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

This study evaluated the relationship between end-tidal carbon dioxide (EtCO2) measured with a portable capnometer and PaCO2 in respiratory disease patients. We retrospectively reviewed patients whose EtCO2, measured with a portable capnometer using a mouthpiece, 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. 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). In the clinical setting, the relationship between EtCO2 and PaCO2 was poor in patients with respiratory disease, especially 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.