Counter-Current Oxygen Flux From Arterioles to Venules in the Human Retinal Circulation

Abstract

Purpose:: The theory of counter-current mechanisms predicts that there is diffusion of oxygen from retinal arterioles into adjacent retinal venules. A possible counter-current exchange was investigated by measuring hemoglobin oxygen saturation (SatO2) in first order retinal arterioles and venules at different distances from the optic disc.

Methods:: An automatic non-invasive retinal oximeter was used to measure the SatO2. The oximeter yields fundus images with isosbestic and non-isosbestic wavelengths of light simultaneously and calculates the optical density ratio, which is approximately linearly related to SatO2. We performed oximetry in 10 healthy subjects. The oxygen saturation was measured in arterioles and venules approximately 0.3 vertical disc diameters (DD) from the edge of the optic disc and at a site approximately two DD from the disc. Differences in SatO2 were analyzed with a double sided, paired t-test for both arterioles and venules.

Results:: The arteriolar SatO2 was 98±5% 0.3 DD from the optic disc (mean ± SD; n=10) and 95±5% two DD from the optic disc (p=0.01). The venular SatO2 was 57±4% two DD from the optic disc and increased to 61±6% 0.3 DD from the optic disc (p=0.02).

Conclusions:: The oxygen saturation of the venous blood increases as it approaches the optic disc. This increase may be explained by the presence of a counter-current exchange of oxygen from the retinal arteriole to the adjacent venule.

Keywords: retina • blood supply • imaging/image analysis: non-clinical