Estimation of retinal oxygen transients from measurements made in the vitreous humor

Measurements of the oxygen tension in the vitreous humor close to the retina were used to estimate the values at the retinal surface in the cat. The retina was modelled as an infinite plane in a semi-infinite medium and the analytical solution was obtained for the transient following a change in breathing gas from room air to 95% O₂/5% CO₂. The solution was, compared with experimental measurements. Our measurements indicate that at steady state, breathing room air, the oxygen tension at the surface of the retina is between 15 and 20 mmHg and the gradient from it averages -2·8 mmHg/mm. Transient oxygen tension measurements at several locations within 1·7 mm of the retinal surface agreed well with our mathematical model which could then be used to predict the transient at the retinal surface. In this way, our model can be used to estimate retinal transients from measurements in the preretinal vitreous humor and the distortion introduced by not measuring on the retinal surface itself can be eliminated. The model predicts that the distortion would be relatively small if the electrode were less than about 100 μm from the retina, provided the retinal transient had a time constant greater than a minute. In the steady state, the error would be less than 0·3 mmHg at this distance. These results are not only important for oxygen but apply, with minor adjustments, to all small molecules and ions which passively diffuse from the retina into the vitreous humor.