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Automatic Retinal Oximetry

¹From the Departments of Ophthalmology and ⁵Anesthesiology, University of Iceland, National University Hospital, Reykjavík, Iceland; the ²Department of Ophthalmology, Indiana University/Purdue University School of Medicine, Indianapolis, Indiana; the ³Faculty of Electrical and Computer Engineering, University of Iceland, Reykjavik; the ⁴Department of Ophthalmology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; the ⁶Eye Department, National University Hospital of Copenhagen, Denmark; and the ⁷Institute for Technology Development, Stennis Space Center, Mississippi.

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 O₂ 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 · 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 O₂ in inhaled air is changed.

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