Background:In patients with pulmonary disease there is value in determining a patient's carbon dioxide (CO2) level. Arterial CO2 (PaCO2) levels are often used to determine if a patient has hypercarbia, as part of UNOS guidelines for lung transplantation, and it can be used to determine physiologic dead space (VD/VT ratio) at rest and during exercise. Obtaining direct measurement of PaCO2 requires an arterial blood gas sample, which is expensive, painful, and with potential risks. Our pulmonary function and exercise lab evaluated a noninvasive method of estimating the PaCO2 using a transcutaneous monitor (TCM) to measure PtcCO2. We were particularly interested in using these devices during our cardiopulmonary exercise testing to aide in calculating VD/VT ratio during rest and exercise. The first step was to evaluate how well the non-invasive PtcCO2 correlated to the PaCO2 at rest. METHOD: Adult patients presenting to the outpatient pulmonary lab to have an arterial blood gas sample collected were placed on a COMBI M54 (TCM 4 series) transcutaneous CO2 monitor (Radiometer Medical, Denmark) prior to arterial puncture. The patients were measured at rest. We set the TCM metabolic correction factor setting at 4 and set the probe temperature to 42C for all measurements. The probe was placed on the skin just below the clavicle using the Radiometer fixation ring. The monitor was kept in place for 10 min to ensure a stabilized reading prior to arterial puncture. We compared PaCO2 values to simultaneously recorded PtcCO2 values using a Bland-Altman plot. Results:We measured 110 PaCO2 levels with simultaneously recorded PtcCO2 levels. A Bland-Altman plot of the comparison is represented in the figure. The mean PaCO2 - PtcCO2 difference (bias) was -0.01 Torr and the standard deviation for the difference was 1.44 Torr. There appeared to be no relationship between the PaCO2 values and the PaCO2 - PtcCO2 difference. Conclusions:PtcCO2 matched PaCO2 almost identically in patients at rest. We will testing PaCO2 - PtcCO2 difference while patients are undergoing exercise testing with arterial catheters in place to determine if the values match under those conditions as well. PtcCO2 is an accurate, noninvasive way to estimate PaCO2. PtcCO2 could facilitate measurement of complex parameters such as VD/VT without the need for arterial puncture.
II et al. (Mon,) studied this question.