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Estimation of Human Corneal Oxygen Consumption by Noninvasive Measurement of Tear Oxygen Tension While Wearing Hydrogel Lenses

¹ From the Borish Center for Ophthalmic Research, Indiana University, School of Optometry, Bloomington, Indiana; and the ² Eye Physiology and Ocular Prosthetics Laboratory, School of Optometry, University of Alabama at Birmingham, Birmingham, Alabama.

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 · mL O₂/mL · sec · 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 PO₂was 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/cm³ · 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 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/cm³ · 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.