[Lung function diagnostics for the small airways]

In the recent years growing interest has focused on the involvement of the distal airways (internal diameter < 2 mm) in obstructive lung diseases and other pulmonary conditions. Inflammation in the small airways seems to play a major role in severe and uncontrolled asthma as a major determinant of airflow obstruction. Thus, small airways represent an important target for inhalation therapy. Currently there is no accepted single lung function parameter to detect small airway dysfunction. Various invasive and non-invasive techniques have been described. In future, non-invasive lung function testing will gain more importance. Using spirometry or body plethysmography, lung function parameters such as the ratio of forced vital capacity to slow vital capacity (FVC/SVC) and the residual volume (RV) can provide information about air trapping in small airway disease. Recent data show that techniques such as impulse oscillometry, nitrogen washout testing and analysis of exhaled nitric oxide are promising tools to assess involvement of the small airways. Impulse oscillometry is a sensitive method to calculate peripheral airway resistance, nitrogen washout allows one to detect air trapping and inhomogeneous ventilation in the distal lung, and the alveolar nitric oxide concentration represents a marker of peripheral inflammation. Further studies are needed to validate these functional tests or their combination for diagnosis and assessment of treatment response in pulmonary diseases involving small airways.