Table. 1—

Usefulness and limitations of explorations of distal airways in asthma

SpirometryFEF25–75%Useful if FVC normalLarge variations in normal values
FEF50%Useful if FVC normalLarge variations in normal values
FVC/SVCPotentially usefulFurther studies required
PlethysmographyRV/TLCEasy to measureIndirect link to distal airwaysFurther studies required
Raw and sGawDo not specifically reflect distal airway abnormalities
Interruption of tidal breathing#Airway resistanceNot specific for distal airways
Forced oscillations#Distal airway resistanceGood sensitivity/specificityNot widely available
Nitrogen washout#
    Single breathClosing volume and closing capacityGood sensitivity/specificity
Not widely available
    Multiple breathScond and SacinGood sensitivity/specificity
Not commercially available
Exhaled NO#
    Constant expiratory flowFeNODo not specifically reflect distal airway abnormalities
    Multiple expiratory flowCalvNOFurther studies required
HRCT#Lung attenuation (semiquantitative)Not reproducible
Lung attenuation (quantitative)Requires expert radiologists and specific software/spirometric gating
Other imaging techniques#Not fully determinedHigh cost
    MRI (hyperpolarised gas)Low availability
    Technegas SPECTFew studies
BronchoscopyDirect measurement of airway resistanceInvasive
Research procedure
Transbronchial biopsiesInvasive
Airway remodelling and inflammatory cells
Limited to single-centre research studies
Inflammatory cells and mediators
  • FEF25–75%: mean forced expiratory flow between 25% and 75% of forced vital capacity (FVC); FEF50%: forced expiratory flow at 50% of FVC; SVC: slow vital capacity; RV: residual volume; TLC: total lung capacity; Raw: airway resistance; sGaw: specific airway conductance; Scond: ventilation inhomogeneity in conductive zones of the lungs; Sacin: ventilation inhomogeneity in acinar zones of the lungs; FeNO: exhaled nitric oxide fraction; CalvNO: alveolar nitric oxide concentration; HRCT: high-resolution computed tomography; MRI: magnetic resonance imaging; SPECT: single photon emission computed tomography; PET: positron emission tomography; BALF: bronchoalveolar lavage fluid. #: these techniques have been used in research protocols or single-centre studies but their use for assessment of distal airways requires expertise that is not available in all centres.