Comparison of End-Tidal, Arterial, Venous, and Transcutaneous P _{CO ₂}

Abstract

BACKGROUND: We investigated the measurement of end-tidal partial pressure of carbon dioxide (P ETCO 2 ) with a capnometer in patients with respiratory failure, and we determined whether this technique could provide an alternative to measurement of P aCO 2 using arterial blood gas analysis in the clinical setting. METHODS: We measured P ETCO 2 in subjects with hypoxemic and hypercarbic respiratory failure using a capnometer. We simultaneously measured P aCO 2 , venous partial pressure of carbon dioxide (P v̄CO 2 ), and transcutaneously measured partial pressure P CO 2 (P tcCO 2 ). 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 ETCO 2 and P aCO 2 , between P tcCO 2 and P aCO 2 , and between P v̄CO 2 and P aCO 2 . Bland-Altman analysis of P ETCO 2 and P aCO 2 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 ETCO 2 and P aCO 2 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 ETCO 2 and P aCO 2 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 ETCO 2 can be measured simply using a capnometer, and P ETCO 2 measurements can estimate P aCO 2 . 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 ETCO 2 might be useful for screening for hypercarbic respiratory failure in the clinical setting.