Summary of functional tests available at bedside for small airways assessment in chronic obstructive pulmonary disease

FEF25–75%Spirometric flow interval at 25–75% of FVCNo added clinical value from other spirometric indices
Nitrogen washoutNitrogen washout in pulmonary alveoli reflecting ventilation heterogeneitySBW relates to SAD
MBW detects heterogeneity at conductive and acinar levels
Forced oscillation techniquePeriodic pressure variations to measure airways impedance to flowLow-frequency component probes small airways
Early sensitive and effort-independent tool for detection of flow limitation
Specific sensitivity to bronchodilation

FEF25–75%: forced expiratory flow at 25–75% of forced vital capacity (FVC); SBW: single-breath washout; SAD: small airways disease; MBW: multiple-breath washout.