Restrictive Lung Disorders: Lung Volume and Capacity Findings

Table 3-2 presents an overview of the lung volume and capacity findings characteristic of restrictive lung disorders. Restrictive lung volumes and capacities are associated with pathologic conditions that alter the anatomic structures of the lungs distal to the terminal bronchioles (i.e., the alveoli or the lung parenchyma). Table 3-3 provides some of the more common restrictive anatomic alterations of the lungs and examples of respiratory disorders that cause them. Restrictive lung disorders result in an increased lung rigidity, which in turn decreases lung compliance. When lung compliance decreases, the ventilatory rate increases and the tidal volume (V_T) decreases (see Figure 2-23).

Obstructive Lung Disorders: Lung Volume and Capacity Findings

Table 3-4 provides an overview of the lung volumes and capacity findings characteristic of obstructive lung disorders. These lung volume and capacity findings are associated with pathologic conditions that alter the tracheobronchial tree. Table 3-5 provides some of the more common obstructive anatomic alterations of the lungs and examples of respiratory disorders that cause them.

In obstructive lung disorders, the gas that enters the alveoli during inspiration (when the bronchial airways are naturally wider) is prevented from leaving the alveoli during expiration (when the bronchial airways narrow). As a result, the alveoli become overdistended with gas, a condition known as air trapping. Figure 3-1 provides a visual comparison of obstructive and restrictive lung disorders.

Indirect Measurements of the Residual Volume and Lung Capacities Containing the Residual Volume

Because the residual volume (RV) cannot be exhaled, the RV and the lung capacities that contain the RV (functional residual capacity [FRC] and total lung capacity [TLC]) are measured indirectly in the clinical setting by one of the following methods:

Forced Vital Capacity

The forced vital capacity (FVC) is the total volume of gas that can be exhaled as forcefully and rapidly as possible after a maximal inspiration. In the healthy individual, the total expiratory time (TET) necessary to perform a FVC is 4 to 6 seconds. In obstructive lung disease (e.g., chronic bronchitis or emphysema), the TET increases because of the increased airway resistance and air trapping associated with the disorder. TETs of more than 10 seconds have been reported in these patients. In the normal individual the FVC equals the vital capacity (VC). Clinically, the lungs are considered normal if the FVC and the VC are within 200 mL of each other. In the patient with obstructive lung disease the FVC is lower than the VC because of increased airway resistance and air trapping with maximal effort (Figure 3-2).

A decreased FVC also is a common clinical manifestation in the patient with a restrictive lung disorder (e.g., pneumonia, acute respiratory distress syndrome, atelectasis). This decrease is mainly a result of the fact that restrictive lung disorders reduce the patient’s ability to fully expand the lungs, thus reducing the VC necessary to generate a good FVC exhalation. However, the TET required to perform an FVC exhalation is usually normal or even less than normal because of the high lung elasticity (low lung compliance) associated with restrictive disorders.

A number of pulmonary function tests can be extrapolated from a single FVC maneuver. The most common tests are as follows:

Forced Expiratory Volume Timed

The maximum volume of gas that can be exhaled over a specific period is the forced expiratory volume timed (FEV_T). This measurement is obtained from an FVC measurement. Commonly used time periods are 0.5, 1.0, 2.0, 3.0, and 6.0 seconds. The most commonly used time period is 1 second (forced expiratory volume in 1 second [FEV₁]). In the normal adult the percentages of the total volume exhaled during these time periods are as follows: FEV_0.5, 60%; FEV₁, 83%; FEV₂, 94%; and FEV₃, 97%. In obstructive disease the FEV_T is decreased because the time necessary to exhale a certain volume forcefully is increased (Figure 3-3). Although the FEV_T may be normal in restrictive lung disorders (e.g., pneumonia, acute respiratory distress syndrome, atelectasis), it is commonly decreased because of the decreased VC associated with restrictive disorders (similar to the FVC in restrictive disorders). The FEV_T progressively decreases with age.

Forced Expiratory Volume in 1 second/Forced Vital Capacity (FEV₁/FVC) Ratio

The FEV₁/FVC ratio compares the amount of air exhaled in 1 second with the total amount exhaled during an FVC maneuver. Because the FEV₁/FVC ratio is expressed as a percentage, it is commonly referred to as the forced expiratory volume in 1 second percentage (FEV_1%). Simply stated, the FEV₁/FVC ratio provides the percentage of the patient’s total volume of air forcefully exhaled (FVC) in 1 second. As discussed earlier in the FEV_T section, the normal adult exhales 83% or more of the FVC in 1 second (FEV₁). Therefore the FEV₁/FVC ratio should also be 83% or greater under normal circumstances. The FEV_1% progressively decreases with age.

Clinically, the FVC, FEV₁, and FEV_1% are commonly used to (1) assess the severity of a patient’s pulmonary disorder and (2) to determine whether the patient has either an obstructive or a restrictive lung disorder. The primary pulmonary function study differences between an obstructive and a restrictive lung disorder are as follows:

Forced Expiratory Flow 25%-75% (FEF_25%-75%)

The FEF_25%-75% is the average flow rate generated by the patient during the middle 50% of an FVC measurement (Figure 3-4). This expiratory maneuver is used to evaluate the status of medium-to-small airways in obstructive lung disorders. The normal FEF_25%-75% in a healthy man 20 to 30 years of age is about 4.5 L/sec (270 L/min). The normal FEF_25%-75% in a healthy woman 20 to 30 years of age is about 3.5 L/sec (210 L/min). The FEF_25%-75% is somewhat effort-dependent because it depends on the FVC exhaled.

