Hydrogel Contact Lenses Impede Carbon Dioxide Efflux from the Human Cornea

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Abstract
A carbon dioxide-sensitive probe applied directly to the anterior corneal surface was used to investigate whether hydrogel contact lenses act as a barrier to carbon dioxide efflux from the human cornea. The effect of increasing periods of eye closure on carbon dioxide accumulation at the anterior corneal surface was also examined. A calibration curve to convert the CO2 probe output to equivalent carbon dioxide partial pressure (pCO2) was determined by measuring the slope of the probe output obtained after 10 minutes exposure of the corneas of three human subjects to gas mixtures containing CO2 at different nominal partial pressures. The equivalent pCO2 at the anterior corneal surface was subsequently determined for two human subjects following 10 mintues wear of hydrogel contact lenses (HEMA; 38% water content) of various center thicknesses. All lens thicknesses used (0.035 to 0.30mm) produced a significant accumulation of CO2 under the contact lens. A maximum pCO2 of approximately 50mmHg was reached with lenses of 0.1mm or greater center thickness. With short periods of eye closure, CO2 accumulated rapidly at the anterior corneal surface, reaching a steady level of approximately 45mmHg after 10 minutes of eye closure. Our results suggest that hydrogel contact lenses provide a significant barrier to carbon dioxide efflux from the cornea, the effect increasing with lens thickness. We suggest that the consequent chronic decrease in stromal pH, particularly during extended wear, may contribute to both short and long-term contact lens-induced changes in corneal endothelial cell morphology.