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

3. Static (Absolute) Lung Volumes

Measures of the so-called static (or absolute) lung volumes are often informative [1]. The most important are the vital capacity, residual volume, and total lung capacity. The vital capacity (VC) is measured by having the subject inhale maximally and then exhale slowly and completely. This VC is called the slow vital capacity (SVC). Similar to the SVC is the inspiratory vital capacity (IVC). The patient breathes normally and then exhales slowly and completely and inhales maximally. The SVC and the IVC provide similar results. The SVC is used in this book.

With complete exhaling, air still remains in the lung. This remaining volume is the residual volume (RV). The RV can be visualized by comparing the inspiratory and expiratory chest radiographs (Fig. 3-1). The fact that the lungs do not collapse completely on full expiration is important physiologically. With complete collapse, transient hypoxemia would occur because mixed venous blood reaching the lung would have no oxygen to pick up. Furthermore, inflation of a collapsed lung requires very high inflating pressures, which would quickly fatigue the respiratory muscles and could tear the lung, leading to a pneumothorax. This is the problem in infants born with respiratory distress syndrome, in which portions of the lung can collapse (individual acinar units, up to whole lobes) at the end of exhalation.

The RV can be measured and added to the SVC to obtain the total lung capacity (TLC). Alternatively, the TLC can be measured and the SVC subtracted from it to obtain the RV. The value of these volumes is discussed on page 35.

FIG. 3-1. Radiographs obtained from a healthy subject at full inspiration (that is, at total lung capacity; A) and full expiration (B), in which the air remaining in the lung is the residual volume.

FIG. 3-2. Spirogram for a normal subject during various maneuvers compared with that of a subject with obstructive lung disease who shows trapping. Expir, expiration; FVC, forced expiratory vital capacity; Inspir, inspiration; SVC, slow vital capacity.

3A. SLOW VITAL CAPACITY

Normally, the SVC and forced expiratory vital capacity (FVC; discussed in Chapter 2) are identical, as shown in the top panel of Figure 3-2. With airway obstruction, as in chronic obstructive pulmonary disease (COPD) or asthma, the FVC can be considerably smaller than the SVC, as shown in the lower panel of Figure 3-2. The difference between SVC and FVC reflects trapping of air in the lungs. The higher flows during the FVC maneuver cause excessive narrowing and closure of diseased airways in COPD, and thus the lung cannot empty as completely as during the SVC maneuver. Although trapping is of interest to the physiologist, it is of little value as a clinical measure. However, it does explain the possible discrepancies between the volumes of the SVC and the FVC.

3B. RESIDUAL VOLUME AND TOTAL LUNG CAPACITY

Figure 3-3 depicts the static lung volumes that are of most interest. The RV is measured (see page 31) and added to the SVC to obtain the TLC. The expiratory reserve volume (ERV) is the volume of air that can be exhaled after a normal expiration during quiet breathing (tidal breathing). The volume used during tidal breathing is the tidal volume (VT). The inspiratory reserve volume is the volume of air that can be inhaled at the end of a normal tidal inspiration. The sum of the ERV and RV is termed the functional residual capacity (FRC).

FIG. 3-3. Various static (or absolute) lung volumes. Total lung capacity (TLC) is the sum of the residual volume (RV) and slow vital capacity (SVC). The SVC is the sum of the inspiratory reserve volume (IRV), the tidal volume (VT), and the expiratory reserve volume (ERV). The functional residual capacity (FRC) is the sum of the RV and the ERV. Expir, expiration; Inspir, inspiration.

RV is the remaining volume of air in the lung at the end of a complete expiratory maneuver. It is determined by the limits of either the chest wall excursion or airway collapse or compression. In restrictive disorders, the limit of chest wall compression by the chest wall muscles determines RV. In obstructive disorders, the collapse of airways prevents air escape from the lungs, thereby determining the maximal amount exhaled. In obstructive disease, the RV is increased. There is one exception. The RV can be increased in a few young, healthy adults who are unable to completely compress their chest wall. In these cases, a curve like the one shown in Figure 2-6E is produced. The TLC is increased in most patients with chronic obstruction. However, TLC is often not increased in asthma. Finally, for a confident diagnosis of a restrictive process, the TLC must be decreased.

