Retinal Photographic Oximetry: Effects of 100% Oxygen Breathing on Retinal Oxygen Tension and Repeatability of Optical Density Ratios

Abstract

Purpose: : Optical density ratio (ODR) calculation provides non-invasive assessment of hemoglobin (Hb) oxygen saturation (SO₂) in retinal vessels. 100% oxygen breathing (hyperoxia) was conducted in healthy subjects to investigate ODR differences in oxygen tension and repeatability of ODR measurements was assessed.

Methods: : 13 healthy subjects (26.5 ± 3.8years) were evaluated before and five minutes after 100% oxygen breathing. Three measurements, focusing on the major temporal retinal vessels were obtained. The fundus was illuminated by white light, and two frequencies were quantified: 605nm (oxygen sensitive) and 586nm (oxygen insensitive) by spectrographic digital fundus oxymetry. Baseline and hyperoxic ODR in the temporal arteries and veins were compared by Wilcoxon signed rank paired comparisons. For repeatability assessment, 7 (26.5 ± 3.8years) subjects were imaged three months apart (3X each image) for a total of 54 cases. Customized software defined two measurement windows, one on the vessel and one on nearby tissue, both proportional to vessel diameter; minimum (darkest) in the vessel window and maximum (brightest) pixel values were identified. Optical density (OD) was calculated as follows: OD=ln(tissue avg/vessel avg). ODs were calculated at 5nm bandwidth wavelengths centered at 605nm and 586nm. ODR, which is proportional to Hb SO₂, was defined as OD₆₀₅/OD₅₈₆.

Results: : 100% oxygen breathing significantly increased venous ODR by 9.4% (p = 0.009). The significant difference between baseline arterial and venous oxygen tension (p= 0.0427) was also eliminated by hyperoxia. ODR repeatability was assessed by the Coefficient of Variability (CoV; standard deviation/mean) of successive measurements. CoV was 4.4% for all locations. Repeatability was better for veins (CoV=1.4%) than arteries (CoV=7.3%) and improved in anatomical locations with reproducibility <10% (CoV=1.5%).

Conclusions: : It is possible to measure changes in arterial and venous oxygen tensions in the human eye with acceptable reproducibility. Our established analysis method yields repeatable ODR values obtained in medium sized retinal vessels. These methods may enhance our ability to investigate retinal metabolism and understand its role in ocular disease.

Keywords: imaging/image analysis: clinical • oxygen • blood supply