D006400:Hematocrit; D006454:Hemoglobins / Q000032:analysis; D006801:Humans; D018471:Hypergravity; D008297:Male; D008875:Middle Aged; D008889:Military Personnel; D011481:Protective Clothing; D012119:Respiration; D012881:Skin Temperature; D014481:United States; D015431:Weight Loss
chemical_list: D006454:Hemoglobins