Interpretation of Pulmonary Function Tests A Practical Guide, 3. ed

2. Spirometry: Dynamic Lung Volumes

Spirometry is used to measure the rate at which the lung changes volume during forced breathing maneuvers. The most commonly performed test uses the forced expiratory vital capacity (FVC) maneuver, in which the subject inhales maximally and then exhales as rapidly and completely as possible. Of all the tests considered in this book, the FVC test is both the simplest and the most important. Generally, it provides most of the information that is to be obtained from pulmonary function testing. It behooves the reader to have a thorough understanding of this procedure.


The two methods of recording the FVC test are shown in Figure 2-1. In Figure 2-1 A, the subject blows into a spirometer that records the volume exhaled, which is plotted as a function of time, the solid line. This is the classic spirogram showing the time course of a 4-L FVC. Two of the most common measurements made from this curve are the forced expiratory volume in 1 second (FEV1) and the average forced expiratory flow rate over the middle 50% of the FVC (FEF25-75). These are discussed later in this chapter.

The FVC test can also be plotted as a flow-volume (FV) curve, as in Figure 2-1B. The subject again exhales forcefully into the spirometer through a flowmeter that measures the flow rate (in liters per second) at which the subject exhales. The volume and the rapidity at which the volume is exhaled (flow in liters per second) are plotted as the FV curve. Several of the common measurements made from this curve are discussed later in this chapter.

FIG. 2-1. The two ways to record the forced vital capacity (FVC) maneuver.

A. Volume recorded as a function of time, the spirogram. FEV1, forced expiratory volume in 1 second; FEF25-75, average forced expiratory flow rate over the middle 50% of the FVC. B. Flow recorded as a function of volume exhaled, the flow-volume curve. FEF25(50,75), forced expiratory flow after 25% (50%, 75%) of the FVC has been exhaled.

The two curves reflect the same data, and a computerized spirometer can easily plot both curves with the subject exhaling through either a flowmeter or a volume recorder. Integration of flow provides volume, which, in turn, can be plotted as a function of time, and all the measurements shown in Figure 2-1 are also readily computed. Conversely, the volume signal can be differentiated with respect to time to determine flow. In our experience, the FV representation (Fig. 2-1B) is the easiest to interpret and the most informative. Therefore, we will use this representation almost exclusively.

Caution: It is extremely important that the subject be instructed and coached to perform the test properly. Expiration must be after a maximal inhalation, initiated as rapidly as possible, and continued with maximal effort until no more air can be expelled. ''Good'' and ''bad'' efforts are shown later on page 16 in Figure 2-6.


The FVC test is the most important pulmonary function test for the following reason: For any given individual during expiration, there is a unique limit to the maximal flow that can be reached at any lung volume. This limit is reached with moderate expiratory efforts, and increasing the force used during expiration does not increase the flow. In Figure 2-1B, consider the maximal FV curve obtained from a normal subject during the FVC test. Once peak flow has been achieved, the rest of the curve defines the maximal flow that can be achieved at any lung volume. Thus, at FEF after 50% of the vital capacity has been exhaled (FEF50), the subject cannot exceed a flow of 5.2 L/s regardless of how hard he or she tries. Note that the maximal flow that can be achieved decreases in an orderly fashion as more air is exhaled (that is, as lung volume decreases) until at residual volume (4 L) no more air can be exhaled. The FVC test is powerful because there is a limit to maximal expiratory flow at all lung volumes after the first 10 to 15% of FVC has been exhaled. Each individual has a unique maximal expiratory FV curve. Because this curve defines a limit to flow, the curve is highly reproducible in a given subject. Most important, maximal flow is very sensitive to the most common diseases that affect the lung.

The basic physics and aerodynamics causing this flow-limiting behavior are not explained here. However, the concepts are illustrated in the simple lung model in Figure 2-2.

