Diffusion and Monod kinetics model to determine in vivo human corneal oxygen-consumption rate during soft contact lens wear / Modelo de difusión y cinética Monod para la determinación 'in vivo' de la tasa de consumo de oxígeno de la córnea en pacientes con lentes de contacto blandas

Purpose: We present an analysis of the corneal oxygen consumption Qc from non-linear models, using data of oxygen partial pressure or tension (pO2 ) obtained from in vivo estimation previously reported by other authors.1 Methods: Assuming that the cornea is a single homogeneous layer, the oxygen permeability through the cornea will be the same regardless of the type of lens that is available on it. The obtention of the real value of the maximum oxygen consumption rate Qc,max is very important because this parameter is directly related with the gradient pressure profile into the cornea and moreover, the real corneal oxygen consumption is influenced by both anterior and posterior oxygen fluxes. Results: Our calculations give different values for the maximum oxygen consumption rate Qc,max, when different oxygen pressure values (high and low pO2 ) are considered at the interface cornea-tears film (AU)

Objetivo: Presentamos un análisis del consumo de oxígeno de la córnea Qc a partir de modelos no lineales, utilizando los datos de presión parcial del oxígeno (pO2) obtenidos de la estimación in vivo obtenidas previamente por otros autores.1 Métodos: Asumiendo que la córnea es una capa homogénea única, la permeabilidad del oxígeno a través de la misma será la misma, independientemente del tipo de lentes que se utilicen sobre ella. La obtención del valor real de la tasa máxima de consumo de oxígeno Qc,max, es muy importante porque este parámetro está directamente relacionado con el perfil del gradiente de presión dentro de la córnea y, además, el consumo real de oxígeno de la córnea está influenciado tanto por los flujos de oxígeno anterior como posterior. Resultados: Nuestros cálculos aportan diferentes valores para la tasa de consumo máximo de oxígeno Qc,max, al considerar diferentes valores de la presión del oxígeno (pO2 elevado y bajo) en la interfase entre la córnea y la película lagrimal. Conclusión: Estos resultados son relevantes para calcular la presión parcial del oxígeno, que se encuentra en diferentes profundidades dentro del tejido de la córnea por detrás de las lentes de contacto con diferente transmisibilidad al oxígeno (AU)

Diffusion and Monod kinetics model to determine in vivo human corneal oxygen-consumption rate during soft contact lens wear.

PURPOSE: We present an analysis of the corneal oxygen consumption Qc from non-linear models, using data of oxygen partial pressure or tension (P(O2) ) obtained from in vivo estimation previously reported by other authors. (1) METHODS: Assuming that the cornea is a single homogeneous layer, the oxygen permeability through the cornea will be the same regardless of the type of lens that is available on it. The obtention of the real value of the maximum oxygen consumption rate Qc,max is very important because this parameter is directly related with the gradient pressure profile into the cornea and moreover, the real corneal oxygen consumption is influenced by both anterior and posterior oxygen fluxes. RESULTS: Our calculations give different values for the maximum oxygen consumption rate Qc,max, when different oxygen pressure values (high and low P(O2)) are considered at the interface cornea-tears film. CONCLUSION: Present results are relevant for the calculation on the partial pressure of oxygen, available at different depths into the corneal tissue behind contact lenses of different oxygen transmissibility.

Oxygen diffusion and edema with modern scleral rigid gas permeable contact lenses.

PURPOSE: We defined the theoretical oxygen tension behind modern scleral contact lenses (CLs) made of different rigid gas permeable (RGP) materials, assuming different thickness of the tear layer behind the lens. A second goal was to show clinically the effect of the postlens tear film on corneal swelling. METHODS: We simulated the partial pressure of oxygen across the cornea behind scleral CLs made of different lens materials (oxygen permeability Dk, 75-200 barrer) and different thickness (Tav, 100-300 µm). Postlens tear film thicknesses (Tpost-tear) ranging from 150 to 350 µm were considered. Eight healthy subjects were fitted randomly with a scleral lens with a thin and a thick postlens tear layer in two different sessions for a period of 3 hours under open-eye conditions. RESULTS: The CLs with less than 125 barrer of Dk and a thickness over 200 µm depleted the oxygen availability at the lens-cornea interface below 55 mm Hg for a postlens tear film of 150 µm. For a postlens tear film thickness of 350 µm, no combination of material or lens thickness will meet the criteria of 55 mm Hg. Our clinical measures of corneal edema showed that this was significantly higher (P < 0.001, Wilcoxon signed ranks test) with the thicker compared to the thinner Tpost-tear (mean ± SD, 1.66 ± 1.12 vs. 4.27 ± 1.19%). CONCLUSIONS: Scleral RGP CLs must be comprised of at least 125 barrer of oxygen permeability and up to 200 µm thick to avoid hypoxic effects even under open eye conditions. Postlens tear film layer should be below 150 µm to avoid clinically significant edema.