Technique

VC is measured by having the patient inspire maximally and then exhale completely into a spirometer. (See Chapter 11 for a complete discussion of spirometers.) The patient is instructed to perform the maneuver slowly and completely. VC can also be measured from maximal expiration to maximal inspiration. The spirometer does not need to produce a graphic display if only VC is to be measured. However, if IC and ERV are to be determined (see Figure 2-1), some means of recording volume change is required. The graphic display may be a computer screen or a recording device (see Chapter 11). A graphic display allows the technologist to determine that the test is performed correctly (Criteria for Acceptability 2-1). A graphic display is also usually required for reimbursement purposes.

Obtaining a valid slow VC is important. The subdivisions of the VC (IC and ERV) are used in the calculation of residual volume (RV) and total lung capacity (TLC). An excessively large tidal volume or an irregular breathing pattern during the VC maneuver may alter ERV or IC. If either ERV or IC is erroneously recorded, other lung volumes may be incorrectly estimated (see Chapter 4).

IC is measured by having the patient breathe normally for three or four breaths and then inhale maximally. The volume inspired from the resting expiratory level is measured by the computer or from a spirogram. This is usually done as part of a slow VC maneuver. IC may also be calculated by subtracting the ERV from the VC. ERV is measured by having the patient breathe normally for three or four breaths and then exhale maximally. The change in volume from the end-expiratory level to the maximal expiratory level is the ERV. ERV may also be calculated by subtracting the IC from the VC. IC and ERV are usually measured from the same VC maneuver. IC and ERV should be reported as the average of at least three acceptable maneuvers, whereas VC is reported as the largest of at least three acceptable maneuvers.

The accuracy of the IC and ERV measurements depends on the stability of the end-expiratory level, which should vary by less than 100 mL. Three or more tidal breaths should be recorded before the VC maneuver is performed. If the end-expiratory volume is not consistent, IC and/or ERV may be measured incorrectly (i.e., too large or too small). Even if the end-expiratory level is constant, the V_T usually increases when the patient breathes through a mouthpiece with a noseclip in place. This increase in V_T may change the IC or ERV, depending on the patient’s breathing pattern. Erroneous estimates of ERV may affect the calculation of RV, as described in Chapter 4.

Significance and Pathophysiology

Spirometry is indicated in a variety of situations (see Chapter 1). According to the 2005 ATS-ERS recommendations, there is only one contraindication to spirometry but several conditions that could yield a suboptimal test (Box 2-1). However, testing personnel should be aware of other conditions, which should be evaluated before testing, such as recent stroke, eye surgery, thoracic/abdominal surgery or pneumothorax, uncontrolled hypertension or hypotension,and known thoracic, aortic, or cerebral aneurysm.

Normal VC may vary as much as 20% above or below the predicted value in healthy individuals, but it must be above the bottom 5th percentile (lower limit of normal; see Chapter 13) to be considered normal. The ATS-ERS guidelines recommend the use of the National Health and Nutrition Examination Survey III (NHANES III) reference equations (see Evolve website http://evolve.elsevier.com/Mottram/Ruppel/) for patients in the United States ages 8-80 years. For children under 8 years old, the guidelines recommend the equations of Wang and others. However, in 2007, Stanojevic and others extended the NHANES III dataset to age 4 years.

In adults, VC varies directly with height and inversely with age; tall patients have larger VCs than short patients. VC increases up to approximately age 20 and then decreases each year thereafter, with an average decrease of about 25-30 mL/yr. It is usually smaller in women than in men because of differences in body size. Recent evidence indicates that lung volumes may differ significantly according to ethnic origin. VC also varies in individuals, depending on body position or time of day. Interpretation of lung function should consider the key factors of age, sex, height, and race (see Chapter 13).

PF Tip 2-1

Normal values for lung function parameters are obtained by studying healthy subjects. The predicted or reference value for VC is computed using an equation like:

< ?xml:namespace prefix = "mml" />VC=xHeight−yAge−z

where:

x, y, and z are constants

Predicted values may be read from special diagrams called nomograms but are usually calculated by a computer. (See Chapter 13.) The lower limit of normal, representing the lower 5th percentile of the normal distribution curve, is recommended as the cutoff to judge normality.

As Hutchinson originally noted (see Chapter 1), a patient’s vital capacity is directly related to survival, and numerous studies have since supported his conclusion. In a study published in 2011 using data on spirometry and survival from the Atherosclerosis Risk in Communities (ARIC) dataset, 7489 participants with usable spirometry data were compared against measures of ventilatory function after controlling for many other factors likely to be associated with survival. The study concluded that survival was strongly associated with the forced vital capacity over all other spirometry parameters.

