Comparison of mainstream and sidestream capnography in spontaneously breathing adults

Introduction: Capnography provides information on ventilation-perfusion matching and the mechanical status of the respiratory system without the need for a close patient cooperation. While mainstream (MS) and sidestream (SS) techniques were proved to allow monitoring the end-tidal CO2 concentration (ETCO2) and time-domain phase 3 slope (S3T) during mechanical ventilation, the agreement of the capnogram parameters obtained by MS and SS methods has not been tested during spontaneous breathing. Methods: Simultaneous MS and SS time and volumetric capnography were performed on healthy, spontaneously breathing adults (n=33). Correlation and Bland-Altman analyses were used to assess trends and agreements between capnography indices obtained by MS and SS techniques, including ETCO2, S3T and total amount of exhaled CO2 per expiration (VCO2). Results: Excellent correlation and acceptable agreement were observed in ETCO2 (R2=0.96, p<0.001, bias and limits of agreement ‑5.21±11.16 %) and VCO2 (R2=0.99, p<0.001, ‑1.55±16.50 %) measured by MS and SS techniques. While S3T exhibited significant correlation (R2=0.81, p<0.001), the range of agreement was fairly wide (‑59.2±69.1 %). Volumetric slopes and ventilation dead spaces could not be reliably estimated by SS. Conclusions: SS capnography is suitable to monitor the metabolic status by measuring ETCO2 and VCO2 reliably. However, MS capnography is required in spontaneously breathing adults for the quantification of the capnogram parameters interpreted in the volumetric domain, i.e. those reflecting uneven alveolar emptying, ventilation-perfusion matching and ventilation dead space fraction. Funding: OTKA-NKFIH FK134274, Hungarian Pulmonology Foundation