The two methods of recording the FVC test are shown in Figure 2-1. In Figure 2-1A, 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 (FEF) rate over the middle 50% of the FVC (FEF_25-75). 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.

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 13 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 (i.e., 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:

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:

If the four possibilities listed are considered (lung, pleura, chest wall, and 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 a 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 (TLC) 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 shows the FVC and FEV₁ from two normal subjects; the larger subject (A) has the larger 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 may also be reduced, although usually to a lesser degree. Figure 2-4C shows a severe degree of obstruction. The 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.

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 ratio ranges 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 may be very low, barely perceptible. 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

Recently,² an international group has recommended that the largest vital capacity measured during a study be used in the denominator of the ratio. In most cases this will be the FVC, but on occasion it will be a slow vital capacity (SVC). When the SVC exceeds the FVC, a subject with a low normal FEV₁/FVC may be moved into the mild obstructive category. The impact and value of this change are yet to be determined (see Section 14D).