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From the 1 Departments of Biomedical Engineering and 2 Neurobiology and Physiology, and the 3 Institute for Neuroscience, Northwestern University, Evanston, Illinois.
PURPOSE. This article assesses the hypothesis that the high blood flow rate and low O2 extraction associated with the choroidal circulation are metabolically necessary and explores the implications of the spatial relationship between the choroid and the photoreceptors for metabolism in the normal and detached retina.
METHODS. The O2 distribution across the retinal layers was previously measured with O2-sensitive microelectrodes in cat. Profiles were fitted to a diffusion model to obtain parameters characterizing photoreceptor O2 demand. This was a study of simulations based on those parameters.
RESULTS. Photoreceptor inner segments have a high O2 demand (QO2), and they are far (20 to 30 µm) from the choroid. These unusual conditions require a large O2 flux to the inner segments, which in turn requires high choroidal oxygen tension (PO2), high choroidal venous saturation (ScvO2), low choroidal O2 oxygen extraction per unit volume of blood, and a choroidal blood flow (ChBF) of at least 500 ml/100 g-min. Movement of the inner segments further from the choroid, which occurs in a retinal detachment, severely reduces the ability of the inner segments to obtain O2, even for detachment heights as small as 100 µm. Depending on detachment height and assumptions about choroidal and inner retinal PO2 during elevation of inspired O2 (hyperoxia), hyperoxia is predicted to partially or fully restore photoreceptor QO2 during a detachment. CONCLUSIONS. The choroid is not overperfused, but requires a high flow rate to satisfy the normal metabolic demand of the retina. Because the oxygenation of the photoreceptors is barely adequate under normal conditions, detachment has serious metabolic consequences. Hyperoxia is predicted to have clinical benefit during detachment.
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