Drift in vivo of transcutaneous dual electrodes

TcPO2 and tcPCO2 monitoring is widely used but the in vitro drift of the new combined sensors is unknown. We tested the in vivo stability of 3 such electrodes in six adults, compared to nasal endtidal values from a mass spectrometer. Each electrode was remembraned within 5 days and had a 2 point dry gas calibration at 45 degrees C before fixing to the subjects' right arms. TcPO2, tcPCO2 and endtidal values were averaged over four minutes after 30 minutes equilibration, and then at seven subsequent 20 minute intervals. We observed that (1) Endtidal values remained stable (2) tcPCO2 differed from baseline by less than 3 torr (3) tcPO2 rose significantly in all three electrodes by an average of 16 to 21% and, (4) in vivo drift greatly exceeded separately determined in vitro changes. We discuss the possible explanations for the observed results, and conclude that skin permeability changes may play an important role. In the light of the large in vivo tcPO2 drift, transcutaneous dual electrodes are not reliable trend indicators of blood gases in adults.