Non-invasive Monitoring of Oxygen and Carbon Dioxide Tension: Accuracy of Transcutaneous O2 and CO2 and Endtidal CO2

ABSTRACT

PURPOSE: To determine the concordance of transcutaneous CO2 (PtcCO2) versus arterial CO2 (PaCO2), end-tidal CO2 (PetCO2) versus PaCO2, and transcutaneous O2 (PtcO2) versus arterial O2 (PaCO2) among healthy adult volunteers, and to determine the normal values of the PtcCO2/PtcO2 and PtcO2/PaO2 that will be used as early signs of shock or as prognostic factors for critically ill patients. METHODS: We measured the PtcO2, PtcCO2, PetCO2, PaO2, and PaCO2 from 11 healthy volunteers while breathing room air or O2 at a flow rate of 6 L/min via nasal cannula. The PtcO2 and PtcCO2 were measured using a Radiometer's transcutaneous sensor that interfaced with the Solar 8000 patient monitor system. The PetCO2 was measured using a side stream capnometer that sampled air from a nasal catheter. The PaO2 and PaCO2 were measured from arterial blood samples. The concordances of the PtcCO2 versus the PaCO2, the PtcO2 versus the PaO2, and the PetCO2 versus the PaCO2 were analyzed using a Bland-Altman plot. We defined the normal values of the P(a-tc)CO2, PtcO2/PaO2, and PtcCO2/PtcO2. RESULTS: Twenty-two pairs of the PtcCO2 versus PaCO2, PtcO2 versus PaO2, and PetCO2 versus PaCO2 were obtained. The mean (+/-SD) values of the P(a-tc)CO2, P(atc) O2, and P(a-et)CO2 were 0(+/-2.2) mmHg, 35.4(+/-24.1) mmHg, and 1.4(+/-1.3) mmHg, respectively (p=0.947, p<0.001, and p<0.001 by paired t-test, respectively). The P(a-tc)CO2 and P(a-et)CO2 showed a high concordance of 95.5% within a range of +/-4 mmHg. The median (25~75%) values of the PtcCO2/PtcO2 and PtcO2/PaO2 at room air were 54.8%(46.8%~62.7%), respectively. CONCLUSION: The PtcCO2 and PetCO2 had a reliable concordance with the PaCO2. However, the PtcO2 was discordant with the PaO2 and this discordance was increased when inspiring O2. Therefore, the absolute values of the PtcO2 cannot be used as a surrogate measurement of the PaO2. However, because the O2 supply did not increase the PtcCO2, but rather the PtcO2, we can use the trend in the change in the PtcCO2/PtcO2 or PtcO2/PaO2 in shock patients.