Citation: Louwies T, Jaki Mekjavic P, Cox B, et al. Separate and combined effects of hypoxia and horizontal bed rest on retinal blood vessel diameters. Invest Ophthalmol Vis Sci. 2016;57:4927-4932. DOI:10.1167/ iovs.16-19968 PURPOSE. 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 6 SD age ¼ 27 6 6 years; body mass index [BMI] ¼ 23.7 6 3.0 kg m À2 ) participated in a repeated-measures crossover design study comprising three 21-day interventions: normoxic bed rest (NBR; partial pressure of inspired O 2 , P i O 2 ¼ 133.1 6 0.3 mm Hg); hypoxic ambulation (HAMB; P i O 2 ¼ 90.0 6 0.4 mm Hg), and hypoxic bed rest (HBR; P i O 2 ¼ 90.0 6 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 (DCRAE ¼ À7.5 lm; 95% confidence interval [CI]: À10.8 to À4.2; P < 0.0001). Hypoxic ambulation resulted in a persistent 21-day increase in CRAE, reaching a maximum on day 4 (DCRAE ¼ 9.4 lm; 95% CI: 6.0-12.7; P < 0.0001). During HBR, the increase in CRAE was highest on day 3 (DCRAE ¼ 4.5 lm; 95% CI: 1.2-7.8; P ¼ 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 (DCRVE ¼ À7.9 lm; 95 CI: À13.3 to À2.5), and the maximum DCRVE during HAMB (24.6 lm; 95% CI: 18.9-30.3) and HBR (15.2 lm; 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
