Cough in non-cystic fibrosis bronchiectasis

Abstract

Non-cystic fibrosis bronchiectasis (NCFBE) belongs to the spectrum of chronic suppurative lung diseases and is characterised by persistent wet/sputum-productive cough and airway dilatation. Morphological and structural changes in the airways lead to changes in airflow, impair breathing-induced mucus transport and sliding, and reduce the shear forces of cough. Moreover, mucus hyperviscosity contributes to compromised ciliary activity and the pathogenesis of the disease. This mini-review highlights the role of cough in NCFBE, especially with respect to mucus clearance. Cough is the principal backup mechanism when mucus clearance is impaired due to either reduced function of cilia- and breathing-induced mucus transport, or abnormal mucus, or both. The efficiency of cough in overcoming the cohesive and adhesive properties of mucus is determined by both the forces applied to mucus by airflow and the mucus-airway surface properties. In NCFBE, mucus hyperviscosity contributes to impaired mucus clearance and determines disease pathogenesis; therefore, it may be a therapeutic target. The primary objectives of physiotherapy regimens in NCFBE are mucus hydration and the establishment of an optimal expiratory airflow velocity, which exerts shearing forces on the mucus located on the airway surface. Modifying the rheological properties of mucus and enhancing its transport whenever possible (by breathing manoeuvres, ciliary activity and cough) represent prime goals in preventing disease progression and, indeed reversing, bronchiectasis in the early stages of the disease, as well as preventing pulmonary exacerbations.

FIGURE 1

a). Simplified representation of physiological dynamics for mucus clearance. 1) Mucus with normal osmotic properties. 2) The periciliary layer (PCL) allows the cilia to beat without catching in the mucus and acts as a lubricant that allows the mucus to slide along the interface. 3) Normal activity of the bronchial wall in response to breathing and cough. 4) Breathing manoeuvres promotes mucus sliding over PCL. 5) Air flow acts tangentially to the mucus surface. 6) Cough shear forces overcome the cohesive and adhesive properties of mucus and dislodge mucus masses. b). Simplified representation of major alterations in bronchiectatic milieu in non-cystic fibrosis bronchiectasis. 1) Mucus hyperviscosity due to increased organic content and the absence of cilial motility-dependent sensing of mucus concentration. 2) Osmotic pressure of the mucus layer exceeds that of the PCL; the PCL loses water and collapses, compromising normal ciliary activity. 3) Alterations in bronchial wall structure and morphology airways lead to changes in airflow and reduce mucus clearance mediated by tidal breathing. 4) Collapsed PCL generates increased friction because of changes in its surface interface. 5) Changes in air flow reduce the cough shear forces needed to efficiently overcome the cohesive and adhesive properties of stacked mucus. 6) Mucus build-up and adherence to the airway surfaces of inflamed and/or infected intraluminal mucus plaques.