While many have tried to pinpoint an exact value, it is now proposed that this value can’t be fixed.

摘要

Goodlaw surmised in 1948 that the cornea required an anterior supply of oxygen to maintain its physiology and transparency, but he did not quantify it.1 It is now well known that the cornea, like all human tissues, requires oxygen for normal metabolic function. The cornea being avascular, oxygen reaches this tissue primarily from the atmosphere and secondarily from the anterior chamber (aqueous humor) under open-eye conditions. When the eye is closed (during sleep), oxygen is provided both from exposure to the tarsal palpebral conjunctiva as well as from the aqueous humor.2,3With a reduced oxygen supply, the cornea is susceptible to hypoxic complications: corneal swelling (and loss of transparency), corneal stromal acidosis, epithelial punctate staining, limbal hyperemia, and endothelial polymegathism. Therefore, the minimum oxygen tension that allows “normal” oxygen consumption as a direct index of corneal oxygen metabolism is a critical clinical parameter to ascertain. To this end, Polse and Mandell published their research titled “Critical Oxygen Tension at the Corneal Surface” in 1970.4 This landmark study proposed that human corneas would swell if exposed to anterior oxygen tensions below 11 to 19 mmHg, while “normal” corneal thickness would be maintained as long as anterior corneal oxygen tension remained above this threshold. The implication was that there was a specific, well-defined, or “fixed” oxygen value in humans below which corneal metabolism would begin to suffer (leading to corneal swelling, among other complications) if only we could properly identify it. Over the next several decades, many investigators spent much time, effort, thought, and money to try to definitively quantitate that “critical” oxygen value (in mmHg or tension, it was called the “COT”). Efron and Brennan published a meta-analysis documenting and discussing many potential values,5 ranging from the original Polse and Mandell 11 to 19 mmHg metric, to Brennan et al’s finding of 137 mmHg for swelling,6 to 100 mmHg to maintain epithelial mitosis.7 In their meta-analysis, Efron and Brennan speculated that the real figure might even be the sea-level room air oxygen tension of 21% or 155 mmHg.5Quantification of the COT is important clinically as contact lens materials continue to evolve. Contact lens designers (material chemists, engineers, etc.) found it helpful over the years to have a precise and definitive oxygen goal. Once this goal is achieved in a lens design, they then typically focus on other lens characteristics such as lubricity of surfaces, anti-soiling, and anti-microbial properties as well as optics (including enhancing astigmatic and presbyopic corrections). Currently, the increasing use of modern-day scleral lenses and associated hypoxic complications has again highlighted the importance of understanding the COT.