Corneal acidosis during contact lens wear: effects of hypoxia and CO2.

The effects of tear-film hypoxia and contact lens wear on human in vivo stromal pH was tested using a non-invasive fluorometric technique. Hypoxia was produced by exposing the normal open eye to 100% nitrogen gas passed through tight-fitting goggles. Stromal pH dropped from 7.53 +/- 0.02 to 7.34 +/- 0.03 (n = 12, +/- SD) within 90 min of nitrogen gas exposure, t1/2 = 20 min. After removing the goggles, stromal pH returned to baseline in 35 min, t1/2 = 10 min. Wearing a thick hydrogel contact lens which caused a tear PO2 less than or equal to 2 mm Hg with the eyes open, reduced stromal pH from 7.55 +/- 0.02 to 7.15 +/- 0.04 (n = 12, +/- SD) in 80 min, t1/2 = 9.5 min. After removing the lens, baseline pH was reached in 40 min, t1/2 = 4.5 min. The stromal pH differences between hypoxia (N2 only) and contact lens wear were not due to differences in tear temperature between the two procedures (contact lens wear 32 +/- 1.5 degrees C, goggles 33 +/- 1.0 degrees C). However exposing the eye to 95% nitrogen-5% carbon dioxide reduced stromal pH to 7.16 +/- 0.05 (n = 7, +/- SD) in 80 min, t1/2 = 8 min, which was similar to that produced during contact lens wear. These experiments show that contact lens wear causes corneal acidosis by: (1) the production of protons from hypoxic metabolism, and (2) the accumulation of carbon dioxide behind the lens due to low lens CO2 transmissibility.