Pathophysiology of cough.

Mucous secretions are normally removed by ciliary beating. When this defense mechanism is impaired or overwhelmed by increased secretions, cough then becomes an important means of secretion removal. For cough to be effective, the linear velocity of gas traveling through the airways should be high. Since the linear velocity of gas is related to flow and the cross-sectional area of the airways, cough is most effective when expiratory flows are great (effort independent) and dynamic compression (effort dependent) leads to a reduction of the cross-sectional area of the larger downstream airways. Cough failures may be related to either inadequate generation of expiratory flow rates (that is, in obstructive lung disease or inspiratory muscle weakness), failure to dynamically compress the airways (that is, in expiratory muscle weakness or increased collapsibility), alterations in airway geometry (that is, in bronchiectasis), or abnormal quantity or quality of mucous production (that is, in chronic bronchitis).