The effect of oxygen and carbon dioxide cross-sensitivity sensor error in the Eco Medics Exhalyzer D device on measures of conductive and acinar airway function.

Introduction: The multiple breath nitrogen washout (MBNW) test provides important clinical information in obstructive airways diseases. Recently, a significant cross-sensitivity error in the O2 and CO2 sensors of a widely used commercial MBNW device (Exhalyzer D, Eco Medics AG, Duernten, Switzerland) was detected, which leads to overestimation of N2 concentrations. Significant errors in functional residual capacity (FRC) and lung clearance index (LCI) have been reported in infants and children. This study investigated the impact in adults, and on additional important indices reflecting conductive (S cond) and acinar (S acin) ventilation heterogeneity, in health and disease. Methods: Existing MBNW measurements of 27 healthy volunteers, 20 participants with asthma and 16 smokers were reanalysed using SPIROWARE V 3.3.1, which incorporates an error correction algorithm. Uncorrected and corrected indices were compared using paired t-tests and Bland-Altman plots. Results: Correction of the sensor error significantly lowered FRC (mean difference 9%) and LCI (8-10%) across all three groups. S cond was higher following correction (11%, 14% and 36% in health, asthma and smokers, respectively) with significant proportional bias. S acin was significantly lower following correction in the asthma and smoker groups, but the effect was small (2-5%) and with no proportional bias. Discussion: The O2 and CO2 cross-sensitivity sensor error significantly overestimated FRC and LCI in adults, consistent with data in infants and children. There was a high degree of underestimation of S cond but minimal impact on S acin. The presence of significant proportional bias indicates that previous studies will require reanalysis to confirm previous findings and to allow comparability with future studies.

Controlled versus free breathing for multiple-breath nitrogen washout in asthma.

The lack of comparability in indices of ventilation heterogeneity between free- and controlled-breathing MBNW protocols is confirmed in asthma https://bit.ly/3lmri4A.

Controlled versus free breathing for multiple breath nitrogen washout in healthy adults.

Multiple breath nitrogen washout (MBNW) quantifies ventilation heterogeneity. Two distinct protocols are currently used for MBNW testing: "controlled breathing", with targeted tidal volume (V T) and respiratory rate (RR); and "free breathing", with no constraints on breathing pattern. Indices derived from the two protocols (functional residual capacity (FRC), lung clearance index (LCI), S cond, S acin) have not been directly compared in adults. We aimed to determine whether MBNW indices are comparable between protocols, to identify factors underlying any between-protocol differences and to determine the between-session variabilities of each protocol. We performed MBNW testing by both protocols in 27 healthy adult volunteers, applying the currently proposed correction for V T to S cond and S acin derived from free breathing. To establish between-session variability, we repeated testing in 15 volunteers within 3 months. While FRC was comparable between controlled versus free breathing (3.17 (0.98) versus 3.18 (0.94) L, p=0.88), indices of ventilation heterogeneity derived from the two protocols were not, with poor correlation for S cond (r=0.18, p=0.36) and significant bias for S acin (0.057 (0.021) L-1 versus 0.085 (0.038) L-1, p=0.0004). Between-protocol differences in S acin were related to differences in the breathing pattern, i.e. V T (p=0.004) and RR (p=0.01), rather than FRC. FRC and LCI showed good between-session repeatability, but S cond and S acin from free breathing showed poor repeatability with wide limits of agreement. These findings have implications for the ongoing clinical implementation of MBNW, as they demonstrate that S cond and S acin from free breathing, despite V T correction, are not equivalent to the controlled breathing protocol. The poor between-session repeatability of S cond during free breathing may limit its clinical utility.