title: Separate and Combined Effects of Hypoxia and Horizontal Bed Rest on Retinal Blood Vessel Diameters.
abstract: OBJECTIVE
  To assess the separate and combined effects of exposure to prolonged and sustained recumbency (bed rest) and hypoxia on retinal microcirculation.
  
  
  METHODS
  Eleven healthy male subjects (mean ± SD age = 27 ± 6 years; body mass index [BMI] = 23.7 ± 3.0 kg m-2) participated in a repeated-measures crossover design study comprising three 21-day interventions: normoxic bed rest (NBR; partial pressure of inspired O2, PiO2 = 133.1 ± 0.3 mm Hg); hypoxic ambulation (HAMB; PiO2 = 90.0 ± 0.4 mm Hg), and hypoxic bed rest (HBR; PiO2 = 90.0 ± 0.4 mm Hg). Central retinal arteriolar (CRAE) and venular (CRVE) equivalents were measured at baseline and at regular intervals during each 21-day intervention.
  
  
  RESULTS
  Normoxic bed rest caused a progressive reduction in CRAE, with the change in CRAE relative to baseline being highest on day 15 (ΔCRAE = -7.5 μm; 95% confidence interval [CI]: -10.8 to -4.2; P < 0.0001). Hypoxic ambulation resulted in a persistent 21-day increase in CRAE, reaching a maximum on day 4 (ΔCRAE = 9.4 μm; 95% CI: 6.0-12.7; P < 0.0001). During HBR, the increase in CRAE was highest on day 3 (ΔCRAE = 4.5 μm; 95% CI: 1.2-7.8; P = 0.007), but CRAE returned to baseline levels thereafter. Central retinal venular equivalent decreased during NBR and increased during HAMB and HBR. The reduction in CRVE during NBR was highest on day 1 (ΔCRVE = -7.9 μm; 95 CI: -13.3 to -2.5), and the maximum ΔCRVE during HAMB (24.6 μm; 95% CI: 18.9-30.3) and HBR (15.2 μm; 95% CI: 9.8-20.5) was observed on days 10 and 3, respectively.
  
  
  CONCLUSIONS
  The diameters of retinal blood vessels exhibited a dynamic response to hypoxia and bed rest, such that retinal vasodilation was smaller during combined bed rest and hypoxia than during hypoxic exposure.
authors.affiliation: Environmental Risk and Health, Flemish Institute for Technological Research (VITO), Mol, Belgium 2Centre for Environmental Sciences, Hasselt University, Hasselt, Belgium.
authors.forename: Tijs
authors.initials: T
authors.lastname: Louwies
authors.affiliation: Eye Hospital, University Medical Centre Ljubljana, Ljubljana, Slovenia.
authors.forename: Polona
authors.initials: P
authors.lastname: Jaki Mekjavic
authors.affiliation: Centre for Environmental Sciences, Hasselt University, Hasselt, Belgium.
authors.forename: Bianca
authors.initials: B
authors.lastname: Cox
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, Royal Institute of Technology, Kungliga Tekniska Högskolan (KTH), Stockholm, Sweden.
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
authors.affiliation: Department of Automation, Biocybernetics and Robotics, Jozef Stefan Institute, Ljubljana, Slovenia 6Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada.
authors.forename: Igor B
authors.initials: IB
authors.lastname: Mekjavic
authors.affiliation: Human Performance-Rehabilitation Laboratory, Faculty of Physical and Cultural Education, Evelpidon Hellenic Military University, Vari, Greece.
authors.forename: Stylianos
authors.initials: S
authors.lastname: Kounalakis
authors.affiliation: Environmental Risk and Health, Flemish Institute for Technological Research (VITO), Mol, Belgium 2Centre for Environmental Sciences, Hasselt University, Hasselt, Belgium.
authors.forename: Patrick
authors.initials: P
authors.lastname: De Boever