The effect of length and width of retinal vessels on oxygen saturation measurements

Abstract

Purpose : Animal studies have indicated that oxygen is lost directly through the walls of retinal arterioles, to the retinal tissue or to adjacent venules. Aim 1 of the study was to test if oxygen saturation changes along the length of retinal vessels in humans.
Previous studies have also indicated that retinal vessel oxygen saturation changes with vessel diameter and that this effect may be technical in nature. Aim 2 of the study was to test if oxygen saturation varies with diameter of retinal vessels, even after correction for a technical artefact.

Methods : Retinal images were taken with a retinal oximeter (Oxymap T1) of healthy individuals (n=55) and patients with central retinal vein occlusion (CRVO, n=13). Analysis was performed with Oxymap Analyzer software 2.5.1, ver. 10981. Correction for artefactual effects of diameter on saturation measurements was applied.
For aim 1: Oxygen saturation was measured in one main arteriole and one main venule near the optic disc (1.5 disc radii from disc center). Oxygen saturation was measured again in the same vessel segment further away from the optic disc (up to 3.0 disc radii from disc center).
For aim 2: Oxygen saturation was measured in all vessels above 6 pixels in diameter (approx. 56 microns) in an area between 1.5 and 3.0 disc radii from the disc center (see figure).

Results : In healthy retinas, oxygen saturation in arterioles was 90.8%±7.3% near the optic disc and 91.2%±13.9% further from the optic disc (p=0.74). The corresponding values for venules in healthy were 54.0%±17.2% near the disc vs. 56.3%±17.5% further from disc (p=0.060). In CRVO patients, differences along the length of arterioles (p=0.45) or venules (p=0.32) were also not statistically significant.
In healthy retinas, oxygen saturation in arterioles decreased by 0.45 percentage points for per pixel (approx. 9.3 microns) increase in vessel diameter (p<0.0001). The decrease in venules was 1.95 percentage points per pixel increase in vessel diameter (p<0.0001).

Conclusions : Indications of loss of oxygen from retinal arterioles were not found in healthy individuals and also not in patients with CRVO, where blood flow is likely to be slower.
Oxygen saturation is less in wider blood vessels, particularly in venules. Whether this difference is purely physiological or influenced by technical errors requires further study.

This is a 2020 ARVO Annual Meeting abstract.

 

Figure: Oxygen saturation in a healthy retina. Measurements were made between the blue circles.