Oximetry of retinal vessels by dual-wavelength imaging: calibration and influence of pigmentation

A method for noninvasive measurement of HbO₂ saturation(SO₂) inretinal blood vessels by digital imaging was developed and tested.Images of vessels were recorded atO₂-sensitive andO₂-insensitive wavelengths (600 and 569 nm, respectively) by using a modified fundus camera with animage splitter coupled to an 18-bit digital camera. Retinal arterialSO₂ wasvaried experimentally by having subjects breathe mixtures ofO₂ andN₂ while systemic arterial SO₂ wasmonitored with a pulse oximeter. Optical densities (ODs) of vascularsegments were determined using a computer algorithm to track the pathof reflected light intensity along vessels. During graded hypoxia theOD ratio (ODR = OD₆₀₀/OD₅₆₉)bore an inverse linear relationship to systemicSO₂.Compensation for the influence of choroidal pigmentation significantlyreduced variation in the arterialSO₂measurements among subjects. An O₂sensitivity of 0.00504 ± 0.00029 (SE) ODRunits/%SO₂was determined. Retinal venousSO₂ atnormoxia was 55 ± 3.38% (SE). Breathing 100%O₂ increased venousSO₂ by19.2 ± 2.9%. This technique, when combined with bloodflow studies in human subjects, will enable the study of retinalO₂ utilization under experimentaland various disease conditions.