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C.J. Radke, M. Chhabra; Minimum Contact Lens Oxygen Transmissibility (Dk/L) With Monod Kinetics for the Corneal Oxygen Consumption Rate . Invest. Ophthalmol. Vis. Sci. 2005;46(13):904.
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Purpose:To predict the minimum contact lens transmissibility needed to avoid corneal edema in open and closed eye conditions. Methods: A 4–layer (3 layers of cornea, epithelium, stroma and endothelium, and a contact lens) diffusion–reaction model was used to calculate the steady oxygen partial pressure (oxygen tension) profile in the cornea–lens system. Previous estimates of the minimum Dk/L adopt a constant oxygen consumption rate yielding aphysical negative oxygen tensions. As reported by Fatt et al, oxygen consumption must decrease to zero when oxygen tension approaches zero. This metabolic behavior is rigorously described by classical Monod kinetics. Here the Monod expression is used for oxygen consumption rates in each layer of the cornea. Necessary parameters (oxygen permeabilities, thicknesses, maximum oxygen consumption rate, and oxygen tensions at the boundaries) were obtained from accepted literature values. With Monod kinetics, the oxygen tension no longer falls to zero. Hence, we adopt a criterion for the minimum oxygen transmissibility of the contact lens as the critical oxygen tension needed at the anterior corneal surface to avoid corneal edema. Results: The minimum Dk/L of the contact lens was found to be 9 and 29 Barrer/cm in open and closed eye conditions, respectively. These values are lower than the corresponding experimentally observed values of 24 and 87 Barrer/cm by Holden and Mertz. Acidosis in the cornea is known to increase the maximum oxygen consumption rate, and apparently, must be accounted for. Although Monod kinetics for oxygen consumption correctly avoids the aphysical negative oxygen partial pressures in the cornea, a more complete metabolic model of corneal respiration seems necessary. Such a model must include coupled transport of glucose, lactate and hydrogen ions, and carbon dioxide. Conclusions:Adoption of Monod kinetics for oxygen consumption predicts the minimum Dk/L of a contact lens to be lower than the experimentally observed values to avoid corneal edema. However, the effect of acidosis on oxygen consumption rate is not included in our analysis. A more rigorous metabolic model of oxygen diffusion and reaction in the cornea is called for.
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