Direct noninvasive measurement of tear oxygen tension beneath gas-permeable contact lenses in rabbits.

PURPOSE To develop a noninvasive measurement of tear oxygen tension and to determine tear oxygen tension and calculated oxygen flux rates during wear of contact lenses of varying oxygen transmissibility (Dk/L). METHODS The basis of the tear oxygen tension (PO2) measurement is phosphorescence quenching of Pd-meso-tetra-(4-carboxyphenyl) porphine by oxygen. A fiber-optic bundle, positioned approximately 2 mm from a cuvette or from the corneal surface, delivers an excitation flash (< 4 microseconds, 539 +/- 23 nm) and collects the phosphorescence emission (> 645 nm) of the dye at 1 MHz for 2 ms. Porphine (500 microM) + 1% albumin is instilled directly into the tears of a sedated rabbit, and phosphorescence is measured before and after contact lens insertion. RESULTS Phosphorescence intensity decay lifetimes are related inversely to PO2 and follow the Stern-Volmer relationship. Calibration control experiments revealed a temperature effect on lifetime of 1% per degree, dye concentration independence over a 100-fold dilution, and no significant pH effect between 6.8 and 7.9. Phosphorescence lifetimes indicated that tear PO2 beneath contact lenses reached steady state within 3 to 8 minutes. Mean steady state tear PO2 (mm Hg +/- SE; N > 7) beneath contact lenses was: polymethylmethacrylate (Dk/L = 0), tear PO2 = 0.58 +/- 0.22; Oxyflow f30 (Dk/L = 15.3), tear PO2 = 27.7 +/- 3.4; Fluorex 700 Dk/L = 30), tear PO2 = 52.5 +/- 3.8; Oxyflow 151 (Dk/L = 51.9), tear PO2 = 78.7 +/- 23.9. Steady state tear PO2 beneath an Oxyflow f30 after application of topical anesthetic and 10 minutes of lens wear was 41 +/- 1.2 mm Hg, significantly higher than without anesthetic. CONCLUSIONS Phosphorescence-based measurements allow the direct, sensitive, and noninvasive assessment of oxygen availability beneath a contact lens. Oxygen levels beneath permeable contact lenses found by direct measurement are significantly lower than what mathematical models predict. The measured values, however, are close to PO2 estimates using the equivalent oxygen percentage (EOP) technique. Tear PO2 measurements may be used to examine metabolic disturbances as shown by reduced oxygen consumption rates after application of topical anesthetic.