A Simple Model of Oxygen Diffusion Out of the Retinal Artery

The purpose of this study is to explore the possibility that oxygen (O2) diffusion out of the retinal artery (RA) can explain the observed significant (P<0.001) decrease of oxygen saturation (O2Sat) in RA as intraocular pressure (IOP) is raised from 10 to 55 mmHg. Hyperspectral image data from normal monkeys' optic nerve head (ONH) and overlying retinal blood vessels were recorded at IOP settings of 10, 30, 45 and 55 mmHg. Average percent O2Sat values of the RAs were found from the recorded blood spectra by comparing to reference spectra from saturated and desaturated red cell suspensions. Percent O2Sat of the RAs was 78.9% at IOP of 10 mmHg. This decreased to 74.1% at 45 mmHg (P=0.01); and further decreased to 51.5% at IOP = 55 mmHg (P<0.0001). To interpret these results, we developed a simple diffusion model assuming that the RA is surrounded by tissues in equilibrium with venous oxygen tension (PO2). O2 flux across the arterial wall was calculated by Fick’s law. The percentage of O2 diffusing out of the RA were 0.6% at IOP of 10 mmHg, and 38% at IOP of 55 mmHg. Confirmation still requires measurement of blood velocity.