studies (Crapo et al., 1981; Knudson et al., 1983; Morris et al., 1971). Predicted values depend on an individual's height, age, and sex. Tall, young men have the greatest predicted lung function. When an individual's actual function is below predicted values, the magnitude of the abnormality may be described according to various schemes such as that presented in Table 5-8 (Engelberg, 1988).

Spirometry interpretation characterizes the type of abnormality that is present. Typically, asthma is characterized by airflow obstruction, meaning that a disproportionate decrease in FEV1 relative to FVC exists and peak flow rates are reduced. Hypersensitivity pneumonitis is characterized by a reversible restrictive pattern; that is, FEV1 and FVC are reduced in parallel, and peak flow rates may be unchanged despite significant drops in other measures of lung function. Exceptions to these generalizations, however, are well recognized. In asymptomatic or mild asthma, lung function may fall within the normal range. In more severe asthma, symmetric reductions of FEV1 and FVC occasionally occur and may be misinterpreted as restrictive lung disease. Measurement of lung volumes (see below) will show an increased total lung capacity and air trapping. Clinical evaluation using bronchodilators may also result in improvement. The use of flow rates is helpful in limited circumstances and requires more careful interpretation (ATS, 1991). Spirometry may be repeated after administration of bronchodilators; improvement in pulmonary function indicates the presence of reversible airway obstruction, a characteristic feature of asthma.

Spirometry interpretation includes determining whether lung function has changed over time. Comparison with previous tests by the same subject

TABLE 5-8 American Medical Association/American Thoracic Society: Description of Respiratory Impairment

Parameter

None

Mild

Moderate

Severe

FVCa

≥80

60–70

51–59

≤50

FEV1

≥80

60–79

41–59

<40

FEV1/FVC

≥70

50–69

41–59

<40

DLCOb

≥80

60–79

41–59

<40

or

 

 

 

 

VO2maxc ml(kg · min)

>25

20–25

15–20

<15

a Predicted values of FVC are from Crapo and Morris, 1981; Crapo et al., 1981.

b DLCO, diffusing capacity of the lungs for carbon monoxide.

c VO2max, maximum volume of oxygen exhaled.

SOURCES: Engelberg, 1988; Renzetti et al., 1986.



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