Characterization of a signal-correction error in a commercial multiple-breath washout device

Abstract

Background: Multiple-breath washout (MBW) is an established technique to assess functional residual capacity (FRC) and ventilation inhomogeneity in the lung. Indirect calculation of nitrogen concentration requires precise calibration and accurate measurement of gas concentrations.

Aim: We investigated the accuracy of the carbon dioxide gas concentration used for the calculation of nitrogen concentration in a commercial MBW device (EasyOne Pro LAB, ndd Medizintechnik AG, Switzerland) and its impact on outcomes.

Methods: A high-precision calibration gas mixture was used to evaluate carbon dioxide sensor output. We assessed the impact of a corrected CO₂ signal on MBW outcomes in a dataset of healthy children (n=8) and adults (n=10) and children with lung disease (n=8).

Results: The EasyOne device uses a respiratory quotient-based adjustment to correct the measured carbon dioxide signal for potential long-term changes in sensor output. In 89% of measurements, this resulted in an overestimation of expired nitrogen concentrations (range -0.7 to 1.7%), and consequently MBW outcomes. Correction of the CO₂ signal reduced the mean (range) cumulative expired volume by 27.1% (-58.1%; 1.0%), FRC by 11.1% (-21.6%; 1.2%), and lung clearance index by 18.3% (-44.0%; -0.3%). Additionally, within-visit variability was substantially reduced with the corrected signals.

Conclusion: Inadequate signal-correction of the measured CO₂ concentration in the EasyOne device leads to a non-systematic error in expired nitrogen concentrations and overestimation of test outcomes. Two-point calibration of the CO₂ sensor may maintain accurate measurement of gas concentrations and overcome this error.