Figure 2-2A shows the lung at full inflation before a forced expiration. Figure 2-2B shows the lung during a forced expiration. As volume decreases, dynamic compression of the airway produces a critical narrowing that develops in the trachea and produces limitation of flow. As expiration continues and lung volume decreases even more, the narrowing migrates distally into the main bronchi and beyond. Three features of the model determine the maximal expiratory flow of the lung at any given lung volume: lung elasticity (e), which drives the flow and holds the airways open; size of the airways (f); and resistance to flow along these airways.

The great value of the FVC test is that it is very sensitive to diseases that alter the lung's mechanical properties:

1. In chronic obstructive pulmonary disease, emphysema causes a loss of lung tissue (alveoli are destroyed). This loss results in a loss of elastic recoil pressure, which is the driving pressure for maximal expiratory flow. Airways are narrowed because of loss of tethering of lung tissue. This results in increased flow resistance and decreased maximal expiratory flow.

FIG. 2-2. Simple lung model at full inflation (A) and during a forced deflation (B). The lung (a) is contained in a thorax (b) whose volume can be changed by the piston (c). Air exits from the lung via the trachea (d). The lung has elasticity (e), which both drives the flow and plays a role in holding the compliant bronchi (f) open. Critical narrowing (CN) occurs during the FVC maneuver.

2. In chronic bronchitis, both mucosal thickening and thick secretions in the airways lead to airway narrowing, increased resistance to flow, and decreased maximal flow.

3. In asthma, the airways are narrowed as a result of broncho- constriction and mucosal inflammation and edema. This narrowing increases resistance and decreases maximal flow.

4. In pulmonary fibrosis, the increased tissue elasticity may distend the airways and increase maximal flow, even though lung volume is reduced.


Tables and equations are used to predict the normal values of the measurements to be discussed. The best values have been obtained from nonsmoking, normal subjects. The prediction equations we use in our laboratory are listed in the Appendix. The important prediction variables are the size, sex, and age of the subject. Certain races, African American and Asian, for example, require race-specific values. Size is best estimated with body height. The taller the subject, the larger the lung and its airways, and thus maximal flows are higher. Women have smaller lungs than men of a given height. With aging, lung elasticity is lost, and thus airways are smaller and flows are lower. The inherent variability in normal predictive values must be kept in mind, however (as in the bell-shaped normal distribution curve of statistics). It is almost never known at what point in the normal distribution a given subject starts. For example, lung disease can develop in people with initially above-average lung volumes and flows. Despite a reduction from their initial baseline, they may still have values within the normal range of a population.

PEARL: Body height should not be used to estimate normal values for a subject with kyphoscoliosis. Why? Because the decreased height in such a subject will lead to a gross underestimation of the normal lung volume and flows. Rather, the patient's arm span should be measured and used instead of height in the reference equations. In a 40-year-old man with kyphoscoliosis, vital capacity is predicted to be 2.78 L if his height of 147 cm is used, but the correct expected value of 5.18 L is predicted if his arm span of 178 cm is used—a 54% difference. The same principle applies to flow predictions.


The FVC is the volume expired during the FVC test; in Figure 2-1 the FVC is 4.0 L. Many abnormalities can cause a decrease in the FVC.

PEARL: To our knowledge, only one disorder, acromegaly, causes an abnormal increase in the FVC. The results of other tests of lung function are usually normal in this condition. However, persons with acromegaly are at increased risk for development of obstructive sleep apnea as a result of hypertrophy of the soft tissues of the upper airway.

Figure 2-3 presents a logical approach to considering possible causes of a decrease in FVC:

1. The problem may be with the lung itself. There may have been a resectional surgical procedure or areas of collapse. Various other conditions can render the lung less expandable, such as fibrosis, congestive heart failure, and thickened pleura. Obstructive lung diseases may reduce the FVC by limiting deflation of the lung (Fig. 2-3).

FIG. 2-3. Various conditions that can restrict the forced expiratory vital capacity. CHF, congestive heart failure.

