Oxygen consumption of the cornea (Q_C) is typically measured in vitro by placing a dissected cornea into a sealed chamber and measuring the rate of depletion of oxygen in the chamber. In order to determine the variability of Q_C in human subjects, it is necessary to develop a non-invasive technique. Using the phosphorescence quenching of oxygen sensitive dyes, we have developed a simple method for measuring tear oxygen tension while a subject is wearing a contact lens. Figure 1 shows the rate of phosphorescence decay obtained from dye placed in the tears of a rabbit. Figure 2 shows a schematic of the phosphorescence measurement system.

Figure 1. Phosphorescent decay of 500 µM Pd-coproporphyrin complexed with 1% bovine serum albumin. Fifteen µl of solution was placed in the tears. High oxygen is dye solution directly exposed to the air. Low Oxygen is dye solution underneath a PMMA contact lens. Oxygen tension is related to the decay constant of each curve and can be calculated using the Stern-Volmer equation.

Figure 2. Schematic of the phosphorescence measurement system. The light source is a xenon flash triggered by a counter-timer board under program control using a PC. The same trigger initiates both flash discharge and data collection. The flash emission is band pass filtered and focused onto a bifurcated fiber optic bundle. The common end of the fiber optic delivers the flash to the sample and collects the resulting phosphorescent emission. This emission is filtered and captured by a photomultiplier. The signal is amplified and digitized at 1 MHz using a 12 bit Analog-to-Digital converter.

Figure 3 indicates the change in tear oxygen tension in a human subject's eye following eye closure for ten minutes.

Figure 3. Tear oxygen tension under a -6.50D Optima 38 contact lens in a human subject. After lens was inserted, the eye was closed for 10 minutes. Measurements were started as the eye was opening. Simple exponential fit indicated that the starting oxygen tension was 3.9 torr, the steady-state open eye tension was 17.5 torr and t1/2=14 sec.

Because of the oxygen consumption of the cornea, there is a distribution of oxygen tension across the cornea (see Figure 4). The oxygen tension (PO₂) at the surface of the cornea is determined by the oxygen transmissibility (Dk/L) of the contact lens being worn and the Q_C of the cornea (Dk is lens permeability and L is lens thickness). For the same contact lens being worn, a lower tear PO₂ would indicate a higher Q_C for that subject. Thus given the tear PO₂ under a contact lens of known Dk/L, it is possible to estimate Q_C. Q_C (ml O₂/ml s) is calculated from the following equations:

(1)

(2)

(3)

Dk_CL/L_CL is the oxygen transmissibility of the contact lens. P_ant is the oxygen tension at the anterior surface of the lens and is generally assumed to be 155 torr in the open eye and 55 torr in the closed eye. Open eye P_ant is simply the oxygen tension in the atmosphere at sea level. While this would be punctuated by blinking, which is very short (~10 ms) compared to the interblink interval (5-10 s), using a time averaged PO₂ of 155 torr is reasonable. The closed eye P_ant is taken from the average for capillary PO₂ in palpebral conjunctiva.

Figure 4. Illustration of hypothesis that corneal swelling and acidosis while wearing a contact lens are related to individual QC, shown in a schematic cross-section of the cornea. Lines were generated from oxygen diffusion models for two corneas, one having an average QC and one with 30% higher QC, wearing a CL (Dk/t=125x10-9) with the eyes closed. Note that the epithelium is significantly more hypoxic and the stroma anoxic in the cornea with the high QC. Our hypothesis predicts that the individual with the higher QC will show more stromal acidosis as indicated by the lower pH. Since lactate and protons are produced in equal amounts, we predict that the cornea with the higher QC will also show more corneal swelling.

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Contact Dr. Bonanno at: IU School of Optometry 800 E. Atwater Ave. Bloomington, IN 47405-3680 USA Phone: 812-856-5977 Fax: 812-855-7045 E-mail: jbonanno@indiana.edu

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	URL: http://www.opt.indiana.edu/people/faculty/bonanno/oxygen.htm Revised: April 19, 2004
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