Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 24

As we have already discussed, the greater part of airway resistance resides in the central airways. These airways are well supported by cartilage and so generally maintain their calibre even at low lung volumes. The calibre of the small airways, without cartilaginous support, is heavily dependent on lung volume. At lower lung volumes, their calibre is reduced, and resistance is increased. During expiration, therefore, as lung volume declines, the site of principal resistance moves from the large central airways to the small peripheral airways. The PEFR (see Chapter 3) tests expiratory flow at high lung volume and is therefore determined largely by the central airways. The forced expiratory volume in 1 second (FEV₁; see Chapter 3) is also heavily influenced by the central airway, though not as much as PEFR. Specialised lung function tests that measure expiratory flow at lower lung volumes (e.g. FEF _25‐75 and ; see Chapter 3) therefore provide more information about the smaller airways.