Comparison of End-Tidal, Arterial, Venous, and Transcutaneous P CO 2 .

BACKGROUND: We investigated the measurement of end-tidal partial pressure of carbon dioxide (PETCO2 ) with a capnometer in patients with respiratory failure, and we determined whether this technique could provide an alternative to measurement of PaCO2 using arterial blood gas analysis in the clinical setting.

METHODS: We measured PETCO2 in subjects with hypoxemic and hypercarbic respiratory failure using a capnometer. We simultaneously measured PaCO2 , venous partial pressure of carbon dioxide (Pv̄CO2 ), and transcutaneously measured partial pressure PCO2 (PtcCO2 ). We analyzed agreements among these parameters with Bland-Altman analysis. We obtained 30 samples from subjects with hypoxemic respiratory failure and 30 samples from subjects with hypercarbic respiratory failure.

RESULTS: Thirty subjects with hypoxemic respiratory failure and 18 subjects with hypercarbic respiratory failure participated in this study. Significant relationships were found between PETCO2 and PaCO2 , between PtcCO2 and PaCO2 , and between Pv̄CO2 and PaCO2 . Bland-Altman analysis of PETCO2 and PaCO2 in all subjects revealed a bias of 6.48 mm Hg (95% CI 4.93-8.03, P < .001) with a precision of 6.01 mm Hg. Bland-Altman analysis of PETCO2 and PaCO2 with hypoxemic respiratory failure revealed a bias of 5.14 mm Hg (95% CI 3.35-6.93, P < .001) with a precision of 4.80 mm Hg. Bland-Altman analysis of PETCO2 and PaCO2 in subjects with hypercarbic respiratory failure revealed a bias of 7.83 mm Hg (95% CI 5.27-10.38, P < .001) with a precision of 6.83 mm Hg.

CONCLUSIONS: PETCO2 can be measured simply using a capnometer, and PETCO2 measurements can estimate PaCO2 . However, the limits of agreement were wide. Therefore, care providers must pay attention to the characteristics and errors of these devices. These results suggest that measurement of PETCO2 might be useful for screening for hypercarbic respiratory failure in the clinical setting.