Limitations of End-Tidal CO2 Measured with a Portable Capnometer to Estimate PaCO2 for Patients with Respiratory Disease.

OBJECTIVE: This study evaluated the relationship between end-tidal carbon dioxide (EtCO₂) measured with a portable capnometer and PaCO₂ in respiratory disease patients. MATERIAL AND METHODS: We retrospectively reviewed patients whose EtCO₂, measured with a portable capnometer using a mouth- piece, and PaCO₂ were simultaneously assessed at a single center from August 2017 to September 2018. The primary outcome was the relationship between EtCO₂ and PaCO₂. We conducted subgroup analyses in patients with interstitial lung disease (ILD), with and without O₂ supplementation. The relationship between EtCO₂ and PaCO₂ was analyzed using Spearman's rank test and Bland--Altman analysis. RESULTS: A total of 100 patients were registered in this study. There was a moderate correlation between EtCO₂ and PaCO₂ (rho = 0.41). The Bland--Altman plot showed that the mean bias was 0.32 mmHg (95% CI: -1.28 to 1.92), the limits of agreement (LOA) were -15.48 and 16.13 mmHg, and the percent error was 38.49%. The LOA in patients with ILD were -15.12 and 13.75 mmHg. In patients with O₂ supplementation, the mean bias was greater, and the LOA were wider than in those without O₂ supplementation (mean bias: 7.17 vs. -1.18 mmHg, respectively; LOA: -14.29 and 28.62 mmHg vs. -13.82 and 11.46 mmHg, respectively). CONCLUSION: In the clinical setting, the relationship between EtCO₂ and PaCO₂ was poor in patients with respiratory disease, espe- cially in those receiving O₂ supplementation, compared with that reported in previous studies. It may be difficult to precisely estimate PaCO₂ in patients with respiratory disease based on measurements of EtCO₂. [ABSTRACT FROM AUTHOR]