Usefulness and limitations of explorations of distal airways in asthma
| Methods | Parameters | Usefulness/limitations |
| Spirometry | FEF25–75% | Useful if FVC normalLarge variations in normal values |
| FEF50% | Useful if FVC normalLarge variations in normal values | |
| FVC/SVC | Potentially usefulFurther studies required | |
| Plethysmography | RV/TLC | Easy to measureIndirect link to distal airwaysFurther studies required |
| Raw and sGaw | Do not specifically reflect distal airway abnormalities | |
| Interruption of tidal breathing# | Airway resistance | Not specific for distal airways |
| Forced oscillations# | Distal airway resistance | Good sensitivity/specificityNot widely available |
| Nitrogen washout# | ||
| Single breath | Closing volume and closing capacity | Good sensitivity/specificity |
| Not widely available | ||
| Multiple breath | Scond and Sacin | Good sensitivity/specificity |
| Not commercially available | ||
| Exhaled NO# | ||
| Constant expiratory flow | FeNO | Do not specifically reflect distal airway abnormalities |
| Multiple expiratory flow | CalvNO | Further studies required |
| HRCT# | Lung attenuation (semiquantitative) | Not reproducible |
| Lung attenuation (quantitative) | Requires expert radiologists and specific software/spirometric gating | |
| Other imaging techniques# | Not fully determined | High cost |
| MRI (hyperpolarised gas) | Low availability | |
| Technegas SPECT | Few studies | |
| PET | ||
| Bronchoscopy | Direct measurement of airway resistance | Invasive |
| Research procedure | ||
| Transbronchial biopsies | Invasive | |
| Airway remodelling and inflammatory cells | ||
| Limited to single-centre research studies | ||
| BALF | Invasive | |
| 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.