The FRC is primarily of interest to the physiologist. It is the lung volume at which the inward elastic recoil of the lung is balanced by the outward elastic forces of the relaxed chest wall (rib cage and abdomen). It is normally 40 to 50% of the TLC. When lung elasticity is reduced, as in emphysema, the FRC increases. It also increases to a lesser extent with normal aging. With the increased lung recoil in pulmonary fibrosis, the FRC decreases.

PEARL: The FRC is normally less when a subject is supine than when sitting or standing. When a person is upright, the heavy abdominal contents pull the relaxed diaphragm down, expanding both the rib cage and the lungs. In the supine position, gravity no longer pulls the abdominal contents downward; instead, the contents tend to push the diaphragm up, and thus the FRC is decreased. The lower FRC and, hence, smaller lung volume in the supine position may interfere with gas exchange in patients with various types of lung disease and in the elderly. Blood drawn while these subjects are supine may show an abnormally low tension of oxygen in arterial blood. A similar effect often occurs in very obese subjects.

3C. HOW THE RESIDUAL VOLUME IS MEASURED

Usually, the FRC is measured by one of the methods to be described. If the ERV is subtracted from the FRC, the RV is obtained and, as noted prEV1ously, if the RV is added to the SVC, the TLC is obtained (Fig. 3-3).

As shown in Figure 3-2, the SVC may be larger than the FVC in obstructive disease. If the FVC is added to the RV, the TLC will be smaller than if the SVC is used. Conversely, if the FVC is less than the SVC and the RV is calculated by subtracting the FVC from the measured TLC, you will calculate an RV that is high. By convention, and in this book, the SVC is used to compute static lung volume. Alternatively, in the United States, the FVC, not the SVC, is used to compute the FEV1/FVC ratio (ratio of the forced expiratory volume in 1 second to the FVC). European reference equations use FEV1/SVC, also called the Tiffeneau index.

The three most commonly used methods of measuring the FRC (from which the RV is obtained) are nitrogen washout, inert gas dilution, and plethysmography. If these are not available, a radiographic method can be used.

Nitrogen Washout Method

The principle of this procedure is illustrated in Figure 3-4. At the end of a normal expiration, the patient is connected to the system.

Initial volume of N2 in patient = 0.8 (Vx)

Vx = FRC

Final volume of N2 in expired bag = 0.035 (Vb)

Vb = volume of bag = 0.035 (100)

There is no loss of N2 from system

so initial N2 volume = final N2 volume 0.8 (Vx) = (0.035)(100)

Vx = 4.37 L = FRC

FIG. 3-4. Nitrogen washout method of measuring the functional residual capacity (FRC). The initial volume of nitrogen (N2) in the lungs at FRC equals 80% N2 x FRC volume. The N2 volume of the inhaled oxygen (O2) is zero. The volume of N2 washed out of the lung is computed as shown, and the FRC, or Vx, is obtained by solving the mass balance equation, 0.8 (Vx) = 0.035 (Vb).

The lung contains an unknown volume (Vx) of air containing 80% nitrogen. With inspiration of nitrogen-free oxygen and exhalation into a separate bag, all the nitrogen can be washed out of the lung. The volume of the expired bag and its nitrogen concentration are measured, and the unknown volume is obtained with the simple mass balance equation. In practice, the procedure is terminated after 7 minutes and not all the nitrogen is removed from the lung, but this is easily corrected for. This procedure underestimates the FRC in patients with airway obstruction because in this condition there are lung regions that are very poorly ventilated, and hence, they lose very little of their nitrogen. A truer estimate in obstructive disease can be obtained if this test is prolonged to 15 to 20 minutes. However, patients then find the test unpleasant.

Inert Gas Dilution Technique

The concept is illustrated in Figure 3-5. Helium, argon, or neon can be used. The spirometer system contains a known volume of gas (V1). (In Fig. 3-5, Ci is helium with a known concentration.) At FRC, the subject is connected to the system and rebreathes until the helium concentration reaches a plateau indicating equal concentrations of helium (C2) in the spirometer and lung. Because essentially no helium is absorbed, Equations l and 2 can be combined and solved for Vx, the FRC. In practice, oxygen is added to the circuit to replace that consumed by the subject, and carbon dioxide is absorbed to prevent hypercarbia. As with the nitrogen washout technique, the gas dilution method underestimates the FRC in patients with airway obstruction.

