Comparison of End-Tidal, Arterial, Venous, and Transcutaneous PCO2

BACKGROUND: We investigated the measurement of end-tidal partial pressure of carbon dioxide (P-ETCO2) with a capnometer in patients with respiratory failure, and we determined whether this technique could provide an alternative to measurement of P-aCO2 using arterial blood gas analysis in the clinical setting. METHODS: We measured P-ETCO2 in subjects with hypoxemic and hypercarbic respiratory failure using a capnometer. We simultaneously measured P-aCO2, venous partial pressure of carbon dioxide (P-(v) over bar CO2), and transcutaneously measured partial pressure P-CO2 (P-tcCO2). 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 P-ETCO2 and P-aCO2, between P-tcCO2 and P-aCO2, and between P-(v) over bar CO2 and P-aCO2. Bland-Altman analysis of P-ETCO2 and P-aCO2 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 P-ETCO2 and P-aCO2 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 P-ETCO2 and P-aCO2 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: P-ETCO2 can be measured simply using a capnometer, and P-ETCO2 measurements can estimate P-aCO2. 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 P-ETCO2 might be useful for screening for hypercarbic respiratory failure in the clinical setting.