Purpose: Tear oxygen tension beneath contact lenses, measured in a previous study, was found to be lower than mathematical model predictions. In this study, the authors developed a phosphorescence-based technique for measuring corneal O2 consumption (QO2) to determine whether errors in previous determinations of QO2 could explain the discrepancy between measured and predicted tear oxygen tension.
Methods: Corneal oxygen consumption is measured using the phosphorescence quenching of Pd-meso-tetra-(4-carboxyphenyl)porphine by oxygen. Oxygen tension of a pH 7.5 Ringer's solution is measured in an airtight chamber that holds an excised rabbit cornea, and QO2 is calculated from the decreased PO2 in the stirred chamber solution. QO2 rates are measured for the whole trephined rabbit cornea with the epithelium, the endothelium, or both, removed, which makes possible the estimation of the consumption rates of the epithelium, stroma, and endothelium.
Results: Control experiments indicated that the cornea QO2 was constant for 3 hours and that exposure to 2,4-dinitrophenol increased QO2. Mean QO2 rates (microliters of O2/cm2 per hour) were: whole cornea, QO2 = 7.53; epithelium, QO2 = 3.73; stroma, QO2 = 2.97; and endothelium, QO2 = 0.86.
Conclusions: Phosphorescence-based measurements can be used to determine the QO2 of the component layers of the cornea. Estimates of QO2 in this study were similar to previous measurements, indicating that the discrepancy of tear PO2 measures and model predictions are not a result of errors in the QO2 measurements at pH 7.5.