2. The problem may be in the pleural cavity, such as an enlarged heart, pleural fluid, or a tumor encroaching on the lung.

3. Another possibility is restriction of the chest wall. The lung cannot inflate and deflate normally if the motion of the chest wall (which includes its abdominal components) is restricted.

4. Inflation and deflation of the system require normal function of the respiratory muscles, primarily the diaphragm, the intercostal muscles, and the abdominal muscles.

If the four possibilities listed are considered (lung, pleura, chest wall, muscles), the cause(s) of decreased FVC is usually determined. Of course, combinations of conditions occur, such as the enlarged failing heart with engorgement of the pulmonary vessels and pleural effusions. It should be remembered that the FVC is a maximally rapid expiratory vital capacity. The vital capacity may be larger when measured at slow flow rates; this situation is discussed in Chapter 3.

Two terms are frequently used in the interpretation of pulmonary function tests. One is an obstructive defect. This is lung disease that causes a decrease in maximal expiratory flow so that rapid emptying of the lungs is not possible; conditions such as emphysema, chronic bronchitis, and asthma cause this. Frequently, an associated decrease in the FVC occurs. A restrictive defect implies that lung volume, in this case the FVC, is reduced by any of the processes listed in Figure 2-3, except those causing obstruction.

Caution: In a restrictive process, the total lung capacity will be less than normal (see Chapter 3).

Earlier in the chapter, it was noted that most alterations in lung mechanics lead to decreased maximal expiratory flows. Low expiratory flows due to airway obstruction are the hallmark of chronic bronchitis, emphysema, and asthma. The measurements commonly obtained to quantify expiratory obstruction are discussed below.


The FEV1 is the most reproducible, most commonly obtained, and possibly most useful measurement. It is the volume of air exhaled in the first second of the FVC test. The normal value depends on the patient's size, age, sex, and race, just as does the FVC. Figure 2-4A and B show the FVC and FEV1 from two normal subjects; the larger subject (A) has the higher FVC and FEV1.

When flow rates are slowed by airway obstruction, as in emphysema, the FEV1 is decreased by an amount that reflects the severity of the disease. The FVC also may be reduced, although usually to a lesser degree. Figure 2-4C shows a severe degree of obstruction. The 1-second volume (FEV1) is easily identified directly from the spirogram. A 1-second mark can be added to the FV curve to identify the FEV1, as shown in the figure. The common conditions producing expiratory slowing or obstruction are chronic bronchitis, emphysema, and asthma.

In Figure 2-4D, the FEV1 is reduced because of a restrictive defect, such as pulmonary fibrosis. A logical question is, ''How can I tell whether the FEV1 is reduced as a result of airway obstruction or a restrictive process?'' This question is considered next.

FIG. 2-4. Typical spirograms and flow-volume curves during forced expiration. A and B. Normal subjects of different sizes. C. Patient with severe airway obstruction. D. Values typical of a pulmonary restrictive process. The arrows indicate the forced expiratory volume in 1 second (FEV1). The ratios of FEV1 to forced expiratory vital capacity (FEV1/FVC) and the slopes of the flow-volume curves (dashed lines) are also shown.


The FEV1/FVC ratio is generally expressed as a percentage. The amount exhaled during the first second is a fairly constant fraction of the FVC, irrespective of lung size. In the normal adult, the ratioranges from 75 to 85%, but it decreases somewhat with aging. Children have high flows for their size, and thus, their ratios are higher, up to 90%.

The significance of this ratio is twofold. First, it aids in quickly identifying persons with airway obstruction in whom the FVC is reduced. For example, in Figure 2-4C, the FEV1/FVC is very low at 43%, indicating that the low FVC is due to airway obstruction and not pulmonary restriction. Second, the ratio is valuable for identifying the cause of a low FEV1. In pulmonary restriction (without any associated obstruction), the FEV1 and FVC are decreased proportionally; hence, the ratio is in the normal range, as in the case of fibrosis in Figure 2-4D, in which it is 87%. Indeed, in some cases of pulmonary fibrosis, the ratio may increase even more because of the increased elastic recoil of such a lung.

