Rivera R K, Polse K A
Morton D. Sarver Laboratory for Contact Lens and Corneal Research, School of Optometry, University of California, Berkeley, USA.
Optom Vis Sci. 1996 Mar;73(3):178-83. doi: 10.1097/00006324-199603000-00009.
It has been assumed that contact lens wear (CLW) may induce stromal acidosis, which is a result of corneal hypoxia and the accumulation of CO2 (hypercapnia) at the tear-lens interface. However, it has not been directly shown whether hypoxia and hypercapnia are the only causes of CL-Induced corneal acidification. In this study, we provide preliminary data about the relative contributions of hypercapnia and hypoxia to CL-induced stromal acidification by monitoring pH while the cornea was exposed to a hyperbaric oxygen atmosphere. This paradigm minimized if not eliminated the pH effects of lens-induced hypoxia on all but one subject without altering the pH effect of hypercapnia. Seven subjects were fitted with hydrogel lenses; 5 with low O2 transmissibility (Dk/LO2 = 14.0 x 10(-9) (cm/s) (ml O2/[ml x mm Hg)]), and 2 with medium O2 transmissibility (Dk/LO2 = 17.2 x 10(-9) (cm/s) (ml O2/[ml x mm Hg])) lenses. After lens insertion, modified goggles were fitted to control the corneal environment by exposing 1 eye to 20%O2 and 80%N2 (air), and the contralateral eye to 80%O2 and 20%N2 (hyperbaric O2). Corneal thickness (CT) was measured before CL insertion and over 120 min of wear. We assumed that corneal hypoxia was present if CT increased during the test period. Stromal pH was measured using a slitlamp fluorophotometer before lens insertion and at 20-min intervals for a total of 80 min. After 80 min of wearing the low Dk/L lens under hyperbaric exposure, 4 of 5 subjects showed reduced pH (mean delta pH = 0.23 +/- 0.05) and no increase in CT, suggesting that only hypercapnia was contributing to acidosis. For the same lens, but with exposure to air, 4 of 5 subjects showed a larger drop in pH (mean delta pH = 0.62 +/- 0.48) compared to hyperbaric exposure and an increase in CT, indicating that both hypoxia and hypercapnia reduced pH. Subjects wearing the medium Dk/L lens showed a small but equal drop in pH under both air and hyperbaric conditions without changes in CT, suggesting that only hypercapnia was contributing to acidosis. These preliminary results suggest that both mechanisms contribute to the pH shift accompanying CLW and that the contribution of hypercapnia is approximately 30%. Finally, the effects of hypercapnia and hypoxia are dependent on individual metabolic requirements and the transmissibility of the lens to O2 and CO2.
人们一直认为，佩戴隐形眼镜（CLW）可能会导致基质酸中毒，这是角膜缺氧以及泪液 - 镜片界面处二氧化碳积聚（高碳酸血症）的结果。然而，尚未直接表明缺氧和高碳酸血症是否是隐形眼镜诱发角膜酸化的唯一原因。在本研究中，我们通过在角膜暴露于高压氧环境时监测pH值，提供了关于高碳酸血症和缺氧对隐形眼镜诱发基质酸化相对贡献的初步数据。这种模式即使没有消除，也最小化了镜片诱发的缺氧对除一名受试者之外的所有受试者的pH值影响，同时没有改变高碳酸血症的pH值影响。七名受试者佩戴水凝胶镜片；5人佩戴低氧透过率（Dk/LO2 = 14.0×10⁻⁹（cm/s）（ml O2/[ml×mmHg]））的镜片，2人佩戴中等氧透过率（Dk/LO2 = 17.2×10⁻⁹（cm/s）（ml O2/[ml×mmHg]））的镜片。插入镜片后，佩戴改良护目镜，通过将一只眼睛暴露于20%O2和80%N2（空气）中，对侧眼睛暴露于80%O2和20%N2（高压氧）中来控制角膜环境。在插入隐形眼镜之前以及佩戴120分钟期间测量角膜厚度（CT）。我们假设如果在测试期间CT增加，则存在角膜缺氧。在插入镜片之前以及总共80分钟内每隔20分钟使用裂隙灯荧光光度计测量基质pH值。在高压暴露下佩戴低Dk/L镜片80分钟后，5名受试者中有4名显示pH值降低（平均ΔpH = 0.23±0.05）且CT没有增加，这表明只有高碳酸血症导致了酸中毒。对于同一镜片，但暴露于空气中时，5名受试者中有4名显示与高压暴露相比pH值下降幅度更大（平均ΔpH = 0.62±0.48）且CT增加，表明缺氧和高碳酸血症都降低了pH值。佩戴中等Dk/L镜片的受试者在空气和高压条件下均显示pH值有小幅度但相等的下降，CT没有变化，这表明只有高碳酸血症导致了酸中毒。这些初步结果表明，这两种机制都导致了与隐形眼镜佩戴相关的pH值变化，并且高碳酸血症的贡献约为30%。最后，高碳酸血症和缺氧的影响取决于个体的代谢需求以及镜片对O2和CO2的透过率。
