Beyond flux: Total corneal oxygen consumption as an index of corneal oxygenation during contact lens wear

Purpose. The purpose of this study is to compute total corneal oxygen consumption during contact lens wear and consider the concept as an index for describing corneal oxygenation during contact lens wear as opposed to flux, partial pressure, or Dk/t. Method. Estimates of total corneal oxygen consumption were generated using a previously described eight-layer model based on oxygen diffusion equations and using contemporary estimates of tear and corneal layer thicknesses. Relative consumption, expressed as %Q (percentage of normal consumption without contact lens wear), was also calculated for daily wear and continuous wear modes, thereby providing an index of the chronic hypoxic effect of contact lens wear. Results. Corneal oxygen consumption is calculated to converge to the same value of 44.8 nL/cm(3)/sec above a Dk/t of approximately 20 and 300 x 10(-9) (cm/sec)(mL0(2)/mL/mm Hg) for the open and closed eye scenarios, respectively. Lenses with Dk/t values of 15 and 50 x 10(-9) (cm/sec)(mL0(2)/mL/mm Hg) allow about 96% of normal long-term total oxygen consumption without a contact lens in place for daily wear and continuous wear, respectively. Conclusion. Total corneal oxygen consumption is based on physical parameters, represents a direct index of corneal oxygen metabolism and thus cellular energy (ATP) production, allows direct comparison between two lenses or a lens and nonlens-wearing state, and eliminates the ambiguity of other measures, thereby providing an attractive means of describing corneal oxygenation.