Considerations in Contact Lens Use Under Adverse Conditions Proceedings of a Symposium (1991) / Chapter Skim
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Hypoxia
Hypoxia
Pages 14-23
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From page 14... ...
The amount of oxygen available for consumption by the cells of a given tissue is determined by the driving force or partial pressure of oxygen entering the tissue. The partial pressure of oxygen, known more commonly in biology as oxygen tension, is the proportion of the total pressure exerted by a gas mixture attributable to its oxygen component; it is a direct function of the number of oxygen molecules present.
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From page 15... ...
the cornea's dominant role in contact lens wear as the principal biological substratum on which a contact lens resides in situ: and (d) the barrier a contact lens forms between the cornea and its anterior oxygen supply, which restricts oxygen flow into the cornea, thus reducing tissue oxygen tension, often enough to levels below the critical hypoxia threshold to make hypoxia probably the most common adverse ocular response associated with contact lens wear.
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17 To ._ o ca ._ P
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From page 18... ...
This difference in water content between the cornea and its surroundings produces an osmotic gradient resulting in a net influx of water flowing into the cornea. The cornea maintains its relatively dehydrated, or deturgescent, state primarily through an active pumping mechanism located in the endothelium, the single layer of cells that forms the cornea's posterior surface.
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From page 19... ...
If the cornea's atmospheric oxygen supply drops below approximately 74mmHg (Holder et al., 1984) , as frequently occurs during contact lens wear, the cornea becomes hypoxic and its epithelial cells begin to respire anaerobically.
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20 ;ca o a a o a: o v TIC o C)
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From page 21... ...
Studies of the corneal swelling response to the wearing of extended-wear lenses (Holder et al., 1983; Holden and Mertz, 1984) have shown that significant levels of corneal edema occur during the overnight wear of these lenses, but suggest that this might be a minimally tolerable situation so long as the lenses transmit enough oxygen during daytime open-eye wear to allow overnight swelling to subside to baseline levels.
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From page 22... ...
Lin 1974 Oxygen tension in the cornea are-examination. Experimental Eye Research 18:357-365.
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From page 23... ...
Sanderson 1984 The minimum precorneal oxygen tension to avoid corneal edema. Investigative Ophthalmology and Visual Science 25~4~:476~80.
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