title: Intraocular pressure and cerebral oxygenation during prolonged headward acceleration.
abstract: OBJECTIVE
  Supra-tolerance head-to-foot directed gravitoinertial load (+Gz) typically induces a sequence of symptoms/signs, including loss of: peripheral vision-central vision-consciousness. The risk of unconsciousness is greater when anti-G-garment failure occurs after prolonged rather than brief exposures, presumably because, in the former condition, mental signs are not consistently preceded by impaired vision. The aims were to investigate if prolonged exposure to moderately elevated +Gz reduces intraocular pressure (IOP; i.e., improves provisions for retinal perfusion), or the cerebral anoxia reserve.
  
  
  METHODS
  Subjects were exposed to 4-min +Gz plateaux either at 2 and 3 G (n = 10), or at 4 and 5 G (n = 12). Measurements included eye-level mean arterial pressure (MAP), oxygenation of the cerebral frontal cortex, and at 2 and 3 G, IOP.
  
  
  RESULTS
  IOP was similar at 1 (14.1 ± 1.6 mmHg), 2 (14.0 ± 1.6 mmHg), and 3 G (14.0 ± 1.6 mmHg). During the G exposures, MAP exhibited an initial prompt drop followed by a partial recovery, end-exposure values being reduced by ≤30 mmHg. Cerebral oxygenation showed a similar initial drop, but without recovery, and was followed by either a plateau or a further slight decrement to a minimum of about -14 μM.
  
  
  CONCLUSIONS
  Gz loading did not affect IOP. That cerebral oxygenation remained suppressed throughout these G exposures, despite a concomitant partial recovery of MAP, suggests that the increased risk of unconsciousness upon G-garment failure after prolonged +Gz exposure is due to reduced cerebral anoxia reserve.
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, School of Technology and Health, KTH, Royal Institute of Technology, Berzelius v 13, SE-17165 Stockholm, Sweden. ola.eiken@sth.kth.se.
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, School of Technology and Health, KTH, Royal Institute of Technology, Berzelius v 13, SE-17165 Stockholm, Sweden.
authors.forename: Michail E
authors.initials: ME
authors.lastname: Keramidas
authors.affiliation: Centre for Human and Applied Physiology, School of Medicine, University of Wollongong, Wollongong, NSW, Australia.
authors.forename: Nigel A S
authors.initials: NA
authors.lastname: Taylor
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, School of Technology and Health, KTH, Royal Institute of Technology, Berzelius v 13, SE-17165 Stockholm, Sweden.
authors.forename: Mikael
authors.initials: M
authors.lastname: Grönkvist
mesh_terms: D000054:Acceleration / Q000009:adverse effects*; D000328:Adult; D002560:Cerebrovascular Circulation; D005260:Female; D005625:Frontal Lobe / Q000098:blood supply / Q000378:metabolism* / Q000502:physiology; D006112:Gravitation; D006257:Head / Q000502:physiology; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D010101:Oxygen Consumption; D013312:Stress, Physiological; D014474:Unconsciousness / Q000209:etiology / Q000378:metabolism* / Q000503:physiopathology
keywords: Cerebral anoxia reserve; Cerebral blood flow; G tolerance; G-induced loss of consciousness; Oxyhaemoglobin saturation; Retinal anoxia reserve