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Introduction: 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)., Discussion: 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.

Purpose: Few studies have investigated the effect of systemic hypoxia on the vascular and neural function of the human retina. Such studies can help us understand physiological responses to environmental hypoxia, as well as the pathophysiological mechanisms underlying certain ocular diseases. The objective of this study was to investigate the impact of mild systemic hypoxia on neuroretinal function through photopic flash electroretinogram (fERG) and oscillatory potential (OP) recordings., Methods: The photopic fERGs and OPs were recorded in 18 healthy adults under conditions of mild systemic hypoxia. The retinal responses were recorded before, during, and after a 5-min period of breathing 12% oxygen (O2) in 88% nitrogen. The heart rate (HR), respiratory rate (RR), end-tidal carbon dioxide pressure (Petco2), and O2 saturation (SaO2) were measured throughout testing. The systemic blood pressure (BP) and intraocular pressure (IOP) were measured to derive the ocular perfusion pressure (OPP)., Results: Systemic hypoxia reduced SaO2 and PetCO2, increased HR but did not alter the RR or OPP. The a-wave amplitude and latency were not altered throughout testing. The b-wave amplitude decreased with hypoxia, whereas its latency was not affected. The amplitude of OP1, OP2, and OP4 remained stable throughout testing, whereas the amplitude of OP3 tended to decrease with hypoxia and was increased at the end of testing. The latency of OP1, OP3, and OP4 did not vary. The latency of OP2 was reduced at the end of testing., Conclusions: Our results show that mild systemic hypoxia alters the fERG b-wave and OPs but not the a-wave. This suggests that the outer retina in humans is more resistant to a mild systemic hypoxic stress than the inner retinal layers.

Background: The microgravity environment of spaceflights alters the systemic circulation, decreasing the peripheral resistance, while increasing the heart rate and systemic blood pressure., Hypothesis: Body orientation simulating fluid shifts associated with microgravity during spaceflights affects the neural retinal function., Methods: Fifteen healthy adults between 18 and 26 yr of age participated in this study. Scotopic flash electroretinograms and oscillatory potentials were compared for: 1) baseline, wherein subjects were declined 30 degrees from vertical; and 2) for microgravity simulation where subjects were in a 7 degrees head down tilt for 90 min., Results: Group averaged implicit times for the ERG b-wave, as well as OP3, OP4 and OP5 were prolonged after microgravity simulation, and the group averaged OP index was reduced., Conclusions: The results suggest that microgravity of relatively short duration may cause retinal ischemia manifest as neuroretinal dysfunction with yet undetermined effects on visual and overall performance.

We quantified the changes in macular and choroidal blood flow during body inversion to elaborate their hemodynamic attributes and relate them to our previously reported effects of inversion on visual neural function. Ten healthy subjects took part in the study. The intraocular pressure (IOP) was altered through graded body tilts. Macular and choroidal pulsatile ocular blood flow (POBF) were measured with an Oculix BFS 2000 and a Langham OBF system, respectively. Body inversion caused a systematic decrease in the POBF for all subjects. Systematic changes in macular leukocyte velocity and density were also found with inversion. Six of ten subjects showed a reciprocal relationship between the leukocyte density and velocity as a function of body orientation, while parallel changes in the leukocyte density and velocity were seen during inversion for the remaining subjects. Our results may reflect individual strategies for autoregulation within the macular circulation and an absence of autoregulation in the pulsatile component of the choroidal circulation.