There are numerous causes of a decreased VC (Box 2-2). In general, these fall into the categories of loss of distensible lung tissue (i.e., fibrosis), obstructive lung disease, and reduced chest wall expansion (i.e., neuromuscular, kyphosis).

When the VC is reduced, additional pulmonary function measurements may be indicated. Forced expiratory maneuvers (see section, Forced Vital Capacity) can reveal whether the reduced VC is caused by obstruction. Reduced VC without slowing of expiratory flow is a nonspecific finding. Measurement of other lung volumes (see Chapter 4) may be indicated to determine whether a restrictive defect is present. Measurement of muscle pressures may help determine whether there is a problem with neuromuscular weakness (see Chapter 10).

In adults, VC less than the lower limit of normal (95% confidence limits, see Chapter 13) may be considered abnormal. Interpretation of the measured VC in relation to the reference value should consider the clinical question to be answered (Interpretive Strategies 2-1). The clinical question is often revealed in the history and physical findings of the patient (see Chapter 1). The terms mild, moderate, and severe may be used to qualify the extent of reduction of the VC in a manner similar to that described for FEV₁ (see following section, FEV₁).

Artificially low estimates of the VC may result from poor patient effort. Similarly, inadequate patient instruction may affect the performance of the test maneuver. These errors may be eliminated by applying appropriate criteria (Criteria for Acceptability 2-1). Values for at least two maneuvers should be reproducible within 150 mL (see Chapter 12).

IC and ERV are approximately 75% and 25% of the VC, respectively. Changes in IC or ERV usually parallel increases or decreases in the VC. Increased V_T caused by exertion or acid-base disorders (e.g., metabolic acidosis, respiratory alkalosis) may reduce IRV or ERV. This occurs because end-inspiratory and end-expiratory levels (see Figure 2-1) are altered. A similar pattern is commonly seen when patients breathe into a spirometer through a mouthpiece with noseclips in place. Changes in IC or ERV are of minimal diagnostic significance when considered alone. Reduction of IC or ERV is consistent with restrictive defects. Obese patients typically show a decrease in ERV, usually resulting in a low VC. A reduced ERV is one of the earliest lung function changes in obese patients.

Technique

FVC is measured by having the patient, after inspiring maximally, expire as forcefully and rapidly as possible into a spirometer (see Chapter 1). The patient should inspire completely. The inhalation should be rapid but not forced. There should be little, if any, pause (less than 1-2 seconds) at maximal inspiration; a prolonged pause (4-6 seconds) may decrease flow during the subsequent expiration.

The volume expired may be read directly from a volume-time recording (Figure 2-2). This method is used by some small portable spirometers. More commonly, the maneuver is displayed on a computer monitor or liquid crystal display screen. The computer analyzes the signal from the spirometer, then calculates and displays the FVC. A spirometer that produces a graphical tracing (either volume-time or flow-volume) is essential for clinical

laboratory purposes to allow a visual inspection of the maneuver (Figure 2-3) and is also necessary for reimbursement purposes. Devices providing only numerical data may be helpful for simple screening. Whether used for diagnosis or monitoring, all spirometers should meet the criteria proposed by the ATS-ERS (see Chapter 12). The FVC maneuver depends on patient effort. Not all patients may be able to perform it acceptably (Criteria for Acceptability 2-2).

FEV₁ (and other FEV_T values) may be measured by timing the FVC maneuver over the described intervals. Historically, this was done by recording the FVC spirogram on graph paper moving at a fixed speed. The FEV for any interval could then be read from the graph, as shown in Figure 2-4. Most modern spirometers time the FVC maneuver using a computer. The computer then calculates and displays the FEV₁ or other FEV_T intervals. The spirometer should provide a volume-time display of each maneuver (Figure 2-5). A graphic representation allows the monitoring of patient effort at the beginning of the test. Accurate measurement of FEV₁ (and other FEV_T intervals) depends on the determination of the start-of-test (Figure 2-6). Computerized spirometers detect the start-of-test as a change in flow or volume above a certain threshold. The computer then stores volume and flow data points in memory and calculates the FEV₁.

