Spirometry is useful in the diagnosis of lung diseases. On the basis of pulmonary function tests, lung disorders can be classified as restrictive or obstructive. In restrictive disorders, such as pulmonary fibrosis, the vital capacity is reduced to below normal. The rate at which the vital capacity can be forcibly exhaled, however, is normal. In disorders that are exclusively obstructive, by contrast, the vital capacity is normal because lung tissue is not damaged. In asthma for example, the vital capacity is normal, but expiration is more difficult and takes a longer time because bronchoconstriction increases the resistance to air flow. Obstructive disorders are therefore diagnosed by tests that measure the rate of expiration. One such test is the forced expiratory volume (FEV), in which the percentage of the vital capacity that can be exhaled in the first second (FEVi) is measured (fig. 16.17). An FEVi that is significantly less than 80% suggests the presence of obstructive pulmonary disease.
Maximum expiration = 1 L
Maximum expiration = 1 L
■ Figure 16.17 The one-second forced expiratory volume (FEV|) test. The percentage in (a) is normal, whereas that in (b) may indicate an obstructive pulmonary disorder such as asthma or bronchitis.
reactions in which immunoglobulin E (IgE) is produced (see chapter 15), by exercise (in exercise-induced bronchoconstriction), by breathing cold, dry air, or by aspirin (in a minority of asthmatics).
Asthma is often treated with glucocorticoid drugs, which inhibit inflammation. A new antileukotriene drug jj also is now available to suppress the inflammatory response. Epinephrine and related compounds stimulate beta-adrenergic receptors in the bronchioles, and by this means promote bronchodilation. Therefore, epinephrine was frequently used as an inhaled spray to relieve the symptoms of an asthma attack. It has since been learned that there are two subtypes of beta receptors for epinephrine, and that the subtype in the heart (called P1) is different from the one in the bronchioles (P2). Capitalizing on these differences, compounds such as terbutaline have been developed. These compounds can more selectively stimulate the p2-adrenergic receptors and cause bronchodilation without affecting the heart to the extent that epinephrine does.
Bronchoconstriction often occurs in response to inhalation of noxious agents present in smoke or smog. The FEV| has therefore been used by researchers to determine the effects of various components of smog and passive cigarette smoke on pulmonary function. These studies have shown that it is unhealthy to exercise on very smoggy days and that inhalation of smoke from other people's cigarettes in a closed environment can adversely affect pulmonary function.
There is normally a decline in the FEV1 with age, but research suggests that this decline may be accelerated in cigarette smokers. Smokers under the age of 35 who quit have improved lung function; those who quit after the age of 35 slow their age-related decline in FEV| to normal rates.
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