Effect of Carbogen on Retinal Oxygenation in Anesthetized Cats

Abstract

Abstract: : Purpose: Inhalation of carbogen (95% O₂ /5% CO₂) and 100% oxygen has been used for several diagnostic and therapeutic purposes. This work assessed the effects of carbogen on intraretinal oxygenation in comparison with hyperoxia (100% oxygen). Methods: Double-barreled oxygen microelectrodes were used to measure the distribution of oxygen in the central retina of dark-adapted anesthetized cats during normoxia, hyperoxia, and carbogen-breathing. Profiles of oxygen tension (PO₂) as a function of retinal depth were recorded. The outer retinal portions of the profiles were fitted to a three-layer diffusion model to extract outer retinal oxygen consumption values. The average value of inner retinal PO₂ was also computed for each profile. In addition, changes in PO₂ caused by the transient administration of carbogen and 100% oxygen were measured at selected retinal depths. Comparisons were made between normal, hyperoxia, and carbogen-breathing conditions. Results: In normal animals, the shape of PO₂ profiles indicated that the retinal and choroidal circulations both supply the retina with oxygen. During hyperoxia, the average arterial PO₂ was 431.48±15.66 mmHg (average±SD), while during carbogen it was 403.24±23.20 mmHg. The corresponding pH values were 7.40±0.01 and 7.18± 0.04. During hyperoxia and carbogen-breathing, the profile shape was more monotonic, decreasing from the choroid to the vitreous, and the inner retinal and vitreal gradients showed that the choroid supplied oxygen to the inner retina. There was a tendency for both the inner retinal and choroidal PO₂ to be higher during carbogen breathing than during hyperoxia. During the transient administration of carbogen and 100% oxygen, average changes in PO₂ generally increased with depth. Conclusions: The administration of carbogen appears to increase PO₂ in the retina in comparison to 100% oxygen, varying with retinal depth. The vasodilatory mechanisms of carbon dioxide probably are involved in the increased retinal oxygenation.

Keywords: retina • blood supply