Open circuit technique describes an FVC maneuver where the subject inhales maximally, places the mouthpiece or filter into their mouth, and then forcefully exhales until empty. Using this technique, the patient has to coordinate taking the deep breath, holding the breath at TLC, and placing the mouthpiece correctly, which can be a challenge for some subjects. Closed-circuit technique describes a maneuver where the subject breathes tidally on the spirometer, inhales maximally, forcefully exhales, and then may be instructed to inhale forcefully to measure the inspiratory flow and volume. Most computerized spirometers correct for a slow start-of-test by back-extrapolation (see Figure 2-5). Visualization of the volume-time spirogram is the best means of identifying poor initial effort. Some portable spirometers report FEV₁ without a spirogram. Such measurements should be used with caution because it may be difficult to determine whether the maneuver was performed acceptably.

The ratio of the FEV₁ to FVC is expressed as follows:

FEV1%=FEV1FVC  ×  100

FEV_1% is also commonly written as FEV₁/FVC. Both ratios are expressed as percentages, and this percentage is then related to normative values as a percentage of predicted. FEV₁ and FVC should be reported as the maximal values obtained from at least three acceptable FVC maneuvers. The FEV₁/FVC ratio based on these values may be different from the ratio obtained from any single maneuver. If both VC and FVC maneuvers have been performed, it is preferable to use the largest VC in the calculation. Some computerized spirometers calculate only the ratio obtained from FVC maneuvers.

The FEF_25%-75% is measured from an FVC maneuver. The FEF_25%-75% is the average flow during the middle half (from 25% to 75%) of the VC. A computerized measurement of the FEF_25%-75% requires storage of flow and volume data points for the entire maneuver. Calculation of the average flow over the middle portion of the exhalation is simply 50% of the volume expired divided by the time required to get from the 25% point to the 75% point. To manually calculate the FEF_25%-75%, a volume-time spirogram is used. The points at which 25% and 75% of the vital capacity have been expired are marked on the curve (see Figure 2-6). A straight line connecting these points can be extended to intersect two timelines 1 second apart. The flow (in liters per second) can then be read directly as the vertical distance between the points of intersection. Instantaneous flows, such as the FEF_50% or FEF_75%, cannot be read directly from a volume-time display but can be measured using a flow-volume curve (see later section, Flow-Volume Curve).

The FEF_25%-75% depends on the FVC. Large FEF_25%-75% values may be derived from maneuvers that produce small FVC measurements because the “middle half” of the volume is actually gas expired at the beginning of expiration. This effect may be particularly evident if the patient terminates the FVC maneuver before exhaling completely. When the FEF_25%-75% is used for assessing the response to bronchodilator or bronchial challenge, the effect of changes in the absolute lung volumes must be considered. Measuring the FEF_25%-75% at the same lung volumes in the comparison tests is called the isovolume technique. Isovolume corrections are usually applied when the FVC changes by more than 10% (indicating a change in TLC or RV). This technique requires that lung volumes (see Chapter 4) be measured in conjunction with flows. The isovolume technique may also be used with other flow measurements that are dependent on FVC.

The largest FEF_25%-75% is not necessarily the value reported. The FEF_25%-75% is recorded from the maneuver with the largest sum of FVC and FEV₁. Flows must be corrected to BTPS.

Acceptability and Repeatability for Spirometry Results

Criteria used to judge the acceptability of test results from the FVC maneuver include the following:

Data from all acceptable maneuvers should be examined. The largest FVC and the largest FEV₁ should be reported, even if the two values are from different test maneuvers. The reported FEV₁/FVC ratio is taken from these values. Flows that depend on the FVC (e.g., the FEF_25%-75%) should be taken from the single best test maneuver, which is the maneuver with the largest sum of FVC and FEV₁ (Table 2-1). The reported flow-volume curve is also taken from the single best test maneuver.

A common problem may occur when using these criteria to produce a spirometry report. If a single volume-time or flow-volume tracing is included in the final report, it may not contain the FVC or FEV₁ that appears in the tabular data. It is advisable to maintain recordings, or raw data, for all acceptable maneuvers. Other methods of selecting the best test have been suggested and are sometimes used. PEF may be used to assess patient effort for an FVC maneuver. Selecting the effort with the largest PEF may cause errors if FVC and FEV₁ are not also evaluated.

