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Editors Selection IGR 24-1
Aqueous Humor: Oxygen tension
Comment by Carla Siegfried on:
51682 Aqueous Oxygen Tension in Glaucomatous and Nonglaucomatous Eyes, Sharifipour F; Yazdani S; Pakravan M et al., Journal of Glaucoma, 2013; 22: 608-613
The role of intraocular oxygen tension (pO2) has been investigated in recent years in light of both theories of ischemia in glaucomatous optic neuropathy, as well as a potential source of oxidative stress for the outflow pathways and the retinal ganglion cells. Sharifipour et al. compared aqueous oxygen tension in eyes with various types of glaucoma (primary open-angle, pseudoexfoliation and neovascular) to non-glaucomatous eyes with senile cataracts prior to initiation of surgery. Enrolled subjects were not receiving any supplemental inhaled oxygen during the sampling of 0.2 ml of aqueous humor and 1 ml of arterial blood drawn into heparinized plastic syringes. Oxygen, carbon dioxide and pH of the samples were measured using a blood-gas analyzer. They noted that aqueous pO2 was higher than arterial pO2 in all study groups except primary openangle glaucoma eyes, suggesting a contribution of atmospheric oxygen. A significant negative correlation was observed between IOP and aqueous pO2.
As the authors note, a variety of techniques have been used to measure aqueous oxygen tension-polarographic oxygen (Clark) electrodes and fiberoptic oxygen sensors as well as blood-gas analyzers. There are several issues of concern regarding the technique of extracting aqueous humor for oxygen measurements- particularly air contamination and metabolic consumption of oxygen. Of particular relevance to air contamination is the type of syringes used for transporting samples. Plastic, with increased pore size and density, serves as a poor barrier to oxygen as compared to glass, especially with delays for analysis and transfer of samples into the blood gas analyzer. The use of a blood gas analyzer has not been validated as an instrument for aqueous humor measurements and their results are significantly higher than other studies. In addition, placement of aqueous humor specimens on ice, routinely performed for blood specimens, may also falsely increase the pO2 measurement.
Due to these important issues of measurement technique, it is difficult to interpret the overall conclusions of this study correlating IOP and aqueous pO2 with support for the role of ischemia in glaucoma development. Additionally, neovascular glaucoma (ischemic), was associated with high pO2 and high IOP, not supporting the author's theory. Further studies of the potential role of pO2 in the development of glaucoma require validation of technique with known controls, especially as such a broad range of oxygen tension levels have been described in the literature.Comments
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