Thus, in regard to the question of how to determine whether airway obstruction or a restrictive process is causing a reduced FEV1, the answer is to check the FEV1 /FVC ratio. A low FEV1 with a normal ratio usually indicates a restrictive process, whereas a low FEV1 and a decreased ratio signify a predominantly obstructive process.

In severe obstructive lung disease near the end of a forced expiration, the flows maybeverylow, barelyperceptible. Continuation of the forced expiration can be very tiring and uncomfortable. To avoid patient fatigue, one can substitute the volume expired in 6 seconds, the FEV6, for the FVC in the ratio. Normal values for FEV1/FEV6 were developed in the third National Health and Nutrition Examination Survey (NHANES III) [1].

PEARL: Look at the FV curve. If significant scooping or concavity can be seen, as in Figure 2-4C, obstruction is usually present (older normal adults usually have some degree of scooping). In addition, look at the slope of the FV curve, the average change in flow divided by the change in volume. In normal subjects, this is roughly 2.5 (2.5 L/s per liter). The normal range is approximately 2.0 to 3.0. In the case of airway obstruction (Fig. 2-4C), the average slope is lower, 1.1. In the patient with fibrosis (Fig. 2-4D), the slope is normal to increased, 5.5. The whole curve needs to be studied.

Caution: Recall that a low FEV1 or FVC and a normal FEV1 /FVC ratio usually indicate restriction. However, a subset of patients with a low FEV1 and normal FEV1/FVC ratio also have a normal total lung capacity, which rules out significant restriction. This is termed a nonspecific ventilatory limitation (see pages 37-39, including Fig. 3-8).


Figure 2-5 shows the other most common measurements of maximal expiratory flow, generally referred to as forced expiratory flow (FEF). All of these measurements are decreased in obstructive disease.

FEF25-75 is the average FEF rate over the middle 50% of the FVC. This variable can be measured directly from the spirogram. A microprocessor is used to obtain it from the FV curve. Some investigators consider the FEF25-75 to be more sensitive than the FEV1 for detecting early airway obstruction, but it has a wider range of normal values.

FEF50 is the flow after 50% of the FVC has been exhaled, and FEF75 is the flow after 75% of the FVC has been exhaled.

Peak expiratory flow (PEF), which is also termed maximal expiratory flow (FEFmax), occurs shortly after the onset of expiration. It is reported in either liters per minute (PEF) or liters per second (FEFmax). The PEF, more than the other measures, is very dependent on patient effort—the patient must initially exhale as hard as possible to obtain reproducible data. However, with practice, reproducible results are obtained. Inexpensive portable dEV1ces allow patients to measure their PEF at home and so monitor their status. This method is particularly valuable for patients with asthma.

As shown in Figure 2-5, these other measures, just as with the FEV1, can be reduced in pure restrictive disease. Again, the FV curve and the FEV1/FVC ratio must be considered.


Although this text is not intended to consider test performance (the results are assumed to be accurate), the FVC test must be performed correctly. Generally, judgment about the performance can be made from the FV curve. Occasionally, less-than-ideal curves may be due to an underlying problem such as muscle weakness.

FIG. 2-5. Other measurements of maximal expiratory flow in three typical conditions—normal, obstructive disease, and pulmonary restrictive disease. The average forced expiratory flow (FEF) rate over the middle 50% of the forced expiratory vital capacity (FVC) (FEF25 - 75) is obtained by measuring the volume exhaled over the middle portion of the FVC maneuvers and dividing it by the time required to exhale that volume. FEF25, FEF after 25% of the FVC has been exhaled; FEF50, flow after 50% of FVC has been exhaled; FEF75, flow after 75% of FVC has been exhaled; PEF, peak expiratory flow.