FIG. 3-5. Helium dilution technique of measuring the functional residual capacity (FRC). Before the test, no helium (He) is present in the lungs (Vx), and there is a known volume of He in the spirometer and tubing—the concentration of He (C1) times the volume of the spirometer and the connecting tubes (V1). At equilibrium, the concentration of He (C2) is uniform throughout the system. The mass balance equation can now be solved for the FRC (Vx).

Plethysmography

The principle of plethysmography is simple. The theory is based on Boyle's law, which states that the product of the pressure (P) and volume (V) (PV) of a gas is constant under constant temperature (isothermal) conditions. The gas in the lungs is isothermal because of its intimate contact with capillary blood. The technique is shown in Figure 3-6 with the standard constant-volume body plethysmograph. An attractive feature of this technique is that several measurements of RV and TLC can be obtained quickly. This is not possible with the washout and dilution methods because the alveolar gas composition must be brought back to the control state before these tests can be repeated, a process that often takes 10 to 20 minutes in patients with COPD. The plethys- mographic method measures essentially all the gas in the lung, including that in poorly ventilated areas. Thus, in COPD, the FRC, RV, and TLC obtained with this method are usually larger and more accurate than those obtained with the gas methods. Often the TLC of a patient with COPD is 2 to 3 L more with plethysmography.

FIG. 3-6. The equipment and the measurements needed to measure the functional residual capacity (FRC) by using a body plethysmograph and applying Boyle's law (Eq 1). The subject is seated in an airtight plethysmograph and the pressure in the plethysmograph (Ppleth) changes with changes in lung volume. When the subject stops breathing, alveolar pressure equals barometric pressure (Pb). Consider what happens if the valve at the mouth is closed at the end of a quiet expiration, that is, FRC, and the subject makes an expiratory effort. Alveolar pressure increases by an amount (AP) that is measured by the mouth gauge, P. Lung volume decreases as a result of gas compression, there being no airflow, and hence Ppleth decreases. The change in Ppleth provides a measure of the change in volume ( AV), as follows. With the subject momentarily not breathing, the piston pump is cycled and the known volume changes produce known changes in Ppleth. These measurements provide all the data needed to solve the above equation for Vf. The final equation is simplified by omitting AP from the quantity (Pb + AP). Because AP is small (~20 cm H2O) compared with Pb (~1,000 cm H2O), it can be neglected. PV, product of pressure and volume.

Radiographic Method

If the above-described methods are not available, radiographic methods can provide a good estimate of TLC. Posterior-anterior and lateral radiographs are obtained while the subject holds his or her breath at TLC. TLC is estimated by either planimetry or the elliptic method [2]. The radiographic technique compares favorably with the body plethysmographic method and is more accurate than the gas methods in patients with COPD. It is also accurate in patients with pulmonary fibrosis. The technique is not difficult but requires that radiographs be obtained at maximal inspiration.

3D. SIGNIFICANCE OF RESIDUAL VOLUME AND TOTAL LUNG CAPACITY

Knowledge of the RV and TLC can help in determining whether a restrictive or an obstructive process is the cause of a decrease in FVC and FEV1. This distinction is not always apparent from the flow-volume (FV) curves. The chest radiographs may help when obvious hyperinflation or fibrosis is present.

As noted in section 2F, page 12, the FEV1/FVC ratio usually provides the answer. However, in a patient with asthma who is not wheezing and has a decreased FVC and FEV1, both the FEV1 /FVC ratio and the slope of the FV curve may be normal. In this case the RV should be mildly increased, but often the TLC is normal.

The TLC and RV are increased in COPD, especially emphysema. Usually the RV is increased more than the TLC, and thus the RV/TLC ratio is also increased. The TLC and RV are also increased in acromegaly, but the RV/TLC ratio is normal.

