医学课件：角膜与氧气

1、1,OXYGEN AND THE CORNEA,2,3,4,几乎全透明 无血管,1-D1,5,空气和呼吸 air vs. breath,6,Cornea needs oxygen,Adequate oxygen Normal breath Healthy cornea Inadequate oxygen Corneal changes Illness of cornea,7,CORNEAL INTEGRITYrequires:,OXYGEN 15% - 20.9% for regular function 1.1% to prevent suppression of epithelial mi

2、tosis 8% to prevent sensitivity loss 5% to prevent glycogen depletion CO2 ELIMINATION Essential to avoid pH and metabolic changes GLUCOSE Main source: anterior chamber,8,PARTIAL PRESSURE OF OXYGENThe Standard Atmosphere,Temperature: 15 C Pressure: 760 mm Hg Water vapour pressure 15 C: 12.79 mm Hg At

3、mospheres dry component: 747.21 mm Hg Oxygen concentration: 20.95% Oxygen partial pressure (pO2): 156.54 mm Hg pO2 value commonly used: 155 mm Hg,9,Oxygen makes up about 21% of the earths atmosphere,10,CORNEAL PERMEABILITY,Water Endothelial permeability is greater than that of the epithelium Oxygen

4、Derived from the atmosphere The most important metabolite Carbon dioxide Permeability is 7X that of oxygen,94071-,90,S.PPT,OPEN EYE,OPEN EYE,55 mm Hg,O,2,O,2,O,2,O,2,O,2,CO,2,CO,2,155 mm Hg,21 礚 CO,2,/cm cornea/h,2,O,2,11,Where does the Oxygen come from?,12,角膜氧供,內皮,Descemets,上皮,泪液,角膜基质,终末血管,血管,大气,房水

5、,O2,O2,13,睁 眼,55 毫米 Hg,O,2,O2,O2,CO2,155 毫米 Hg,5L O /cm 角膜/h,2,2,21 L CO2 /cm 角膜/h,2,O,2,14,闭眼,O2,CO2,15,OXYGEN SUPPLY TO THE EYE,Atmosphere Ophthalmic artery Anterior chamber Limbus Palpebral conjunctiva,16,SOURCES OF CORNEAL OXYGEN,17,OXYGEN SUPPLY FACTORS,Contact lenses RGP or SCL Material Thickn

6、ess Fitting,Altitude,18,CLOSED EYE,Oxygen tension approx. 55 mm Hg Corneal swelling approx. 3.5% after 8 hours (no lens wear) Rapid deswelling after eye opening Altered metabolic activity? Reduced O2 requirements?,19,EFFECTS OF EYE CLOSURE,Reduced oxygen supply (palpebral conjunctiva) Decreased tear

7、 osmolality Increased CO2 tension Decreased tear and stromal pH Increased corneal temperature (37C),20,metabolism,With oxygen 1 glucose 36ATP Without oxygen 1 glucose 2ATP,三羧酸循环 无氧糖酵解 磷酸戊糖途径,21,三羧酸循环 Review: 生化,22,Demanded oxygen,Low oxygen Epithelial microcyst Polymegethism Corneal edema Endothelia

8、l blebs,23,How much minimum oxygen the corneal needs for keeping its healthy How do we know that,24,MEASURING CORNEAL OXYGEN DEMAND,INVASIVE: Polargraphic oxygen sensor (O2 uptake) Glycogen reserves Cell mitosis Loss of sensitivity (aesthesiometry) NON-INVASIVE: Corneal swelling (pachometry, goggles

9、) Corneal changes (slit-lamp biomicroscopy),25,MEASURING CORNEAL OXYGEN CONSUMPTION,Membrane saturated with air Cornea is applanated by sensor O2 diffuses into anterior cornea Rate of depletion of O2 from membrane estimated,(Technique pioneered by Hill and Fatt, 1964),26,Important factors,氧通透性oxygen

10、 permeability, Dk 氧传导性oxygen transmissibility, DK/t 等效氧分压equivalent oxygen percentage, EOP,用于材料的指标,27,Permeability = Dk Where D = diffusion coefficient of the material and k = solubility of the gas in the material,28,GAS DIFFUSION,Molecules migrate through microvoids (intramolecular spaces) within t

11、he materials matrix,29,30,透氧性取决于镜片厚度,镜片中心厚度: 0.11 mm / 边缘厚度 0.12 mm,31,透氧性取决于镜片厚度,镜片中心厚度: 0.15 mm / 边缘厚度: 18 mm,32,氧分压?,氧分压21%,33,EQUIVALENT OXYGEN PERCENTAGE,ADVANTAGES In vivo measurement Determined by oxygen thirst Normal lens wearing environment SCL vs RGP lens comparisons possible Static vs dyn

12、amic values Correlation between contact lens transmissibility (Dk/t) and EOP Can derive an EOP curve for a material,DISADVANTAGES O2 consumption rate affected by more than just contact lens wear Potential for error Invalid assumptions Relationship to other measuring techniques may not be direct,34,临

13、界氧需求(COR),Critical oxygen requirement The minimum oxygen for keeping the corneal healthy. Data various P17,35,CORNEAL OXYGENATION WITH CONTACT LENSES,36,GAS MOVEMENT THROUGH CONTACT LENSES,Composition of polymer Temperature effect Partial pressure of gas at lens surface Lens thickness Boundary layer

14、 effect,37,38,GAS SOLUBILITY,Sorption of a gas within the material Similar to a sponge which absorbs and holds water Gas is dissolved, or solubilized, into the material,39,CONTACT LENS PERMEABILITY to OXYGEN,Permeability = Dk Where D = diffusion coefficient of the material and k = solubility of the

15、gas in the material,One of the most important properties of a contact lens material An inherent material property not a function of lens thickness, shape or back vertex power (BVP),40,OXYGEN TRANSMISSIBILITY,Based on material permeability (Dk) Related to material thickness (t) Transmissibility = Dk/

16、t Relevant clinically,Dk t,41,OXYGEN TRANSMISSIBILITY,In vitro measurements: Dk/t In vivo (indirect) measurements: Overnight corneal swelling EOP Corneal oxygen demand following lens removal,42,DYNAMIC OXYGEN SUPPLY,Post-lens tear film exchange rates: SCL 1% per blink RGP 15 20% per blink Blinking R

17、ate Quality (completeness) RGP vs SCL fitting characteristics,43,44,Fluorosilicone and silicone hydrogel backbones - very high oxygen transmissibility,Microscopic “channels” that allow ions and water to move through the lens - critical to lens movement on the eye,THE APPROACHTighe, BCLA, May 1999,45

18、,TRANSMISSIBILITY CLASSIFICATIONSoft LensesRGP Lenses,Low 2550,46,CORNEAL OXYGEN REQUIREMENTSSCL Daily Wear,For zero daytime swelling: Dk/t = 24.1 2.7 x 10 9 EOP of 9.9%,47,CORNEAL SWELLINGRGP Daily Wear,Material (8 hours wear) Swelling (%) PMMA6 Low Dk3 - 4 Mod Dk1 High Dk0,48,CORNEAL SWELLINGRGP Overnight Wear,Material (8 hours wear) Swelling (%) Low Dk10 - 13 Mod Dk7 - 9 High Dk5 - 6,49,CORNEAL SWELLINGSCL Overnight Wear,Material (8 hours wear) Swelling (%) Low water12 Mid water10 High water11 Novel polymer 4 S