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J. Ning, D. A. Rice, B. Khoobehi, J. M. Beach, E. Puissegur; A Simple Model of Oxygen Diffusion Out of the Retinal Artery. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4613.
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© ARVO (1962-2015); The Authors (2016-present)
To explore the possibility that diffusion of oxygen (O2) from retinal artery (RA) alone can explain the significant (P<0.001, about 35%) decrease of oxygen saturation (SO2) observed in RA as intraocular pressure (IOP) is raised from 10 to 55 mmHg in monkeys.
A simple model assumes that RA is surrounded by tissues in equilibrium with venous oxygen partial pressure (PO2). O2 in blood flowing along the RA diffuses through the arterial wall. Two cases were assumed: 1) O2 diffuses from the RA paralleling the retinal vein (RV) on the optic disk, through a flat layer with the size of the RA’s diameter, wall thickness and a length of 1.5 mm. 2) O2 diffuses from RA surrounded by tissue of venous PO2 before penetration into the optic disk, through a circular layer with the maximum transport distance of about 10 mm. Assuming that arterial and venous PO2 are 100 and 50 mmHg respectively, the O2 diffusion coefficient and solubility at body temperature and 1 atm pressure were used. O2 flux across the arterial wall was calculated by Fick’s law. We assumed that retinal arterial blood flow is in steady state with normal velocity (V) of 9.28 mm/s, and with V = 1mm/s at IOP of 55 mmHg. The amount of O2 flowing in arterial blood is the product of oxygen concentration and blood flow (F). F = the product of blood velocity and the area of the artery. The total O2 in 100 ml of blood is slightly more than 20.1 vol % at saturation.
The percentages of O2 diffusing out of the retinal arterial wall in cases 1 and 2 are about 0.058% and 1.21%, respectively, and about 1.27% together. This is insignificant at normal arterial blood velocity. As IOP rises to 55 mmHg, O2 lost via diffusion totals at most 12%. To have a decrease of 35% in retinal arterial SO2, the retinal arterial blood velocity needs to be less than 0.34 mm/s. At 55 mmHg IOP, we observed retinal arterial pulsation suggesting that the RA flow is much decreased.
The significant decrease of SO2 in RA caused by increase in IOP at 55 mmHg can only be explained by diffusion alone if the arterial blood velocity is less than 0.34 mm/s. Confirmation of this still requires measurement of blood velocity. Otherwise, diffusion of O2 out of the RA cannot explain the observed SO2.
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