Spirometry may be performed in either the sitting or standing position for adults and children, although the sitting position is recommended for safety, especially in adults. There is some evidence that FEV₁ may be larger in the standing position in adults and in children younger than 12 years of age. The position used for testing should be indicated on the final report. The patient should keep their head slightly elevated and should keep false teeth in if they fit well. The use of noseclips is recommended for spirometric measurements that require rebreathing, even if just for a few breaths. Spirometers that record only expiratory flow may require the patient to place the mouthpiece into the mouth after maximal inspiration. If this is the case, noseclips are usually unnecessary. Care should be taken, however, that the patient places the mouthpiece into the mouth before beginning a forced expiration. Failure to do so may result in an undetectable loss of volume. It may be impossible to calculate the volume of back-extrapolation from a tracing that displays expiratory flow only. Spirometers that use mechanical recorders should have pen or paper moving at recording speed when the forced expiration begins. Recorders that trigger pen or paper movement with exhalation may be unable to accurately record the start-of-test. Such spirometers often underestimate the FEV₁. Most spirometers use computer-generated graphics, avoiding the problems associated with mechanical recorders.

See Interpretive Strategies 2-2. FVC usually equals VC in healthy individuals. In patients without obstruction, FVC and VC should be within 150 mL of each other. FVC and VC may differ if the patient’s effort is variable or if significant airway obstruction is present (i.e., FEV₁/FVC less than 0.70). FVC is often lower than VC in patients with obstructive diseases if forced expiration causes airway collapse. This pattern is often seen in emphysema because of a loss of tethering support of the airways. Large pressure gradients across the walls of the airways during forced expiration collapse the terminal portions of the airways. Gas is trapped in the alveoli and cannot be expired. This causes the FVC to appear smaller than the VC. The FVC can appear larger than the VC if the patient exerts greater effort on the forced maneuver.

FVC can be reduced by mucus plugging and bronchiolar narrowing, as is common in chronic

bronchitis, chronic or acute asthma, bronchiectasis, and cystic fibrosis. Reduced FVC is also present in patients whose trachea or mainstem bronchi are obstructed. Tumors or diseases affecting the patency of the large airways can produce this result.

Some obstructed patients have a relatively normal FVC in relation to their predicted values. However, the time required to expire their FVC (forced expiratory time or FET) is usually prolonged. Healthy adults can expire their FVC within 4 to 6 seconds. Normal children and adolescents may exhale their FVC in less than 4 seconds. Patients with severe obstruction (e.g., those with emphysema) may require 15 seconds or more to exhale completely (Figure 2-7). Accurate measurement of FVC in such individuals may be limited by how long the spirometer can collect exhaled volume. Some spirometers allow only 15 seconds of volume recording. This is usually long enough to diagnose airway obstruction. However, the FVC and FEV_1% may be inaccurate if the patient continues to exhale for a longer time. The ATS-ERS recommends that spirometers measure FVC for at least 15 seconds (see Chapter 12).

An alternative to measuring FVC in severely obstructed patients is to use FEV₆. Because normal patients can exhale their FVC in 6 seconds, substituting FEV₆ for FVC allows the FEV₁/FEV₆ to be used as an index of obstruction. Using FEV₆ in place of FVC eliminates the necessity of having the patient try to exhale for a long interval. Predicted values for FEV₆ and FEV₁/FEV₆ may be calculated using coefficients in Chapter 13. Because FEV₆ may underestimate FVC, use of FEV₁/FEV₆ instead of FEV₁/FVC may reduce the sensitivity of spirometry to detect airway obstruction, especially in older patients and in those with mild obstruction.

Decreased FVC is seen in the same circumstances as described for reduced VC, as discussed earlier. However, the reduction in FVC may be more evident than that of the slow VC in diseases that affect the bellows function of the chest, which is a key determinant of the force generation involved in the FVC compared to the VC maneuver. Such diseases include neuromuscular disorders as well as chest wall mechanical abnormalities, like kyphoscoliosis and obesity. A reduction in FVC when going from sitting to supine has been shown to be a good indicator of diaphragmatic weakness.

Reduced FVC (or VC) is a nonspecific finding. Values below the 5th percentile are considered abnormal (see Chapter 13). A low FVC may be caused by either obstruction or restriction (see Box 2-2). Interpretation of the FVC in obstructive diseases requires correlation with flows. An FVC that is significantly lower than VC suggests airway collapse and gas trapping. In restrictive patterns, low FVC may indicate the need to assess other lung volumes, particularly total lung capacity (TLC; see Chapter 4). Interpretation of FVC values close to the lower limit of normal depends on the clinical question to be answered. An FVC at the 5th percentile would be interpreted differently in a healthy patient with no symptoms than in a patient with a history of cough or wheezing. An FVC much lower than expected is often accompanied by the complaint of exertional dyspnea.