The FEF_25%-75% progressively decreases in obstructive diseases and with age. The FEF_25%-75% may also be decreased in moderate or severe restrictive lung disorders. This decrease is believed to be caused primarily by the reduced cross-sectional area of the small airways associated with restrictive lung problems. Clinically, the FEF_25%-75% is often used to further confirm—or rule out—the presence of an obstructive pulmonary disease in the patient with a borderline FEV_1% value.

Forced Expiratory Flow 200-1200 (FEF_200-1200)

The FEF_200-1200 measures the average flow rate between 200 and 1200 mL of an FVC (Figure 3-5). The first 200 mL of the FVC is usually exhaled more slowly than at the average flow rate because of (1) the normal inertia involved in the respiratory maneuver and (2) the initial slow response time of the pulmonary function equipment. Because the FEF_200-1200 measures expiratory flows at high lung volumes (i.e., the initial part of the FVC), it provides a good assessment of the large upper airways. The FEF_200-1200 is relatively effort-dependent.

The normal FEF_200-1200 for the average healthy man 20 to 30 years of age is approximately 8 L/sec (480 L/min). The normal FEF_200-1200 in the average healthy woman 20 to 30 years of age is approximately 5.5 L/sec (330 L/min). The FEF_200-1200 decreases in obstructive lung disorders. The FEF_200-1200 is a good test to determine the patient’s response to bronchodilator therapy. In restrictive lung disorders the FEF_200-1200 is usually normal because it measures the early expiratory flow rates during the first part of an FVC maneuver (i.e., when the patient’s VC is at its highest level). The FEF_200-1200 progressively decreases with age.

Peak Expiratory Flow Rate (PEFR)

The PEFR (also known as the peak flow rate) is the maximum flow rate generated during an FVC maneuver (Figure 3-6). The PEFR provides a good assessment of the large upper airways. It is quite effort-dependent. The normal PEFR in the average healthy man 20 to 30 years of age is approximately 10 L/sec (600 L/min). The normal PEFR in the average healthy woman 20 to 30 years of age is approximately 7.5 L/sec (450 L/min). The PEFR decreases in obstructive lung diseases. In restrictive lung disorders, the PEFR is usually normal because it measures the early expiratory flow rates during the first part of an FVC maneuver (i.e., when the patient’s VC is at its highest level). The PEFR progressively decreases with age.

The PEFR also can easily be measured at the patient’s bedside with a hand-held peak flowmeter (e.g., Wright peak flowmeter). The hand-held peak flowmeter is used to monitor the degree of airway obstruction on a moment-to-moment basis and is relatively small, inexpensive, accurate, reproducible, and easy for the patient to use. In addition, the mouthpieces are disposable, thus allowing the safe use of the same peak flowmeter from one patient to another. PEFR measurements should routinely be performed at the patient’s bedside to assess the degree of bronchospasm, effect of bronchodilators, and day-to-day progress. The PEFR results generated by the patient before and after bronchodilator therapy can serve as excellent objective data by which to assess the effectiveness of therapy.

Maximum Voluntary Ventilation (MVV)

The MVV is the largest volume of gas that can be breathed voluntarily in and out of the lungs in 1 minute (Figure 3-7). The normal MVV in the average healthy man 20 to 30 years of age is approximately 170 L/min. The normal MVV in the average healthy woman 20 to 30 years of age is approximately 110 L/min. The MVV progressively decreases in obstructive pulmonary disorders. In restrictive pulmonary disorders, the MVV may be normal or decreased.

Flow-Volume Loop

The flow-volume loop is a graphic illustration of both a forced vital capacity (FVC) maneuver and a forced inspiratory volume (FIV) maneuver. The FVC and FIV are plotted together as two curves that form what is called a flow-volume loop. As shown in Figure 3-8, the upper half of the flow-volume loop (above the zero flow axis) represents the maximum expiratory flow generated at various lung volumes during an FVC maneuver plotted against volume. This portion of the curve shows the flow generated between the TLC and RV.

The lower half of the flow-volume loop (below the zero flow axis) illustrates the maximum inspiratory flow generated at various lung volumes during a forced inspiration (called a forced inspiratory volume [FIV]) plotted against the volume inhaled. This portion of the curve shows the flow generated between the RV and TLC. Depending on the sophistication of the equipment, several important pulmonary function study measurements can be obtained, including the following:

In the normal subject the expiratory flow rate decreases linearly during an FVC maneuver, immediately after the PEFR has been achieved. In the patient with an obstructive lung disease, however, the flow rate decreases in a nonlinear fashion after the PEFR has been reached. This nonlinear flow rate causes a cuplike or scooped-out appearance in the expiratory flow curve when 50% of the FVC has been exhaled. This portion of the flow curve is the FEF_50%, or (Figure 3-9). Table 3-6 summarizes (1) the forced expiratory flow rate and volume measurements and (2) the normal values found in healthy men and women ages 20 to 30 years.

Table 3-6

Normal Forced Expiratory Flow Rate Measurements in Healthy Men and Women 20 to 30 Years of Age

Forced Expiratory Flow Rate Measurement	Men	Women
Forced vital capacity (FVC). A is the point of maximal inspiration and the starting point of an FVC maneuver. Note the reduction in FVC in obstructive pulmonary disease, caused by dynamic compression of the airways.	Usually equals vital capacity (VC) (FVC and VC should be within 200 mL of each other)	Usually equals VC (FVC and VC should be within 200 mL of each other)
Forced expiratory volume timed (FEVT): FEV0.5, FEV1.0, FEV2.0, FEV3.0. In obstructive disorders, more time is needed to exhale a specified volume.	FEV0.5: 60% FEV1.0: 83% FEV2.0: 94% FEV3.0: 97%