FIG. 2-6. Examples of good and unacceptable forced expiratory vital capacity maneuvers. A. Excellent effort. a, rapid climb to peak flow; b, continuous decrease in flow; c, termination at 0 to 0.05 L/s of zero flow. B. Hesitating start makes curve unacceptable. C. Subject did not exert maximal effort at start of expiration; test needs to be repeated. D. Such a curve almost always indicates failure to exert maximal effort initially, but occasionally, it is reproducible and valid, especially in young, nonsmoking females. This is called a rainbow curve. This curve may be found in children, patients with neuromuscular disease, or subjects who perform the maneuver poorly. In B, C, and D, the dashed line indicates the expected curve; the arrow indicates the reduction in flow caused by performance error. E. Curve shows good start, but subject quit too soon; test needs to be repeated. Occasionally, this is reproducible, and this curve can be normal for some young nonsmokers.

F. Coughing during the first second will decrease the forced expiratory volume in 1 second. The maneuver should be repeated. G. Subject stopped exhaling momentarily; test needs to be repeated. H. This curve with a ''knee'' is a normal variant that often is seen in nonsmokers, especially young women.

In Figure 2-6, an excellent effort (A) is contrasted with ones that are unacceptable or require repeating of the test. The three features of the well-performed test are that (1) the curve shows a rapid climb to peak flow (a); (2) the curve then has a fairly smooth, continuous decrease in flow (b); and (3) the curve terminates at a flow within 0.05 L/s of zero flow or ideally at zero flow (c). The other curves in Figure 2-6 do not satisfy at least one of these features.

An additional important criterion is that the curves should be repeatable. Ideally, two curves should exhibit the above-described features and have peak flows within 10% of each other and FVC and FEV1 volumes within 150 mL or 5% of each other. The technician needs to work with the patient to satisfy these repeatability criteria. The physician must examine the selected curve for the contour characteristics. If the results are not satisfactory, the test may be repeated so that the data truly reflect the mechanical properties of a patient's lungs. A suboptimal test must be interpreted with caution because it may suggest the presence of disease when none exists.


The test for maximal voluntary ventilation (MVV) is an athletic event. The subject is instructed to breathe as hard and fast as possible for 10 to 15 seconds. The result is extrapolated to 60 seconds and reported in liters per minute. There can be a significant learning effect with this test, but a skilled technician can often avoid this problem.

A low MVV can occur in obstructive disease, in restrictive disease, in neuromuscular disease, in heart disease, in a patient who does not try or who does not understand, or in a frail patient. Thus, this test is very nonspecific, and yet it correlates well with a subject's exercise capacity and with the complaint of dyspnea. It is also useful for estimating the subject's ability to withstand certain types of major operation (see Chapter 10).

PEARL: In a well-performed MVV test in a normal subject, the MVV is approximately equal to the FEV1 x 40. If the FEV1 is 3.0 L, the MVV should be approximately 120 L/min (40 x 3). On the basis of a rEV1ew of many pulmonary function tests, we set the lower limit of the predicted MVV at FEV1 x 30. Example: A patient's FEV1 is 2.5 L, and the MVV is 65 L/min. The FEV1 x 30 is 75 L/min, and thus, the MVV of 65 L/min leads to a suspicion of poor test performance or fatigue. There are two important pathologic causes for the MVV to be less than the predicted lower limit in an otherwise normal subject: obstructing lesions of the major airways (see section 2K, page 18) and respiratory muscle weakness (see section 9D, page 97). An MVV much greater than FEV1 x 40 may mean that the FEV1 test was poorly performed. However, this product estimate may be less useful in advanced obstructive disease, when the subject's MVV sometimes exceeds that predicted from the FEV1 (see Chapter 15 case 20, page 202).