A low value for FVC may also occur if the patient’s effort is suboptimal. Patients who stop exhaling before achieving an obvious plateau (on a volume-time display) or who fail to take a maximal inhalation will typically have an underestimated FVC. These patients should be encouraged to inhale maximally at the start of the maneuver then exhale completely for 6 seconds (3 seconds for children less than age 10) or until a plateau occurs. Premature termination of the FVC effort may cause the FEV_1% to be overestimated, masking the presence of obstruction.

An important quality issue to recognize is that a large FVC (and its derived parameters) derived from a flow sensor may result from improper zeroing or contamination of the sensor by condensation or mucus. This problem can usually be recognized by poor repeatability of measurements, characterized by progressively increasing test results within a single testing session.

Forced Expiratory Volume (FEV₁)

FEV₁ measures the volume expired over the first second of an FVC maneuver. FEV₁ is reported as a volume, although it measures flow over a specific interval. FEV₁, like FVC, may be reduced in either obstructive or restrictive patterns (Figures 2-7 and 2-8). FEV₁ values may also be reduced because of poor effort or cooperation by the patient.

An obstructive ventilatory defect is characterized by the reduction of maximal airflow at all lung volumes (e.g., the classic concave shape to the flow-volume curve). Flow is limited by airway narrowing during forced expiration. Airway obstruction may be caused by mucus secretion, bronchospasm, and inflammation such as in asthma or bronchitis. Airflow limitation may also result from a loss of elastic support for the airways themselves, as in emphysema. The earliest changes in obstructive patterns may occur in the small airways (i.e., those less than 2 mm), which may or may not be detected by changes in FEV₁.

FEV₁ may also be decreased in large airway obstruction (trachea and bronchi). Tumors or foreign bodies that limit airflow cause the FEV₁ to be reduced. These defects can be identified by flow reductions across the entire forced expiration (see later section, Flow-Volume Curve).

FEV₁ and FEV₁/VC are the most standardized indices of obstructive disease. An obstructive defect is defined best by a reduced FEV₁/VC ratio. The severity of obstructive disease may be gauged by the extent to which FEV₁ is reduced. The ability to work and function in daily life is related to FEV₁and VC. Mortality (likelihood of dying) caused by respiratory disease is similarly related to the degree of obstruction as measured by FEV₁. Patients with markedly reduced FEV₁ values are much more likely to die from chronic obstructive pulmonary disease (COPD), lung cancer, and even cardiovascular disease, including myocardial infarction and stroke. Although FEV₁ correlates with prognosis and severity of symptoms in obstructive lung disease, outcomes for individual patients cannot be accurately predicted.

While the ratio of FEV₁/VC defines obstruction, the severity of obstruction is defined by the degree to which the FEV₁ is reduced. The ATS-ERS Task Force suggests the following classifications of severity:

MildFEV₁ > 70% predicted

ModerateFEV₁ = 60%-69% predicted

Moderately severeFEV₁ = 50%-59% predicted

SevereFEV₁ = 35%-49% predicted

Very severeFEV₁ < 35% predicted

The concept of grading severity based on FEV₁ applies best when the VC is in the normal range. Once the VC is below normal, a concomitant restrictive defect may also be present, and this can be determined only by further measurement of lung volumes, in particular TLC. Because the FEV₁ in restriction is reduced, in part, by the restrictive process itself, the decrement in FEV₁ resulting from the obstructive component of disease can no longer be assumed to reflect the severity of obstruction only. Thus, the severity of obstruction in a combined restrictive and obstructive defect is typically overestimated when basing severity solely on the reduction in FEV₁.

Restrictive processes, such as fibrosis, edema, space-occupying lesions, neuromuscular disorders, obesity, and chest wall deformities, may all cause FEV₁ to be decreased. Reduction in FEV₁ occurs in much the same way as reduction in VC. Unlike the pattern seen in obstructive disease, in which VC is preserved and FEV₁ is reduced, in restriction VC and FEV₁, values are proportionately decreased. Some patients with moderate or severe restriction have an FEV₁ nearly equal to the VC. The entire VC, because it is reduced, is exhaled almost completely in the first second. To distinguish between obstructive and restrictive causes of reduced FEV₁ values, the FEV₁/VC ratio (FEV_1%) and other flow measurements are useful. Further definition of obstruction versus restriction may require the measurement of lung volumes (e.g., functional residual capacity or FRC, and TLC). However, studies have shown that when the FVC and FEV₁/VC ratio are both normal, restriction, as defined by a low TLC, is very unlikely.

FEV₁ is the most widely used spirometric parameter, particularly for the assessment of airway obstruction. FEV₁

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