Peripheral airways in asthma.

The peripheral, or small, airways are usually defined as conducting airways that are less than 2 mm in internal diameter and extend from the noncartilaginous bronchioles to the alveolar ducts. Noninvasively measuring the function of the small airways in isolation is difficult since they make up only about 10% of total airway resistance. Quantitative pathologic studies have shown that both the small and large airways are involved in inflammation and remodeling in asthma. Recent studies also have shown that inflammation involves the alveoli surrounding small airways in asthma and that the distribution of different inflammatory cells across the airway wall varies in both large and small airways. Inhaled treatment that targets the small airways may be more effective than treatment that is deposited more proximally and suggests that treatments in the future need to address the variable distribution of pathology in the bronchial tree in asthma.