PEARL: Some lesions of the major airway (see page 21, the Pearl) cause the MVV to be reduced out of proportion to the FEV1. The same result can occur in patients who have muscle weakness, as in neuromuscular diseases (amyotrophic lateral sclerosis, myasthenia gravis, polymyositis). Thus, all these conditions need to be considered when the MVV is reduced out of proportion to the FEV1.


With spirometer systems that measure both expiratory and inspiratory flows, the maximal inspiratory flow (MIF) can be measured. The usual approach is shown in Figure 2-7A. The subject exhales maximally (the FVC test) and then immediately inhales as rapidly and completely as possible, producing an inspiratory curve. The combined expiratory and inspiratory FV curves form the FV loop. Increased airway resistance decreases both maximal expiratory flow and MIF. However, unlike expiration, in which there is a limit to maximal flow, no mechanism such as dynamic compression limits MIF. Thus, it is very effort-dependent.

For these reasons, measurements of MIF are not widely obtained. They add little, other than cost, to the evaluation of most patients undergoing pulmonary function tests. The main value of testing MIF is for detecting lesions of the major airway.


Obstructing lesions involving the major airway (carina to oral pharynx) are relatively uncommon. When present, however, they can often be detected by changes in the FV loop [2]. This is a very important diagnosis to make.

The identification of these lesions from the FV loop depends on two characteristics. One is the behavior of the lesion during rapid expiration and inspiration. Does the lesion narrow and decrease flow excessively during one or the other phases of respiration? If it does, the lesion is categorized as variable. If the lesion is narrowed and decreases flow equally during both phases, the lesion is categorized as fixed. The other characteristic is the location of the lesion. Is it extrathoracic (above the thoracic outlet) or intrathoracic (to and including the carina but generally not beyond)?

FIG. 2-7. Comparison of typical flow-volume loops (A-C) with the classic flow-volume loops in cases of lesions of the major airway (D-F). FEF50, forced expiratory flow (expir flow) after 50% of the FVC has been exhaled; FIF5o, forced inspiratory flow (inspir flow) measured at the same volume as FEF50.

Figure 2-7 illustrates typical FV loops in normal subjects (Fig. 2-7A), various disease states (Fig. 2-7B and C), and the three classic loops caused by lesions of the major airway (Fig. 2-7D-F). The factors that determine the unique contours of the curves for lesions of the major airway can be appreciated by considering the relationship between the intra-airway and extra-airway pressures during these forced maneuvers.

During forced expiration, the airway pressure in the intrathoracic trachea (Ptr) is less than the surrounding pleural pressure (Ppl), and this airway region normally narrows. The airway pressure in the extrathoracic trachea (Ptr) is higher than the surrounding atmospheric pressure (Patm), and the region tends to stay distended. During forced inspiration, Ptr in the extrathoracic portion is lower than the surrounding pressure (that is, Patm), and therefore this region tends to narrow. In the intrathoracic trachea, the surrounding Ppl is more negative than Ptr, which favors dilatation of this region. In the variable lesions, these normal changes in airway size are greatly exaggerated.

Figure 2-7D shows results with a variable lesion in the extrathoracic trachea. This may be caused by, for example, paralyzed but mobile vocal cords. This is explained by the model in Figure 28 (left). During expiration, the high intra-airway pressure (Ptr) keeps the cords distended and there may be little effect on expiratory flow. Ptr is greater than Patm acting on the outside of this lesion. During inspiration, however, the low pressure in the trachea causes marked narrowing of the cords with the remarkable reduction in flow seen in the inspiratory FV loop because Patm now greatly exceeds airway pressure, Ptr.

The model in Figure 2-8 (right) also explains Figure 2-7E, a variable intrathoracic lesion, for example, a compressible tracheal malignancy. During forced expiration, the high Ppl relative to airway pressure (Ptr) produces a marked narrowing with a dramatic constant reduction in expiratory flow in the FV loop. Yet inspiratory flow may be little affected because Ppl is more negative than airway pressure and the lesion distends.

