Disparity between mainstream and sidestream end-tidal carbon dioxide values and arterial carbon dioxide levels.

BACKGROUND Measuring and monitoring end-tidal carbon dioxide (PETCO2) is an important aspect of caring for critically ill patients. The 2 methods used for PETCO2 measurement are the mainstream and sidestream methods. OBJECTIVE To assess the agreement between PETCO2 measurements performed by mainstream and sidestream methods with the PaCO2 values. METHODS This was a prospective observational study. A total of 114 subjects were enrolled in the study. PETCO2 measurements using mainstream and sidestream methods were performed simultaneously with the arterial blood sampling in subjects who were observed in the emergency department and required arterial blood gas analysis. Agreement between the PETCO2 measurements and the PaCO2 values obtained from arterial blood gas analysis were evaluated using the Bland-Altman method. RESULTS Sixty subjects (52.6%) were female, and the mean age was 60.9 years (95% CI 58.3-63.6). The mean PaCO2 was 35.16 mm Hg (95% CI 33.81-36.51), the mainstream PETCO2 was 22.11 (95% CI 21.05-23.18), and the sidestream PETCO2 was 25.48 (95% CI 24.22-26.75). Bland-Altman analysis showed an average difference between mainstream PETCO2 and PaCO2 values of 13 mm Hg (95% limits of agreement -0.6 to 25.5) and moderate correlation (r = 0.55, P < .001). The average difference between the sidestream PETCO2 and PaCO2 values was 9.7 mm Hg (95% limits of agreement -5.4 to 24.7) and poor correlation (r = 0.41, P < .001). CONCLUSIONS PETCO2 values obtained by mainstream and sidestream methods were found to be significantly lower than the PaCO2 values. There was essentially no agreement between the measurements obtained by 2 different methods and the PaCO2 values.