Oxygen: how well is the closed eye being served?

While the environment of the naturally closed eye substantially challenges the oxygen-dependent processes of the cornea, the presence of a contact lens further exacerbates those stresses. The purposes of this study are: (1) to describe responses of the human cornea under closed eye conditions to a wide range of Dk/t environments as ratios of their corresponding normal, open-eye (physiological) baseline rates; (2) to describe the difference ("hypoxic gap") function that separates those closed-eye responses from parallel responses for the open eye; and (3) to present predictive models for all three (closed-eye, open-eye, and difference) response functions in both graphical and mathematical forms.

Oxygen uptake rates were measured polarographically: for the normal open eye and after both closed- and open-eye wear of six rigid contact lenses of transmissibilities ranging from 0 to 91 x 10(-9) (cm/sec)(ml O2/ml mmHg).

Responses from the closed-eye and open-eye series to those six Dk/t levels were compared both graphically and statistically. Those response series were described by best-fit equations, from which a model for Dk/t effectiveness in meeting corneal oxygen needs under both closed- and open-eye conditions was developed.

Response series for both closed and open eyes were found to be well described by natural log equations (R2 values > 0.99). Statistically, separation of those two series could be demonstrated (p < 0.05) by the Dk/t level of 12.8 x 10(-9) (cm/sec)(ml O2/ml mmHg). While, by the highest Dk/t level observed here of 91 x 10(-9) (cm/sec)(ml O2/ml mmHg), corneal responses for the open eye have already moderated down to just 1.5x the "ideal standard," closed-eye response rates are still averaging 2.7x that baseline. The difference ("hypoxic gap") function remaining between those closed- and open-eye curves appears to be stabilizing in its magnitude by 91 x 10(-9) (cm/sec)(ml O2/ml mmHg) as well.