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

A method for noninvasive measurement of Hb O-2 saturation (So(2)) in retinal blood vessels by digital imaging was developed and tested. Images of vessels were recorded at O-2-sensitive and O-2-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 So(2) was varied experimentally by having subjects breathe mixtures of O-2 and N-2 while systemic arterial So(2) 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 Set. Compensation for the influence of choroidal pigmentation significantly reduced variation in the arterial Sol measurements among subjects. An Oa sensitivity of 0.00504 +/- 0.00029 (SE) ODR units/%So(2) was determined. Retinal venous So(2) at normoxia was 55 +/- 3.38% (SE). Breathing 100% O-2 increased venous So(2) by 19.2 +/- 2.9%. This technique, when combined with blood flow studies in human subjects, will enable the study of retinal Oa utilization under experimental and various disease conditions.