Introduction: Outcomes after lung transplant (LTx) are limited by development of CLAD. Lung function is monitored serially by spirometry. Multiple breath washout (MBW) and Impulse Oscillometry (IOS) are effort-independent alternatives with improved sensitivity to small airway changes.

Methods: MBW and IOS were performed at routine review in double LTx recipients using a closed circuit Innocor MBW system, and a TremoFlo device. Patients were CLAD-free at recruitment with normal FEV₁ (% baseline). Lung clearance index (LCI), total respiratory resistance (R5), small airway resistance (R5-20), lung reactance (X5) and reactance area (AX) were measured. High LCI reflects ventilation inhomogeneity during gas washout.

Results: 40 patients were assessed (19F); mean (SD) age: 50.1 (11.1) years; median (IQR) time since LTx 1.71 (0.67-3.50) years (range 0.52-4.52). Median (IQR) FEV₁ was 94.2(88.5-99.8)% baseline. Mean (SD) LCI at baseline was 8.47 (2.01), 38(95%) were higher than ULN in healthy volunteers. IOS outputs are shown in the table. LCI correlated weakly with AX and R5-20. The significance of correlation with AX was lost after removal and adjustment of outliers.

Conclusions: High rates of abnormal small airway function were identified by both techniques in CLAD-free recipients with preserved FEV₁. IOS and MBW outcomes appear to capture different properties of the peripheral lung after LTx. Abnormalities may reflect donor lung history and/or surgical complications, and longitudinal data are required to assess clinical utility in detecting CLAD.