Criteria for evaluating mucus functions and their disorders in chronic bronchitis

In chronic bronchitis, disorders of the physical and chemical properties of mucus may contribute to bronchial obstruction. The abnormalities are analysed in sputum collected by physiotherapy. Measurements of the rheological properties (apparent viscosity and strain recovery), content analysis of secretory immunoglobulin A (S-IgA) and of serum albumin, and determination of the transport velocity of sputum on the ciliated frog palate provide reliable indices of the functional capacity of bronchial mucosa. The effectiveness of mucociliary clearance and coughing is analysed in patients by measuring the rate of removal of radioactive particles deposited on proximal airways. Different types of pathological secretions may be observed. In subjects with occasional cough and sputum, the secretions are characterized by a high content of S-IgA and serum albumin, and a high viscosity and low elasticity, reflecting their high degree of cross-linking. In chronic bronchitics, the S-IgA content and viscoelastic properties of sputum decrease as the illness progresses. During infection, purulent sputum exhibits high viscosity and low strain recovery. Such modifications of the rheological properties of mucus may impair mucociliary clearance. The role of the elastic component is predominant. Sputum, characterized by very low strain recovery (SR less than 4 units) or conversely by very high strain recovery (SR greater than 15 units), is transported at a low rate by mucociliary clearance. A marked hyperviscosity (no greater than 200 poises (120 N s m-2)) also appears as a limiting factor of the mucociliary clearance.