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Julia Arciero, Alon Harris, Giovanna Guidoboni; Effect of intraocular pressure and arterial blood pressure on oxygen saturation levels in the retina: a theoretical model. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4463.
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Open-angle glaucoma (OAG) is characterized by progressive retinal ganglion cell death and visual field loss. Although elevated intraocular pressure (IOP) is the primary risk factor for OAG, several studies have shown that impaired perfusion and oxygen delivery to retinal ganglion cells may contribute to OAG pathophysiology. In this study, a mathematical model is used to predict blood oxygen saturation in the retina as IOP and mean arterial pressure (MAP) are varied and to assess conditions that may lead to normal or impaired oxygenation.
A theoretical model is used to simulate oxygen delivery to the retina via five compartments of blood vessels. Oxygen is assumed to be delivered by large arterioles (LA), small arterioles (SA), and capillaries (C); oxygen exchange by small venules (SV) and large venules (LV) is neglected. A Krogh cylinder model is used in which each oxygen-delivering vessel runs along the axis of a cylinder representing the exclusive tissue region it supplies. The oxygen demand is assumed to be constant, and the decline in oxygen flux must equal the rate of oxygen consumption, by the conservation of mass. Oxygen saturation levels in the five compartments are predicted for a wide range of MAP (80, 100, and 160 mmHg) while IOP is held at a control (15 mmHg) or elevated (25 mmHg) level.
Model predictions suggest that an isolated measure of oxygen saturation is not sufficient for distinguishing between the multiple factors that influence blood oxygen levels, including IOP and MAP. For example, a MAP of 80 mmHg and IOP of 15 mmHg yield nearly identical model predictions of oxygen saturation in venules as a MAP of 100 mmHg and IOP of 25 mmHg (Fig. 1A-1B). Oximetry data from patients typically indicate a blood oxygen saturation of 90-100% in arterioles and 50-75% in venules (Figure 2). Thus, oximetry data provide direct clinical measures of oxygen saturation, but do not provide a direct identification of the factors that may alter oxygen saturation levels.
Oximetry measures provide important indications of tissue oxygenation, but this study also motivates the need for theoretical models to guide the differentiation and identification of the most relevant factors to consider in order to treat OAG most successfully.
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