Comparison of Mainstream End Tidal Carbon Dioxide on Y-Piece Side Versus Patient Side of Heat and Moisture Exchanger Filters in Critically Ill Adult Patients: a prospective observational study

Introduction: Whether HMEF impairs the accuracy of mainstream EtCO 2 measured on the Y-piece (filtered) side of HMEF compared to that on the patient (unfiltered) side is unclear. Methods: : We conducted a prospective observational method comparison study in a teaching hospital in Japan between July 2019 and December 2019. Critically ill adult subjects receiving mechanical ventilation with HMEF were included. We performed a noninferiority comparison of the accuracy of EtCO 2 measurements on the two sides of HMEF. The accuracy was measured by the absolute difference between PaCO 2 and EtCO 2 . We set the non-inferiority margin at +1 mmHg in accuracy difference between the Y-piece side and the patient side of HMEF. We also assessed the agreement between PaCO 2 and EtCO 2 using Bland-Altman analysis. Results: : Among thirty-seven subjects, the accuracy difference was -0.14 mmHg (95% CI, -0.58–0.29), and the upper 95% CI limit did not exceed the predefined margin of + 1 mmHg, establishing non-inferiority of EtCO 2 on the Y-piece side of HMEF ( P for non-inferiority <0.001). In the Bland-Altman analyses, 95% limits of agreement between PaCO 2 and EtCO 2 were similar in both sides of HMEF (Y-piece side, -8.67–+10.65 mmHg; patient side, -8.93–+10.67 mmHg). Conclusions: : The accuracy of mainstream EtCO 2 measurements on the Y-piece side of HMEF was noninferior to that on the patient side in critically ill adult subjects. Mechanically ventilated adult subjects could be accurately monitored with mainstream EtCO 2 on the Y-piece side of the HMEF unless their tidal volume was extremely low.