MEASURING OXYGEN-TENSION IN THE ANTERIOR-CHAMBER OF RABBITS

PURPOSE. Measuring the concentration of oxygen in the aqueous humor wi thout penetrating the eye would provide a new dimension in understandi ng aqueous humor and corneal dynamics. In this study a preinvasive met hod was developed for determining the cameral oxygen concentration in anesthetized rabbits by measuring the excited-state lifetime of a phos phorescent dye. METHODS. A scanning ocular fluorometer was designed to excite phosphorescence with a brief flash of light and to measure the decay of luminescence for as long as 1000 mu sec after excitation. Th e measurement window was scanned through the depth of the anterior cha mber or fixed at the mid-anterior chamber. A depot of the phosphoresce nt dye Pd-uroporphyrin was injected into the vitreous of eight pigment ed rabbits, and within a few days the dye was measurable in the anteri or chamber. The excited-state lifetime of this dye is inversely correl ated to oxygen concentration and was calibrated by measuring the lifet ime of dye in cuvettes equilibrated with oxygen-nitrogen mixtures. Oxy gen tensions were determined from lifetimes measured in the open eye, under a polymethylmethacrylate (PMMA) contact lens, under two oxygen-p ermeable contact lenses, and immediately after lid closure. RESULTS. O xygen tension in the mid-anterior chamber before placing a PMMA contac t lens was 23 +/- 3 mm Hg (mean +/- SD; n = 6). After 20 minutes of PM MA lens wear, oxygen tension decreased to 4 +/- 2 mm Hg. When the foca l diamond was scanned through the anterior chamber, oxygen tension was 24 +/- 5 mm Hg near the corneal endothelium and decreased to 17 +/- 8 mm Hg near the crystalline lens. Under the PMMA contact lens this gra dient reversed: Oxygen tensions near the endothelium and lens were 3 /- 2 mm Hg and 6 +/- 2 mm Hg, respectively. Lid closure for 10 minutes or longer decreased the mid-anterior chamber oxygen tension from 21 /- 2 mm Hg (n = 19 measurements from seven animals) to 10 +/- 3 mm Hg (n = 15 measurements from five animals). CONCLUSIONS. Measuring excite d-state lifetime of phosphorescent dyes in the anterior chamber provid es a useful method for determining oxygen concentration in vivo, witho ut penetrating the eye. Cameral oxygen tension under PMMA contact lens es are significantly lower than in the uncovered eye. The profile of o xygen tension through the anterior chamber suggests that oxygen is sup plied transcorneally to the aqueous humor.