Predicting the impact of retinal capillary density on retinal vessel and tissue oxygenation using a theoretical model

Abstract

Purpose : A theoretical model of the human retina is used to predict the partial pressure of oxygen (PO₂) and oxyhemoglobin saturation in retinal vessels and tissue for varied levels of capillary density.

Methods : Clinical values of capillary density are obtained in healthy and early glaucoma patients. Specifically, the radial papillary capillary (RPC) density (%) of the small vessels is assessed in the whole image (4.5 mm HD Angio Disc RPC Slab) by optical coherence tomography angiography. A published theoretical model of the human retina, inclusive of a combined heterogeneous representation of retinal arterioles and compartmental representation of capillaries, small venules, and large venules, is adapted to assess the potential impact of capillary density on retinal vessel and tissue oxygenation. A Green’s function method is used to model oxygen transport in the arterioles, and a Krogh cylinder model is used in the capillaries and venules. The model is simulated for capillary density values ranging from 250 to 750 capillaries/mm². Oxygen extraction fraction, defined as the ratio of oxygen consumption to oxygen delivery, is calculated for each model simulation.

Results : Early glaucoma patients are shown to exhibit a 10-12% reduction in capillary density compared to healthy individuals. The model predicts a 6% and 53% decrease in mean PO₂ in retinal vessels immediately downstream of the capillaries when capillary density is decreased from its reference value of 500 capillaries/mm² by 10% and 50%, respectively (Figure 1, column 1). The corresponding predicted impact of these capillary density reductions on oxyhemoglobin saturation at the downstream end of the capillaries is also shown (Figure 1, column 2). Model predictions of the increase in oxygen extraction fraction as capillary density is decreased are given in Figure 2.

Conclusions : Our clinical observations showed a moderate (10-12%) reduction in capillary density in early glaucoma patients, while larger reductions in capillary density have been previously identified in advanced glaucoma patients. Quantifying the impact of decreased capillary density on oxygenation of the retina provides a translatable framework for understanding the metabolic impact of capillary loss.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.