Considerations in Contact Lens Use Under Adverse Conditions Proceedings of a Symposium (1991) / Chapter Skim
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Environmental Gases and Contact Lens Wear
Environmental Gases and Contact Lens Wear
Pages 3-13
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Without oxygen from the air, corneal swelling will occur, with resultant corneal clouding, decreased visual acuity, discomfort, and other long-term adverse effects. The cornea requires about 5 pi O2/mm2 cornea/hour (Hill and Fatt, 1963; Larke et al., 1981~.
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~ ~ I 20 _ O 0 20 I I I STEAK ~ 2 31 1 ~ ~ ~ I ~ 1 1 1 1 ~! I. I I I I 1 1 1 I ~ 1 1 ~ ~ 1 1 ~ I 1 1 1 1 1 1 '1 40 60 80 120 100 140 160 180 200 1 TIME - SECONDS FIGURE 1 Oxygen tension under a rigid PMMA lens with different blink rates.
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Another important factor is how much of the required oxygen does the available contact lens materials provide. In terms of oxygen percentage to prevent corneal changes, values from 1.5 percent up to 18 percent have been quoted (see Table 1~.
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Table 2 gives the Dk/L values for different water contents and lens thicknesses. TABLE 1 Estimates of the Critical Oxygen Requirements (COR)
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of hydrogel lenses of various specific water contents (expressed in percentages)
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Dk/L 38% 38% 38% 55% 55% 55% 75% 75% 75% 0.035 0.06 0.12 0.035 0.06 0 12 0.06 0.12 0.20 21.1 12.3 6.2 44.7 26.1 13.0 63.1 31.5 18.9 If water is lost from the lens due to dehydration, the oxygen transmissibility will decrease accordingly. If new hydrogel materials are developed that do not depend solely on water content for oxygen diffusion, the hydrogel lenses will allow for extended wear without the present physiological problems.
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With eye closure for 20 minutes, the pH dropped to 7.39 and returned to 7.54 within 10 to 15 minutes of eye opening. With a gas mixture in a goggle consisting of 6.7 percent CO2, 7.1 percent O2, and N2, the pH dropped to 7.29.
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FIGURE 5 Carbon dioxide partial pressure at the anterior corneal surface immediately following short periods of eye closure. SOURCE: Holden et al.
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0.05 0.10 0.1 5 0.20 0.25 0.30 LENS CENTER THICKNESS (mm) 11 FIGURE 6 Carbon dioxide partial pressure at the anterior corneal surface immediately following 10 minutes wear of hydrogel lenses of venous center thicknesses.
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Mertz 1984 Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Investigative Ophthalmology 25~10~:1161-1167.
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Ross, and J Jenkins 1987 Hydrogel contact lenses impede carbon dioxide efflux from the human cornea.
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