Relating Vis-OCT Measurements of Retinal SO2 to Microelectrode Measurements of PO2

Abstract

Purpose : To determine how much of the O₂ from the retinal circulation supplies the outer retina rather than the inner retina by comparing measurements of retinal oxygenation via both visible light optical coherence tomography (vis-OCT) and O₂-sensitive microelectrodes in rat retina. The inner retinal metabolic rate of oxygen (IR-MRO₂) is the amount of oxygen extracted from the retinal circulation. It can be obtained by multiplying retinal blood flow by the difference in O₂ saturation (sO₂) in retinal arteries and veins. IR-MRO₂ is generally larger than the actual O₂ utilization of the inner retina (IR-QO₂), because some O₂ extracted from the retinal circulation supplies the avascular outer retina (R-OR-QO₂). Subtracting R-OR-QO₂ from IR-MRO₂ yields the amount of oxygen used by the inner retina, IR-QO₂.

Methods : Adult Long-Evans rats, anesthetized with IV ketamine/xylazine were used in this study. We measured IR-MRO₂ with vis-OCT. We used a dual-circular scan pattern 1.5 mm in diameter centered around the optic nerve head, acquiring 100 B-scans of 8192 A-lines each. We averaged sO₂ and summed retinal blood flow across all arteries and all veins to obtain arterial and venous sO₂, and total retinal blood flow. Following vis-OCT imaging, in the same animals, O₂ microelectrodes were used to measure gradients of PO₂ across the retina. Fitting a previously developed diffusion model to the gradients allowed us to obtain the amount of O₂ used by the outer retina (OR-QO₂) and the fraction supplied by the retinal circulation (R-OR-QO₂). Measurements of blood PO₂ ensured that conditions were the same in both types of measurements.

Results : We calculated OR-QO₂ and R-OR-QO₂ under normoxic conditions. OR-QO₂ and R-OR-QO₂ were higher under dark-adapted conditions than light adaptation, as expected. In the dark-adapted retina, OR-QO₂ is roughly equivalent to the IR-MRO₂ value of 3.3 mlO₂-min^-1-100g^-1, with the outer retina receiving approximately 15% of the total oxygen extracted from the retinal circulation. Thus, IR-QO₂ is about 2.8 mlO₂-min^-1-100g^-1.

Conclusions : Under normoxic, dark-adapted conditions, approximately 15% of the oxygen extracted from the retinal circulation supplies the outer retina. Therefore, it is important to apply a correction when using IR-MRO₂ as a surrogate for IR-QO₂.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.