Differences in lung clearance index and functional residual capacity between two commercial multiple-breath nitrogen washout devices in healthy children and adults

Multiple-breath nitrogen washout (MBNW) and its clinical parameter lung clearance index (LCI) are gaining increasing attention for the assessment of small airway function. Measurement of LCI relies on accurate assessment of functional residual capacity (FRC). The EasyOne Pro LAB (ndd) and Exhalyzer D (EM) are two commercially available MBNW devices. The aim of the study was to compare these two devicesin vitroandin vivoin healthy subjects with regard to FRC, LCI and secondary outcome parameters and to relate FRCMBNWto FRC measured by body plethysmography (pleth) and helium dilution technique. MBNW measurements were performed using a lung model (FRC between 500 and 4000 mL)in vitroand in 38 subjects aged 6–65 years followed by helium dilution and plethin vivousing fixed and relaxed breathing techniques.In vitroaccuracy within 5% of lung model FRC was 67.3% for ndd, FRC was >5% higher for EM in all tests.In vivo, FRCplethranged from 1.2 to 5.6 L. Mean differences (limits of agreement) between FRCplethand FRCMBNWwere −7.0%, (−23.2 to 9.2%) and 5.7% (−11.2 to 22.6%) using ndd and EM, respectively. FRCnddwas consistently lower than FRCEM(−11.8% (−25.6 to 2%)). LCI was comparable between the two devices (−1.3% (−21.9 to 19.3%)). There was a difference of >10 % in LCI in 12 of 38 subjects. Using the most recent software updates, both devices show relevant deviations in FRC measurement bothin vitroandin vivoand individual differences in LCI in a significant proportion of subjects. The devices are therefore not interchangeable.