Chronic obstructive pulmonary disease (COPD) is comprised primarily of two related diseases – chronic bronchitis and emphysema. In both diseases, there is chronic obstruction of the flow of air through the airways and out of the lungs, and the obstruction generally is permanent and progressive over time.
Asthma also is a pulmonary disease in which there is obstruction to the flow of air out of the lungs, but unlike chronic bronchitis and emphysema, the obstruction in asthma usually is reversible. Between "attacks" of asthma the flow of air through the airways usually is good.
There are exceptions, however. In some patients with COPD, the obstruction can be partially reversed by medications that enlarge or dilate the airways (bronchodilators) as with asthma. Conversely, some patients with asthma can develop permanent airway obstruction if chronic inflammation of the airways leads to scarring and narrowing of the airways. This process is referred to as lung remodeling. These asthma patients with a fixed component of airway obstruction are also considered to have COPD.
There also is frequent overlap among COPD patients. Thus, patients with emphysema may have some of the characteristics of chronic bronchitis. Similarly, patients with chronic bronchitis also may have some of the characteristics of emphysema.
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