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Light and Choroidal PO₂ Modulation of Intraretinal Oxygen Levels in an Avascular Retina

From the Centre for Ophthalmology and Visual Science, The University of Western Australia, Nedlands, Perth, Western Australia.

Abstract

PURPOSE. To determine the influence that choroidal oxygen level and outer retinal oxygen demand have on oxygen availability to the inner retina when the choroid is the only source of retinal oxygenation. This condition prevails in avascular retinas and in vascularized retinas suffering vascular occlusion.

METHODS. Oxygen-sensitive microelectrodes were used to measure the oxygen tension as a function of depth in the naturally avascular retina of anesthetized and mechanically ventilated guinea pigs (n = 6). Choroidal PO₂ was manipulated by varying the ventilation gas mixture, and outer retinal oxygen consumption was modulated by light-dark adaptation. Individual PO₂ profiles were fitted to a multilayer mathematical model of PO₂ distribution, and pairs of profiles at different choroidal PO₂ levels, or under light and dark conditions, were fitted to an intraretinal PO₂ difference model. Both models reflect the purely choroidal supply of retinal oxygenation.

RESULTS. An increase in choroidal PO₂ produced an equivalent increase in all retinal layers. Light induced a decreased oxygen consumption in the region of the inner segments of the photoreceptors, which resulted in a significant increase in PO₂ in this layer, flowing on unattenuated to all inner retinal layers. The intraretinal PO₂ distribution and the light- and ventilatory-induced changes in PO₂ were consistent with theoretical predictions of the mathematical models.

CONCLUSIONS. The present experimental studies confirm that when the choroid is the only source of retinal oxygenation, the full effect of increased choroidal oxygen level or reduced uptake in the outer retina passes through to the inner retinal layers if the oxygen utilization by the inner retina remains constant.

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