Measurement of blood flow through the retinal circulation of the cat during normoxia and hypoxemia using fluorescent microspheres

The most successful method for measuring absolute blood flow rate through t he retinal circulation has been the use of radioactive microspheres. The pu rpose of this study was to develop a microsphere method that did not have t he drawbacks associated with radioactivity and to use this method to make m easurements of retinal blood flow in the cat. Blood flow measurements were made by injecting 15-mum-diameter polystyrene microspheres into the left ve ntricle of anesthetized, artificially ventilated cats. These microspheres w ere labeled with one of three fluorescent dyes. Retinal blood flow measurem ents were made by determining the number of spheres that were embedded in t he retina and comparing them to the number found in a reference sample. Sph eres in the retina were counted by making retinal whole mounts and taking r etinal images with a CCD camera mounted on an epifluorescence microscope eq uipped with filter sets appropriate for imaging the dyes used to label the spheres. Blood flow measurements made under normal conditions showed a mean retinal blood flow of 19.8 +/- 12.4 ml/min 100 g tissue (mean +/- SD; n=15 cats). Since the retinal circulation perfuses only the inner half of the r etina, the effective flow rate in that region is about twice this value. RB F increased during hypoxemia. (PaO2<42 mm Hg) to 336% of the normoxic value on average. Analysis of sphere deposition patterns showed that the central retina had a higher blood flow than the peripheral retina, although this d ifference was significant only during hypoxemia. We conclude that even with a relatively small number of spheres deposited in the retina, the techniqu e can reveal important properties of the retinal circulation. (C) 2001 Acad emic Press.