A method for noninvasive measurement of Hb O₂ saturation (SO₂) in retinal blood vessels by digital imaging was developed and tested. Images of vessels were recorded at O₂-sensitive and O₂-insensitive wavelengths (600 and 569 nm, respectively) by using a modified fundus camera with an image splitter coupled to an 18-bit digital camera. Retinal arterial SO₂ was varied experimentally by having subjects breathe mixtures of O₂ and N₂ while systemic arterial SO₂ was monitored with a pulse oximeter. Optical densities (ODs) of vascular segments were determined using a computer algorithm to track the path of reflected light intensity along vessels. During graded hypoxia the OD ratio (ODR = OD₆₀₀/OD₅₆₉) bore an inverse linear relationship to systemic SO₂. Compensation for the influence of choroidal pigmentation significantly reduced variation in the arterial SO₂ measurements among subjects. An O₂ sensitivity of 0.00504 ± 0.00029 (SE) ODR units/%SO₂ was determined. Retinal venous SO₂ at normoxia was 55 ± 3.38% (SE). Breathing 100% O₂ increased venous SO₂ by 19.2 ± 2.9%. This technique, when combined with blood flow studies in human subjects, will enable the study of retinal O₂ utilization under experimental and various disease conditions.

hemoglobin oxygen saturation; spectrophotometry

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