title: Seven-degree head-down tilt reduces choroidal pulsatile ocular blood flow.
abstract: BACKGROUND
  We investigated the effects of head-down tilt (HDT), which simulates microgravity during spaceflights, on the choroidal pulsatile ocular blood flow (POBF). This investigation is important because alterations in the choroidal blood flow can affect the function of retinal rods and cones that rely totally on the choroid for metabolites.
  
  
  METHODS
  Nineteen healthy adults between 20 and 38 yr of age participated in this study. The POBF was compared for: 1) baseline, wherein subjects were declined 30 degrees from vertical; 2) microgravity simulation where subjects were in a 7 degrees HDT for 2 min; 3) 90 min of the 7 degrees HDT; and 4) recovery, i.e., back at 30 degrees for 2 min.
  
  
  RESULTS
  The group averaged POBF (Mean +/- SEM values: 828.43 +/- 48.88 microL x min(-1)) decreased immediately during the 2-min microgravity simulation (582.18 +/- 43.62 microL x min(-1)), remained at that inferior level at the 90-min mark of HDT (542.26 +/- 45.35 microL x min(-1)), and came back toward baseline POBF during the recovery period (760.11 +/- 46.03 microL x min(-1)) (p = 0.0001).
  
  
  CONCLUSIONS
  The results show that simulated-microgravity of relatively short duration induces retinal hypoperfusion throughout the microgravity interval through the reduction in the POBF. This finding may have important implications regarding visual performance in space crewmembers placed in prolonged microgravity environments.
authors.affiliation: Ecole d'optométrie, Université de Montréal, Montréal, Québec, Canada. helene.kergoat@umontreal.ca
authors.forename: Hélène
authors.initials: H
authors.lastname: Kergoat
authors.forename: John V
authors.initials: JV
authors.lastname: Lovasik
mesh_terms: D000328:Adult; D002829:Choroid / Q000098:blood supply*; D005260:Female; D018475:Head-Down Tilt; D006801:Humans; D018472:Hypogravity / Q000009:adverse effects*; D008297:Male; D012039:Regional Blood Flow; D017949:Retinal Cone Photoreceptor Cells / Q000098:blood supply; D017948:Retinal Rod Photoreceptor Cells / Q000098:blood supply; D013026:Space Flight; D013997:Time Factors