Can variability in corneal metabolism explain the variability in corneal swelling?

PURPOSE: To examine if the variability in hypoxic corneal swelling in non-contact lens wearing and contact lens wearing groups can be explained by the variability in corneal metabolic activity or endothelial function. METHODS: Corneal thickness was measured by Orbscan before and after 2 hr of closed-eye hydrogel lens wear. De-swelling rates (percentage recovery per hour) were determined by continued corneal thickness measurements until open-eye steady-state thickness was reached. Corneal oxygen consumption was estimated by measuring tear oxygen tension while wearing hydrogels of known oxygen transmissibility. The change in stromal pH because of hypoxia was measured by ratio fluorescence of stromal fluorescein. RESULTS: Corneal swelling was inversely correlated with open- or closed-eye tear oxygen tension while wearing the test hydrogels. Corneal swelling was directly correlated with the change in stromal pH during hypoxic lens wear. Unexpectedly, corneal swelling was also inversely correlated with the percent recovery per hour. These parameters could not explain all the swelling variability. The contact lens wearing group showed significantly less swelling and change in stromal pH with the same lenses. However, percent recovery per hour and tear oxygen tension was the same. CONCLUSION: Corneal swelling is associated with corneal metabolic activity and endothelial function; however, other factors must also be involved. Contact lens wearers do not adapt by modifying oxygen consumption but probably by using glucose more efficiently.

Estimation of human corneal oxygen consumption by noninvasive measurement of tear oxygen tension while wearing hydrogel lenses.

PURPOSE: To devise a procedure for direct estimation of corneal oxygen consumption in human subjects. METHODS: Tear oxygen tension (PO2) was measured at the posterior surface of two standard hydrogel contact lenses (38% water, 0.2 and 0.06 mm thick, oxygen transmissibility [Dk/t] = 4.2 and 14 x 10(-9) cm x mL O2/mL x sec x torr) and one newly available hydrogel-silicone polymer lens (Dk/t = 99 x 10(-9)). The oxygen-sensitive dye, Pd-meso-tetra (4-carboxyphenyl) porphine, bound to bovine serum albumin, was incubated with the lenses overnight. The lenses, coated with the protein-dye complex, were placed on four subjects' eyes, and tear PO2 was measured in the open eye and after 5 minutes of eye closure, using a time-domain phosphorescence measurement system. Given the tear PO2, lens Dk/t, and corneal thickness, oxygen consumption (Q(C), in mL O2/cm(3) x sec) could be calculated from established oxygen diffusion models. RESULTS: Protein-dye complex bound to the lens surface enabled reporting of tear PO2 for long periods. As expected, estimated tear PO2 was higher in subjects wearing lenses with higher Dk/t: mean open-eye PO2 = 30.6 +/- 3.1 and 8.1 +/- 1.3 torr for the thin and thick hydrogel lenses, respectively, and 97.6 +/- 22.9 torr for the hydrogel-silicone lens. After 5 minutes of eye closure, tear PO2 was significantly reduced and reached a new steady state in approximately 20 seconds after eye opening. Fitting a single exponential model to the data and extrapolating to t = 0 provided an estimate of PO2 under the closed lid for the thin hydrogel (PO2 = 7 +/- 2.3 torr) and the hydrogel-silicone lens (PO2 = 22.6 +/- 4 torr). After 5 minutes of eye closure with the thick hydrogel lens, tear PO2 remained constant for approximately 10 seconds after eye opening (mean PO2 = 3.9 +/- 0.7) before increasing to a new steady state. This delay could be accounted for by the time needed for oxygen to diffuse to the posterior surface of the lens. Calculated Q(C) ranged from 2.2 x 10(-4) to 3.7 x 10(-6) mL O2/cm(3) x sec) at the highest and lowest PO2s, respectively, and is comparable to previous in vitro and in vivo estimates. CONCLUSIONS: Tear PO2 behind hydrogel lenses can be measured in human subjects using the phosphorescence of the porphyrin-protein complex bound to the lens surface. The method is simple, fast, reliable, and noninvasive, allowing quick and direct estimates of Q(C). In addition to contact lens wear, this method should be useful for examining the effects of disease, surgery, or topical drugs on the corneal oxygen consumption rate.