title: Effect of acute hypercapnia during 10-day hypoxic bed rest on posterior eye structures.
abstract: To gain insights into microgravity-induced ophthalmic changes (microgravity ocular syndrome), and as part of a project investigating effects of future planetary habitats, we investigated the effect of acute hypercapnia following 10-day bed rest and hypoxia on posterior eye structures. Female subjects (N = 7) completed three 10-day experimental interventions: 1) normoxic bed rest [NBR; partial pressure of inspired O2 (PiO2 ) = 132.9 ± 0.3 Torr]; 2) hypoxic ambulatory confinement (HAMB; PiO2 = 90.4 ± 0.3 Torr); and 3) hypoxic bed rest (HBR; n = 12; PiO2 = 90.4 ± 0.3 Torr). Before and on the last day of each intervention, optical coherence tomography (OCT) of the optic disk was performed, and the thicknesses of the retinal nerve fiber layer (RNFL), retina, and choroid were measured. OCT examinations were conducted with the subjects breathing the prevailing normocapnic breathing mixture (either normoxic or hypoxic) and then following a 10-min period of breathing the same gas mixture, but with the addition of 1% CO2 Choroidal thickness was greater during both bed-rest conditions (NBR and HBR) compared with the ambulatory (HAMB) condition (ANOVA, P < 0.001). Increases in RNFL thickness compared with baseline were observed in the hypoxic trials (HBR, P < 0.001; and HAMB, P = 0.021), but not the normoxic trial (NBR). A further increase in RNFL thickness (P = 0.019) was observed after the 10-min hypercapnic trial in the NBR condition only. The fact that choroidal thickness was not affected by Po2 or Pco2, but increased by bed rest, suggests a hydrostatic rather than a vasoactive effect. The increments in RNFL thickness were most likely associated with local hypoxia and hypercapnia-induced dilatation of the retinal blood vessels.
authors.affiliation: Eye Hospital, University Medical Centre Ljubljana, Ljubljana, Slovenia; polona.jaki@guest.arnes.si.
authors.forename: Polona
authors.initials: P
authors.lastname: Jaki Mekjavic
authors.affiliation: Eye Hospital, University Medical Centre Ljubljana, Ljubljana, Slovenia;
authors.forename: Eva
authors.initials: E
authors.lastname: Lenassi
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, Royal Institute of Technology, Stockholm, Sweden; and.
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
authors.affiliation: Department of Automation, Biocybernetics and Robotics, Ljubljana, Slovenia.
authors.forename: Igor B
authors.initials: IB
authors.lastname: Mekjavic
mesh_terms: D000328:Adult; D001510:Bed Rest / Q000009:adverse effects*; D002245:Carbon Dioxide / Q000378:metabolism; D002829:Choroid / Q000098:blood supply / Q000378:metabolism / Q000503:physiopathology*; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000378:metabolism / Q000503:physiopathology*; D000860:Hypoxia / Q000378:metabolism / Q000503:physiopathology*; D009898:Optic Disk / Q000378:metabolism / Q000503:physiopathology; D010100:Oxygen / Q000378:metabolism; D012119:Respiration; D012160:Retina / Q000378:metabolism / Q000503:physiopathology*; D055351:Retinal Neurons / Q000378:metabolism / Q000502:physiology; D041623:Tomography, Optical Coherence / Q000379:methods; D014893:Weightlessness; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen
keywords: hypercapnia; hypoxia; ophthalmology; planetary habitats; space life sciences