Figure 2-7F shows the characteristic loop with a fixed, orifice-like lesion. Such a lesion—a napkin-ring cancer of the trachea or fixed, narrowed, paralyzed vocal cords—interferes almost equally with expiratory and inspiratory flows. The location of the lesion does not matter because the lesion does not change size regardless of the intra-airway and extra-airway pressures.

FIG. 2-8. Model explaining the pathophysiology of the variable lesion of the major airway. Patm, atmospheric pressure acting on the extrathoracic trachea; Ppl, pressure in the pleural cavity that acts on the intrathoracic trachea; Ptr, lateral, intratracheal airway pressure.

Various indices have been used to characterize these lesions of the major airway. Figure 2-7 shows the ratio of expiratory to inspiratory flow at 50% of the vital capacity (FEF50/FIF50). The ratio dEV1ates most dramatically from the other curves in the variable lesion in the extrathoracic trachea (Fig. 2-7D). The ratio is nonspecific in the other lesions. The unique FV loop contours of the various lesions are the principal diagnostic features. Once a lesion of the major airway is suspected, confirmation by direct endoscopic visualization or radiographic imaging of the lesion is required.

Caution: Because some lesions may be predominantly, but not absolutely, variable or fixed, intermediate patterns can occur, but the loops are usually sufficiently abnormal to raise suspicion.

The spirograms corresponding to the lesions in Figure 2-7D through are not shown because they are not nearly as useful as the FV loops for detecting these lesions. Some of the clinical situations in which we have encountered these abnormal FV loops are listed in Table 2-1.

PEARL: If an isolated, significant decrease in the MVV occurs in association with a normal FVC, FEV1, and FEF25-75, or if the MVV is reduced well out of proportion to the reduction in the FEV1, a major airway obstruction should be strongly suspected. A forced inspiratory vital capacity loop needs to be obtained. Of course, an inspiratory loop is also mandated if there is a plateau on the expiratory curve (Fig. 2-7E and F). Not all laboratories routinely measure inspiratory loops. The technician needs to be asked whether stridor was heard during the MVV—it often is. In most such cases at our institution, these lesions are identified by technicians who find a low, unexplained MVV; may hear stridor; obtain the inspiratory loop; and hence make the diagnosis. Another consideration is whether the patient has a neuromuscular disorder, as discussed in section 9D, page 97.

TABLE 2-1. Examples of lesions of the major airway detected with the flow-volume loop

Variable extrathoracic lesions

Vocal cord paralysis (due to thyroid operation, tumor invading recurrent laryngeal nerve, amyotrophic lateral sclerosis, post-polio)

Subglottic stenosis

Neoplasm (primary hypopharyngeal or tracheal, metastatic from primary lesion in lung or breast)


Variable intrathoracic lesions

Tumor of lower trachea (below sternal notch)



Wegener's granulomatosis or relapsing polychondritis

Fixed lesions

Fixed neoplasm in central airway (at any level)

Vocal cord paralysis with fixed stenosis

Fibrotic stricture


Small airway disease, that is, disease of the peripheral airways, is an established pathologic finding. However, it has been difficult to develop tests that are specific indicators of small airway dysfunction. Tests such as density dependence of maximal expiratory flow and frequency dependence of compliance are difficult to perform and relatively nonspecific. (They are not discussed here.) Chapter 8 discusses the closing volume and the slope of phase III. The slope of phase III is very sensitive but relatively nonspecific. The data that may best reflect peripheral airway function are the flows measured at low lung volumes during the FVC tests. These include the FEF25-75, FEF50, and FEF75 (see Fig. 2-5, page 15), but these tests do have a wide range of normal values.


The typical test patterns discussed are summarized in Table 2-2. Because test results are nonspecific in lesions of the major airway, they are not included, the most diagnostically useful measure being the contour of the full FV loop.


