Measurement of in vivo human corneal stromal pH: open and closed eyes

The pH sensitive fluorescent properties of fluorescein were utilized to noninvasively measure human in vivo stromal pH in the normal open eye and following eye closure. Stromal pH after either 20 or 90 min of eye closure was 7.39 +/- 0.01 (n = 12, +/- SEM) and returned to 7.54 within 10-15 min of opening the eye, t1/2 = 3 min. Eye closure was simulated by exposing the eye to a gas mixture of 7.1% O2, 6.7% CO2, balance N2, which was passed through tight-fitting goggles. This gas mixture resulted in a steady-state stromal pH of 7.29 +/- 0.02 within 10-15 min, t1/2 = 2.2 min. The time course of the return of stromal pH to open eye levels after removal of the goggles, t1/2 = 2.3 min, was similar to that after eye opening. The extent of the pH change however, was 0.1 pH units greater with the test gas. Exposure of the eyes to 5% CO2 (CO2 concentration of blood) and balance air, produced a stromal pH of 7.38 + 0.01 (n = 6, +/- SEM), which closely matches that following eye closure suggesting that conjunctival [CO2] is 5% and is the major component controlling stromal pH when the eyes are closed.