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A cough is an explosive expiration that provides a protective mechanism for clearing the tracheobronchial tree of secretions and foreign material.(1) Cough is one of the most common symptoms for which patients seek medical help. Reasons include discomfort from the cough itself, interference with normal lifestyle, and fear of the connection with cough to cancer or AIDS.
Coughing may be initiated either voluntarily or reflexively. The sequence of a cough starts with a deep inspiration and is followed by glottic closure, relaxation of the diaphragm, and muscle contraction against a closed glottis. This action causes a positive intrathoracic pressure and narrowing of the trachea. Once the glottis opens, air is forced rapidly through the trachea. The abrupt airway vibrations and high airflow velocity that the cough produces are highly effective in mobilizing secretions; they also cause the characteristic sound of a cough.(2)
Cough receptors located in the pharynx, stomach, external auditory canal, diaphragm, nose, and bifurcations of the large airways of the tracheobronchial tree initiate the cough reflex.(3) Cough can be initiated by inhalation of airway irritants such as smoke, dust, or fumes, or by aspiration of such things as gastric contents, upper airway secretions, or foreign bodies. When the irritations are due to upper airway secretions, as is seen in postnasal drip, or gastric contents, as in gastroesophageal reflux disease (GERD), the initiating factor may go unrecognized and the cough may be persistent. Prolonged exposure to such irritants often causes airway inflammation, which itself can trigger cough and sensitizes the airway to other irritants.
Any disorder that causes inflammation, irritation, constriction, or compression of airways can lead to coughing. Airway inflammation commonly results from bacterial or viral infection. In viral bronchitis, airway inflammation sometimes persists long after resolution of typical acute symptoms, thereby producing a prolonged cough lasting for several weeks. The duration of a cough is a vital clue to its etiology. The most common causes of chronic cough are postnasal drip, bronchial asthma, GERD, chronic bronchitis, and bronchiectasis.(4)
Asthma, which is associated with bronchoconstriction and airway inflammation, commonly causes cough. Generally, the clinical setting suggests when a cough is secondary to asthma; however, some patients present with a cough in the absence of dyspnea or wheezing.
The smoking of cigarettes, pipes, and cigars irritates the airways and reduces the efficiency of the ciliated cells that move particle-laden mucus upwards for expectoration, causing a characteristic hacking "smoker's cough." Other conditions associated with coughing include neoplasm infiltrating the airway wall, granulomas associated with tuberculosis, or endobronchial sarcoidosis, or compression of airways resulting from extrinsic masses, including aortic aneurysms, mediastinal tumors, and lymph nodes. Congestive heart failure or congenital heart defects may also be associated with a cough, and a non-productive cough is associated with the use of ACE inhibitors in 5-20 percent of the patients taking them. Lung diseases potentially producing chronic cough include interstitial lung disease, pneumonia, and lung abscess.
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