Rather than merely memorizing patterns such as those listed in Table 2-2, another approach that is very useful is to visually compare the individual FV curve to the normal predicted curve (see Chapter 14).

TABLE 2-2. Typical patterns of impairment FEF25 - 75, forced expiratory flow rate over the middle 50% of the FVC; FEF50, forced expiratory flow after 50% of the FVC has been exhaled; FEV1, forced expiratory volume in 1 second; FV, flow-volume; FVC, forced expiratory vital capacity; MVV, maximal voluntary ventilation; N, normal; PEF, peak expiratory flow; I, decreased; f, increased.


1. If pulmonary fibrosis is suspected as the cause of restriction, diffusing capacity (see Chapter 4) and total lung capacity (see Chapter 3) should be determined.

2. If muscle weakness is suspected as a cause of restriction, maximal respiratory pressures should be determined (see Chapter 9).

3. For assessing the degree of emphysema, total lung capacity and diffusing capacity (see Chapters 3 and 4) should be determined.

4. If asthma is suspected, testing should be repeated after bronchodilator therapy (see Chapter 5).

In Figure 2-9A, the dashed curve is the patient's normal, predicted FV curve. As a first approximation, this curve can be viewed as defining the maximal expiratory flows and volumes that can be achieved by the patient. In other words, it defines a mechanical limit to ventilation, and all expiratory flows are usually on or beneath the curve (that is, within the area under the curve).

Assume that chronic obstructive pulmonary disease develops in the patient with the normal, predicted curve in Figure 2-9A, and then the curve becomes that shown in Figure 2-9B. At a glance, this plot provides a lot of information. First, the patient has lost a great deal of the normal area (the shaded area) and is confined to breathing in the reduced area under the measured curve. Clearly, severe ventilatory limitation is present. The concave shape of the FV curve and the low slope indicate an obstructive process. Before one even looks at the values to the right, it can be determined that the FVC and PEF are reduced and that the FEV1, FEV1/FVC ratio, FEF25-75, and FEF50must also be reduced. Because the MVV is confined to this reduced area, it too will be decreased. The numbers in the figure confirm this.

FIG. 2-9. The gestalt approach to interpreting pulmonary function data when the predicted and observed flow-volume curves are available. The shaded area between the predicted and measured curves (B and C) provides a visual index of the degree of ventilatory limitation, there being none for the normal subject in A. B is typical of severe airway obstruction. C is typical of a severe pulmonary restrictive process. FEF25-75, forced expiratory flow rate over the middle 50% of the FVC; FEF50, forced expiratory flow after 50% of the FVC has been exhaled; FEVj, forced expiratory volume in 1 second; FVC, forced vital capacity; MVV, maximal voluntary ventilation.

Next, consider Figure 2-9C, in which the patient has interstitial pulmonary fibrosis. Again, a glance at the plot reveals a substantial loss of area, indicating a moderately severe ventilatory limitation. The steep slope of the FV curve and the reduced FVC are consistent with the process being restrictive. A reduced FEV1 but a normal FEV1/FVC ratio can also be determined, and the flow rates (FEF25-75 and FEF50) can be expected to be normal to reduced. The MVV will be better preserved than that shown in Figure 2-9B because high expiratory flows can still develop, albeit over a restricted volume range. The numbers confirm these conclusions.

The gestalt approach is a very useful first step in analyzing pulmonary function data. The degree of ventilatory limitation can be defined according to loss of area under the normal predicted FV curve, the shaded areas in Figure 2-9B and C. We arbitrarily define an area loss of 25% as mild, 50% as moderate, and 75% as severe ventilatory limitation.


1. Hankinson JL, Odencrantz JR, Fedan KB. Spirometric reference values from a sample of the general U.S. population. Am J Respir Crit Care Med 159:179-187,1999.

2. Miller RD, Hyatt RE. Obstructing lesions of the larynx and trachea: clinical and physiologic characteristics. Mayo Clin Proc 44:145-161, 1969.

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