By definition, the TLC is reduced in restrictive disease, and usually the RV is also reduced. The diagnosis of a restrictive process cannot be made with confidence unless there is EV1dence of a decreased TLC. The EV1dence may be the direct measure of TLC or the apparent volume reduction seen on the chest radiograph, or it may be suggested by the presence of a very steep slope of the FV curve (see Fig. 2-4).

PEARL: Lung resection for lung cancer or bronchiectasis decreases the RV and TLC, but this is an unusual restrictive process. Because there is often associated airway obstruction, the RV/TLC may be abnormally high. Furthermore, an obstructive process will be apparent because of the shape of the FV curve and a decreased FEV1/FVC ratio. This is a mixed restrictive-obstructive pattern.

3E. EXPANDING THE GESTALT APPROACH TO ABSOLUTE LUNG VOLUME DATA

Figure 3-7 shows the FV curves from Figure 2-9 as a means to consider what changes might be expected in the absolute lung volumes. Figure 3-7A represents findings in a normal subject: TLC of 7 L, RV of 2 L, and RV/TLC ratio of 29%.

Figure 3-7B shows a severe ventilatory limitation due to airway obstruction. In addition to the reduced flows, TLC and RV are expected to be increased, RV more than TLC, so that the RV/TLC ratio will also be abnormal. These expectations are confirmed by the values on the right of the figure. However, the effect of lung resection in COPD needs to be considered (see section 3D).

FIG. 3-7. Further application of the gestalt approach introduced in Figure 2-9, page 24. Note that the area between the predicted (dashed line) and observed (solid line) flow-volume curves is not shaded.

A. Normal pattern. B. Severe obstruction. C. Severe pulmonary restriction. (The numbers in parentheses are the percentage of predicted normal.) RV, residual volume; TLC, total lung capacity.

The FV curve in Figure 3-7C is consistent with severe ventilatory limitation due to a restrictive process. This diagnosis requires the TLC to be decreased, and the RV/TLC ratio is expected to be essentially normal. The values on the right of the figure confirm these expectations.

A question in regard to Figure 3-7C is, What is the cause of this restrictive process? The answer to this question requires rEV1ew of Figure 2-3 (page 10), in which all but the obstructive diseases need to be considered. Most restrictive processes can be evaluated from the history, physical examination, and chest radiograph. In fibrosis, diffusing capacity (discussed in Chapter 4) is expected to be reduced and radiographic changes EV1dent. Poor patient effort can be excluded by evaluating the FV curve (see Fig. 2-6, page 16) and by noting that the patient gives reproducible efforts.

A curve similar to that in Figure 3-7C but with reduced peak flows is found in patients with normal lungs in whom a neuromuscular disorder such as amyotrophic lateral sclerosis or muscular dystrophy develops. In this case, the maximal voluntary ventilation is often reduced (see section 2I, page 17). In addition, with this reduction in the FVC, the maximal respiratory muscle strength is reduced, as discussed in Chapter 9. Interestingly, patients with bilateral diaphragmatic paralysis can present with this pattern. However, these patients differ in that their dyspnea becomes extreme, and often intolerable, when they lie down.

Some massively obese subjects also show the pattern in Figure 3-7C. They have a very abnormal ratio of weight (in kilograms) to height2 (in meters), the body mass index (BMI), which has become the standard index for obesity. A BMI more than 25 is considered overweight. Anyone with a BMI of 30 or more is considered obese. In our laboratory, we find that a BMI more than 35 is associated with an average reduction in FVC of 5 to 10% (unpublished data). There is a large variation, however: Some obese individuals have normal lung volumes, and others are more severely affected. This difference maybe related to fat distribution or to the relationship between fat mass and muscle mass [3].

Figure 3-8 shows two curves in which the FEV1 and FVC are reduced and the FEV1/FVC ratio is normal. Both are consistent with a restrictive process. However, in both cases the TLC is normal. Therefore, the diagnosis of a restrictive process cannot be made. In this case, the term nonspecific ventilatory limitation is applied (see section 2F, pages 12-14).

