title: G-protection mechanisms afforded by the anti-G suit abdominal bladder with and without pressure breathing.
abstract: BACKGROUND
  G protection afforded by the abdominal bladder of a pneumatic anti-G suit is usually attributed to counteraction of G-induced caudad displacement of the heart and pooling of blood in the abdominal veins. The study examined whether the abdominal bladder might provide G protection also via other mechanisms.
  
  
  METHODS
  Each subject was exposed to +Gz loads while sitting relaxed, wearing a full-coverage anti-G suit modified to permit separate pressurization of the abdominal and leg bladders. In two experimental series (N = 8, N = 14), subjects were breathing at positive airway pressure (PPB); in a third series, five subjects were breathing at atmospheric airway pressure. Intrathoracic pressures were estimated by use of esophageal catheters.
  
  
  RESULTS
  During PPB at high G loads, intrathoracic pressure was higher with than without the pressurized abdominal bladder. In 7 of the 14 subjects, basilar intrathoracic pressure exceeded airway pressure during PPB when the abdominal bladder was pressurized. The mean arterial pressure response at high G loads was higher in this subset of subjects (55 +/- 23 mmHg) than in the subjects in whom airway pressure exceeded intrathoracic pressure (41 +/- 27 mmHg). Without PPB at increased G load, the intrathoracic pressure gradient was higher with than without the pressurized abdominal bladder.
  
  
  CONCLUSIONS
  During PPB, the abdominal bladder acts as an airway counterpressure, thereby facilitating pressure transmission from the airways to the thorax and hence improving G protection. It also appears that in several individuals, pressure may be transmitted from the abdominal bladder to the thorax and heart.
authors.affiliation: Department of Environmental Physiology, School of Technology and Health, Royal Institute of Technology, Stockholm, Sweden. ola.eiken@sth.kth.se
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
authors.forename: Eddie
authors.initials: E
authors.lastname: Bergsten
authors.forename: Mikael
authors.initials: M
authors.lastname: Grönkvist
mesh_terms: D000328:Adult; D000337:Aerospace Medicine; D001794:Blood Pressure; D006112:Gravitation; D005676:Gravity Suits; D006439:Hemodynamics; D006801:Humans; D008297:Male; D008875:Middle Aged; D011175:Positive-Pressure Respiration; D011312:Pressure; D055815:Young Adult