Transcutaneous PCO 2 Is Nearly Identical to Arterial CO 2 in Adult Outpatients

Abstract

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/V_T 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/V_T 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/V_T without the need for arterial puncture.

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