Standardization of Retinal Vessel Oximetry and the Influence of Vessel Diameter

Abstract

 

Purpose

 

The purpose of the study was to develop and test a standardized method for determining the mean retinal vessel oxygen saturation within an eye.

 

Methods

 

The retinal oximeter (Oxymap, Reykjavik, Iceland) simultaneously captures fundus images at 570nm and 600nm. Custom made software analyzes the images and estimates retinal vessel oxygen saturation. Twenty healthy individuals (age 20 to 30 years) were measured according to the analysis standard shown in the figure. Blood flow in the large vessels is related to the fourth power of the diameter and an adjustment was made accordingly to calculate the weighted mean saturation for the whole retina. Additionally, ten images of one individual were measured to estimate the stability of the measurements.

 

Results

 

The table shows the average saturation values, with and without weighing with diameter in the power of four (n=20, mean±SD). The standard deviation for repeated measurements of arterioles in the same eye was 0.30 percentage points for simple means and 0.31 percentage points for means weighted with diameter in the power of four. The corresponding numbers for venules were 1.29 percentage points (simple means) and 1.94 percentage points (weighted means). Retinal venous oxygen saturation decreased by 2.0 percentage points per pixel (≈9μm) of increased diameter (mixed model linear regression, p<0.0001). No significant relationship was detected between arteriolar saturation and diameter (-0.3 percentage points per pixel, p=0.11)

 

Conclusions

 

Weighing the average saturation with the fourth power of vessel diameter, which is a surrogate for weighing with blood flow, yields considerably lower venous saturation, compared to a simple mean. Repeatability of the measurements is good, both with and without weighing the mean. Retinal venous oxygen saturation decreases with vessel diameter, even after correction for artificial effects.

 

 

Figure. Analysis protocol. All vessels above minimum diameter and minimum length are measured close to the optic disc.

 

 

Keywords: 688 retina • 635 oxygen • 550 imaging/image analysis: clinical