Automatic retinal oximetry

PURPOSE. To measure hemoglobin oxygen saturation (SO2) in retinal vessels and to test the reproducibility and sensitivity of an automatic spectrophotometric oximeter. METHODS. Specialized software automatically identifies the retinal blood vessels on fundus images, which are obtained with four different wavelengths of light. The software calculates optical density ratios (ODRs) for each vessel. The reproducibility was evaluated by analyzing five repeated measurements of the same vessels. A linear relationship between SO2 and ODR was assumed and a linear model derived. After calibration, reproducibility and sensitivity were calculated in terms of SO2. Systemic hyperoxia (n = 16) was induced in healthy volunteers by changing the O-2 concentration in inhaled air from 21% to 100%. RESULTS. The automatic software enhanced reproducibility, and the mean SD for repeated measurements was 3.7% for arterioles and 5.3% venules, in terms of percentage of SO2 (five repeats, 10 individuals). The model derived for calibration was SO2 = 125 - 142 center dot ODR. The arterial SO2 measured 96% +/- 9% (mean +/- SD) during normoxia and 101% +/- 8% during hyperoxia (n = 16). The difference between normoxia and hyperoxia was significant (P = 0.0027, paired t-test). Corresponding numbers for venules were 55% +/- 14% and 78% +/- 15% (P < 0.0001). SO2 is displayed as a pseudocolor map drawn on fundus images. CONCLUSIONS. The retinal oximeter is reliable, easy to use, and sensitive to changes in SO2 when concentration of O-2 in inhaled air is changed.