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

Abstract

A method for noninvasive measurement of Hb O2 saturation (So 2) 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 So 2 was varied experimentally by having subjects breathe mixtures of O2 and N2 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 So 2. Compensation for the influence of choroidal pigmentation significantly reduced variation in the arterial So 2measurements among subjects. An O2sensitivity of 0.00504 ± 0.00029 (SE) ODR units/%So 2was determined. Retinal venous So 2 at normoxia was 55 ± 3.38% (SE). Breathing 100% O2 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 O2 utilization under experimental and various disease conditions.