Measurement of heated skin O2 diffusion conductance and PO2 sensor induced O2 gradient.

A large cathode heated transcutaneous PO2 electrode was used to measure the resistance of skin to diffusion of O2 from the capillary bed to the surface. With a non-O2-consuming tcPO2 electrode, one would read ideal tcPO2, Pi, which approximates PaO2 between 50 and 100 mmHg due to approximate cancellation of the rise by heating and the fall by skin O2 consumption. When a large cathode tcPO2 electrode is used, its O2 consumption causes a PO2 gradient across skin resistance, Rs, which depends on electrode membrane resistance RM (with Mylar), or RT (with Telfon). Surface PO2 will fall below Pi, and assuming no change in capillary PO2: PT/Pi = RT/(RT + Rs) and PM/Pi = RM/(RM + Rs) (1) 12 micrometer Telfon is 50 times as permeable to O2 as 6 micrometer Myler. When an electrode, covered alternately with these two membranes, was mounted on the same skin site in six subjects, the mean observed PT/PM was about 0.3. Substituting in equation (1) yields RT/Rs = 0.4, Rm/Rs = 20, and PM/Pi = 0.95. Using the known O2 permeability of Teflon, we computed skin O2 conductance to be 6.6 pmol O2/(cm2 .s .kPa), or 15 nl O2/(cm2 .s .atm). This method thus provides a way of determining ideal tc PO2 and stirring effect, phi, of other electrodes and various membranes.