tcPCO2 electrode design, calibration and temperature gradient problems.

Transcutaneous PCO2 electrodes habe been constructed and evaluated on adults and children. Glass pH and silver reference electrodes were used at 44--45 degrees C, with either circulating water and a copper jacket, or with internal electrical heating. The skin surface PCO2 at 44 degrees C is about 1.33 times PaCO2 plus 3 mmHg when measured with electrodes calibrated in gas at 44 degrees C. Three temperature effects combine in this ratio: Heating raises blood PCO2 4.5%/degrees C, skin metabolism adds about 3 mmHg, and the cooling of the electrode active surface by skin increases electrode reading. Response time to step changes of PaCO2 was about 3 min to 63%, of which 1.2 min was sensor delay, the remainder skin CO2 washout. It was found important to use ethylene glycol-water mixtures rather than water for electrolyte to avoid bubble generation and drift. Heat transfer through the pH glass electrode has been increased by enlarging the internal silver electrode to virtually fill the entire glass electrode. Time required for initial vasodilation and stabilization is similar to that of tcPO2 electrodes, and accuracy of determination of PaCO2 appears to be better than +/- mmHg.