FIG. 3-8. A and B. Examples of nonspecific ventilatory impairment in which the forced expiratory volume in 1 second (FEV1) and forced expiratory vital capacity (FVC) are reduced proportionately, giving a normal FEV1/FVC ratio, and the total lung capacity (TLC) is normal. The numbers in parentheses are the percentage of predicted normal. Note that residual volume (RV) is increased. This should not be confused with the prEV1ously discussed obstructive disorders in which RV is also increased.

Sometimes a more definitive diagnosis can be made. For example, Figure 3-8A shows a parallel shift of the FV curve. Ventilatory limitation is mild to moderate. This finding is common in mild asthma [4]. The TLC is normal, and the RV and RV/TLC are mildly increased. The history may be consistent with asthma with or without wheezing. The subject often has a higher than normal increase in expiratory flows on the FV curve after use of an inhaled bronchodilator. If this does not occur, a methacholine challenge test is often recommended in an attempt to uncover a possible asthmatic process. These procedures are discussed in Chapter 5.

Figure 3-8B is a nonspecific ventilatory limitation of moderate degree. In this case, the slope of the FV curve is increased, but there is no clinical EV1dence of parenchymal involvement, and the pulmonary diffusing capacity (Dlco,see Chapter 4) is normal, as is the TLC. This pattern can also occur in patients with relatively quiescent asthma. A thorough history and physical examination may uncover the problem. The response to a bronchodilator may be marked, or results of the methacholine challenge test may be positive.

We recently studied a random sample of 100 patients with the nonspecific ventilatory limitation pattern. All had a TLC by plethysmography and a diffusing capacity within normal limits; thus, restriction was ruled out. There were 62 men and 38 women 20 years or older. Airway hyperreactivity based on bronchodilator response or methacholine challenge was present in 56. Fifty of the subjects were obese. Chronic obstructive disease was present in 16%. Forty-one subjects had multiple tests, and 56% of these had, on one occasion, either an obstructive pattern or a normal test result. Thus, airway hyperreactivity and obesity are commonly associated with the nonspecific pattern, and the pattern may vacillate between normal and obstructive. If this nonspecific pattern is found, testing for airway hyperreactivity should be done by either the bronchodilator or the methacholine method, and occasionally both may be indicated. Normal predicted values of the FV curves are relied on heavily. The values used in our laboratory are given in the Appendix.

Table 3-1 is an expansion of Table 2-2 (page 23): the TLC, RV, and RV/TLC ratio are added.

TABLE 3-1. 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; RV, residual volume; TLC, total lung capacity; ??, decreased; ??, increased.

PEARL: Assume you had estimates of TLC by the plethysmographic method and the dilution method (nitrogen or helium). If the plethys- mographic TLC exceeds that of the dilution method, you have an estimate of the volume of poorly ventilated lung, which is characteristic of airway obstruction. In section 4C, page 43, another estimate of poorly ventilated volume is described when the Dlco is measured.

PEARL: What determines RV? As healthy adults exhale slowly and completely, airway resistance increases dramatically at very low volumes as the airways narrow (see Fig. 7-4, page 78). When airway resistance approaches infinity, no further exhalation occurs and RV is reached. At this point, the small, peripheral airways are essentially closed. An increase in RV is sometimes the first sign of early airway disease. An interesting exception to this description of how RV in adults is determined can occur in children and young adults. Their FV curve shows an abrupt cessation of flow with a contour similar to that seen in Figure 2-6E (page 16). An increase in airway resistance does not cause exhalation to cease. Rather, the respiratory muscles are not strong enough to compress the chest wall and abdomen any further. This increase in RV is not abnormal and usually disappears with growth and aging.

REFERENCES

1. Gibson GJ. Lung volumes and elasticity. Clin Chest Med 22:623-635, 2001.

2. Miller RD, Offord KP. Roentgenologic determination of total lung capacity. Mayo Clin Proc 55:694-699,1980.

3. Cotes JE, Chinn DJ, Reed JW. Body mass, fat percentage, and fat free mass as reference variables for lung function: effects on terms for age and sex. Thorax 56:839-844, 2001.

4. Olive JT Jr, Hyatt RE. Maximal expiratory flow and total respiratory resistance during induced bronchoconstriction in asthmatic subjects. Am Rev Respir Dis 106:366-376, 1972.



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