Performance of transcutaneous PO2 and PCO2 dual electrodes in adults

Three commercially-available combined PO2-PCO2 electrodes were assessed in vitro, and in adults breathing air, hypoxic and hypercapnic mixtures, for speed of response, correlation with end-tidal gas tensions and drift. Differences in the 90% response time of the individual electrodes were more marked in vitro than in vivo. Changes in end-tidal gas tensions were reflected by proportionate changes in transcutaneous oxygen and carbon dioxide (PtcO2 and PtcCO2) but, in the individual subject, PtcO2 and PtcCO2 were generally poor indicators of the end-tidal values. During steady-state recordings, the PtcO2 signal drifted upwards by more than 12 mm Hg during 140 min in vivo recording in all three electrodes, without changes in either PtcCO2 or end-tidal values. The dual electrodes tested provide non-invasive estimates of qualitative, but not quantitative, change in blood-gas tensions and are likely to have only a limited role to play in adult anaesthetic practice.