Choroidal oximetry with a noninvasive spectrophotometric oximeter

The purpose of the study was to establish a new technology to measure hemoglobin oxygen saturation in human choroidal vasculature with a noninvasive spectrophotometric oximeter.

The fundus camera-based oximeter captures dual-wavelength oximetry images of the fundus and calculates optical density ratio (ODR), which is inversely related to hemoglobin oxygen saturation. Sixteen healthy and lightly pigmented individuals were imaged during normoxia and six during both normoxia and pure oxygen breathing (hyperoxia). ODR was measured for choroidal vessels, vortex veins, and retinal arterioles and venules.

ODR was 0.10 ± 0.10 (mean ± SD) for choroidal vessels, 0.13 ± 0.12 for vortex veins, 0.22 ± 0.04 for retinal arterioles, and 0.50 ± 0.09 for retinal venules. Inhalation of pure oxygen lowered ODR levels in all vessel types; the decrease was 0.035 ± 0.028 in choroidal vessels (P = 0.029, paired t-test), 0.022 ± 0.017 in the retinal arterioles (P = 0.022, paired t-test), and 0.246 ± 0.067 in retinal venules (P = 0.0003, paired t-test).

The ODR can be measured noninvasively in the choroidal vessels of lightly pigmented individuals and is significantly lower in choroidal vessels than in retinal arterioles. This may suggest higher oxygen saturation but is also compatible with the reduced contrast of choroidal vessels at both wavelengths that is expected from scattering of light within the choroid. The decrease of ODR during hyperoxia was significant for all vessel types, which confirms that the oximeter is sensitive to changes in oxygen saturation in both choroidal and retinal vessels.