Alan S Bell, Philip J Lawrence, Dave Singh, Alexander Horsley

Lung clearance index (LCI), derived from multiple-breath washout (MBW), is a well-established assessment of ventilation inhomogeneity in cystic fibrosis but has not been widely applied in other conditions characterized by heterogeneous airways disease, such as COPD. The aim of this study was to evaluate the sensitivity, repeatability, and practicality of LCI in patients with COPD. Fifty-four COPD patients completed MBW using nitrogen as the washout tracer gas (MBWN2, measured using an Exhalyzer™ device), spirometry, and plethysmography. Twenty patients repeated MBWN2, MBWSF6 (using a separate Innocor™ gas analyzer to measure washout of the exogenous trace sulphur hexafluoride), and spirometry at a second visit ≥24 hours later. Mean (SD) COPD LCI measured by nitrogen washout (LCIN2) was 12.1 (2.2); mean (SD) LCI Z-score 5.8 (2.0). LCIN2 increased across Global Initiative for Obstructive Lung Disease stages 1 to 3 and was abnormal (Z-score >1.65) in all COPD patients, including those with forced expiratory volume in 1 second (FEV1) ≥80% predicted. LCI was repeatable (median intra-test coefficient of variation 4.1%) and reproducible (limits of agreement -1.8 to 1.6) after mean of 16 days. Functional residual capacity (FRC) measurements were significantly greater using nitrogen than SF6 or plethysmography: mean FRC measured by nitrogen washout (FRCN2) 139% predicted versus FRC measured by plethysmography 125% predicted, p<0.0001. LCI is most suitable as a measure of early airways disease in COPD in those with well-preserved FEV1, with similar repeatability and limitations to that observed in cystic fibrosis. Using the Exhalyzer system to perform MBWN2, however, appeared to substantially over-read FRC. This discrepancy needs addressing before FRCN2 measurements made using this device can be reliably deployed.