The relationship between corneal oxygen tension and hypoxic corneal edema.

PURPOSE

We sought to develop a direct, noninvasive measurement of tear oxygen tension behind soft contact lenses, and to use the tear oxygen tension data to determine if subjects with greater comeal oxygen consumption experience more comeal edema under hypoxic conditions. The relationship between tear oxygen tension and comeal thickness was also analyzed to determine if thicker comeas have higher metabolic rates.

METHODS

Tear oxygen tension measurement was based on the quenching of a phosphorescent dye by oxygen. By coating a soft contact lens with the dye and placing it in the eye, the rate of decay of phosphorescence could be measured and the tear oxygen tension calculated. Corneal oxygen consumption could then be calculated from the tear oxygen tension. Central corneal swelling was measured after 3 hours of hypoxia induced by contact lens wear under closed-eye conditions.

RESULTS

Seven right eyes of seven subjects were tested. Baseline central corneal thickness ranged from 517 to 616 microns. After 3 hours of hypoxia, central corneal swelling ranged from 24.5 to 74.3 microns (3.9% to 13.4%). Steady-state open-eye tear PO2 behind the lens varied between 42.7 and 67.1 torr Pearson correlation suggested possible inverse relationship between open-eye tear oxygen tension and central comeal swelling during hypoxia (r = -0.711, p = 0.0734).

CONCLUSIONS

Phosphorescent-based measurements provided a measure of tear oxygen tension in human beings. Low tear film oxygen tensions (high corneal metabolic demand) appeared to predict greater amounts of swelling in subjects' corneas. No relation was found between corneal thickness and tear oxygen tension, suggesting that epithelial and endothelial metabolic rates are more important than stromal thickness in determination of comeal oxygen demand.