Choroidal Blood Flow Measurements in Zero Gravity (space-like) Environment Using Laser-Doppler Flowmetry

Abstract

Abstract: : Purpose: To investigate the effects of reduced ("zero") and higher gravity environment on the choroidal blood flow. In space flight or reduced gravity environment, bodily fluids shift to the upper extremities of the body. In terms of ocular parameters, intraocular pressure (IOP) changes significantly. A significant number of astronauts report changes in visual acuity (VA) during orbital flight. To date, this remains of unknown etiology. Primarily, could choroidal engorgement mechanism and secondarily some curvature/shape-change mechanism of the cornea or lens be responsible for this change in VA? Since the choroid has no baroreceptors to autoregulate the choroid during fluid shifts as does the systemic vascular system, the choroid possibly remains engorged thereby pushing the macula forward, causing a hyperopic shift of the eye. Our experiments could help answer this hypothesis and facilitate planning for long-duration space missions. Methods: A low power (~100 microwatt) CW solid-state laser operating at ~780 nm is used for frequency shift measurements in a head-mounted miniature laser-Doppler flowmeter (Oculix). The velocity, volume, and flow parameters are calulated from the Doppler shift resulting from red blood cells flowing through the choroid. At the time of this writing, measurements on five human volunteers were completed in accordance with the approved NASA IRB protocol for human testing. The experiments were conducted on-board a wide body aircraft (KC-135 ) during parabolic flight trajectories (0-G to 2G environment). Arterial blood pressure (BP) was also measured continuously (beat-to-beat). Results: Both systolic and diastolic arterial BP decreases in reduced gravity and increases at higer gravity levels. The choroidal blood velocity consistently show an increasing trend in zero gravity. Conclusions: The choroidal blood flow increases in zero gravity. But more experiments are needed to confirm the hypothesis presented above. Acknowledgements: RRA would like to acknowledge the support from NASA-NEI/NIH Interagency Agreement, the John Glenn Biomedical Engineering Consortium, and the Collaborative Research Agreement between the National Center for Microgravity Research (NCMR) at NASA GRC and the Institut de Recherche en Ophthalmologie (IRO) in Switzerland. Fabrice Moret was a visiting research fellow at NCMR supported by the Swiss National Science Foundation.  

Keywords: choroid • blood supply • clinical (human) or epidemiologic studies: sys