Outcome differences in multiple-breath washout devices are explained primarily by sensor characteristics

Abstract

Nitrogen multiple breath washout (MBW) is an established technique for the assessment of ventilation inhomogeneity in the lungs. Poor agreement among commercially available devices for MBW has been described, (Poncin, W. et al. J Cyst Fibros. 16(2):258-266 2017) but it remained unclear whether these differences are due to breathing pattern, sensor characteristics, or outcome computation.

We investigated differences for MBW due to i) sensors and signal processing (e.g. measured/calculated gas-concentrations) and ii) subsequent computation of MBW outcomes between two commercial devices: Exhalyzer D (eco; Ecomedics AG, Duernten, Switzerland), and EasyOne Pro Lab (ndd; ndd Medizintechnik AG, Zurich, Switzerland). We performed simultaneous measurements of side-stream sampled gas in healthy individuals (n=8) and splicing of signals between the devices’ processing streams to exclude the influence of breathing pattern.

Overall functional residual capacity (FRC) was (mean±SD) 13.3±4.2% higher, and lung clearance index (LCI) 17.2±7.0% higher in eco vs ndd. Separate analysis of steps i) and ii) showed that sensor characteristics in both eco and ndd lead to an overestimation of expiratory N2 concentration. The degree of overestimation varies between devices, resulting in a higher FRC (+12.1±4.0%) and higher LCI (+18.1±7.1%) in eco vs ndd. Independently of sensor characteristics, outcome algorithms resulted in comparable FRC (+1.1±1.2%) and comparable LCI (-0.7±2.0%) in eco vs ndd.

Identification of the exact cause of differences between devices and software algorithms allows precise recommendations to the manufacturers to increase transparency and improve comparability.