Rationale: Nitrogen multiple-breath washout is an established technique to assess functional residual capacity and ventilation inhomogeneity in the lung. Accurate measurement of gas concentrations is essential for the appropriate calculation of clinical outcomes. Objectives: We investigated the accuracy of oxygen and carbon dioxide gas sensor measurements used for the indirect calculation of nitrogen concentration in a commercial multiple-breath washout device (Exhalyzer D, Eco Medics AG, Duernten, Switzerland) and its impact on functional residual capacity and lung clearance index. Methods: High precision calibration gas mixtures and mass spectrometry were used to evaluate sensor output. We assessed the impact of corrected signal processing on multiple-breath washout outcomes in a dataset of healthy children and children with cystic fibrosis using custom analysis software. Results: We found inadequate correction for the cross sensitivity of the oxygen and carbon dioxide sensors in the Exhalyzer D device. This results in an overestimation of expired nitrogen concentration, and consequently multiple-breath washout outcomes. Breath-by-breath correction of this error reduced the mean (SD) cumulative expired volume by 19.6 (5.0)%, functional residual capacity by 8.9 (2.2)%, and lung clearance index by 11.9 (4.0)%. It also substantially reduced the level of the tissue nitrogen signal at the end of measurements. Conclusions: Inadequate correction for cross sensitivity in the oxygen and carbon dioxide gas sensors of the Exhalyzer D device leads to an overestimation of functional residual capacity and lung clearance index. Correction of this error is possible and could be applied by re-analyzing the measurements in an updated software version.

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Rationale: Nitrogen multiple breath washout (N2MBW) is an established technique to assess functional residual capacity (FRC) and ventilation inhomogeneity in the lung. Accurate gas sensors are essential for the appropriate calculation of clinical outcomes. Objectives: We investigated the accuracy of oxygen and carbon dioxide sensors used for the indirect measurement of nitrogen in the Eco Medics Exhalyzer D N2MBW device and characterized the impact of potential sensor errors on FRC and the lung clearance index (LCI). Methods: Technical gas mixtures and mass spectrometry were used to evaluate sensor accuracy. We assessed the impact of potential sensor errors and correction on FRC and LCI in a dataset of healthy children and children with cystic fibrosis using custom analysis software. Results: We found a systematic error in the gas sensors of the Exhalyzer D device involving inadequate correction for the cross sensitivity between the oxygen and carbon dioxide sensors. This error results in an overestimation of expired nitrogen concentration, and consequently FRC and LCI outcomes. Breath-by-breath correction for this sensor error non-systematically reduced mean (SD) FRC by 8.9 (2.2)% and LCI by 11.9 (4.0)%. It also resulted in almost complete disappearance of the tissue nitrogen signal at the end of the measurement. Conclusions: An error in the cross sensitivity correction between the oxygen and carbon dioxide gas sensors of the Exhalyzer D device leads to an overestimation of FRC and LCI. Correction of this error is possible but needs to be applied breath-by-breath by re-analysing the measurements in an updated software version.