Pulmonary Function Tests

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Chapter 9

Pulmonary Function Tests

Donna Frownfelter

Pulmonary function tests are usually ordered when patients go to their primary physician or a pulmonary specialist with complaints of shortness of breath, cough, decreased activity, and dyspnea. When the history is taken, patients’ complaints are due to their breathing problem, for example they are short of breath walking up stairs or dressing. As dyspnea begins to affect their daily life more and more, people will often make lifestyle changes, such as selling a two-story house and moving to a ranch house with no stairs or altering the way they dress (e.g., wearing housecoats or jogging pants that are easier to put on and take off). Often, people think that their breathing problems are just part of getting older, and they try to change their lifestyle to accommodate their limitations and avoid doing things that make them short of breath. In many cases, by the time a patient comes to the care provider with these complaints, he or she is already showing signs of moderate to severe chronic obstructive pulmonary disease (COPD). As will be discussed in this chapter, this is a very good reason for considering office spirometry to be a “fifth vital sign.” Suggestions have been made that all smokers and any nonsmoker over 40 years of age should have office spirometry. If the values are abnormal, even though patients may not be experiencing noticeable lifestyle changes, the appropriate drug therapy regimen can be implemented and patients who are smokers will have a definite motivation to quit smoking.

Pulmonary function tests (PFTs) help in the evaluation of the mechanical function of the lungs.¹ They are based on researched norms, taking into account sex, height, and age. For example, there are predicted values for a male, age 65, who is 6 feet tall.^2,3 Race and ethnic differences also play roles in the reference values and need to be taken into account for diagnostic and research purposes.^4–6

When the patient performs the test, actual results (observed) are compared with the predicted value expected of a person of that gender, height, and age to see whether he or she falls within the “normal” range or has a restrictive, obstructive, or a mixed component, based on the tests. If the patient is not within the normal range, a bronchodilator is given, and the test is repeated to see whether there is significant improvement with medication. Basically, the pulmonary function tests are categorized as volume, flow, or diffusion studies. Diagnosis of pulmonary disease or dysfunction and improvement with treatment are evaluated after interpreting a patient’s pulmonary function tests.

Spirometry is the most useful and commonly available test of pulmonary function. Both pulmonologists and primary care physicians commonly use screening spirometry in their offices in order to assess patients and to evaluate the effectiveness of treatment being given to the patient.⁷

Patients with asthma and COPD make up a large portion of the primary care physician’s caseload. COPD is often diagnosed in the moderate to severe stage of the disease. The diagnosis of COPD can be made quite easily with spirometry, taking the patient’s symptoms and history into account. Smokers can be evaluated for early lung disease, and this can serve as a “teachable moment” for them to quit smoking.⁸

Yawn and colleagues (2007)⁹ compared the office spirometry interpretations of pulmonary experts with those of family physicians. They found agreement in 78% of the completed tests. In addition, following spirometry, changes were implemented in the management of 48% of patients. Of interest, there was closer correct interpretation of pulmonary functions between family physicians and pulmonologists in patients who had asthma, versus those with COPD.

Preoperative Pulmonary Evaluation

Preoperative pulmonary evaluation can predict postoperative pulmonary complications.10–12 As people are living longer lives, more older adults will be candidates for surgery. From 1980 to 1995 the rates of cardiovascular surgical procedures in patients over 65 tripled.¹³ In 1997 the performance of 10 of the most common surgical procedures in the United States totaled 1 in 350,0000 operations in the 65- to 84-year-old age group.¹⁴ Considering the comorbidities of an aging population and the concern about complications in older adults, a thorough preoperative pulmonary screening is recommended.¹¹

Risk factors that contribute to postoperative complications include smoking, older age, obesity, poor health, and chronic obstructive pulmonary disease.¹¹ Additional procedure-related risk factors include the site of surgery (abdominal, chest wall versus extremity, duration of surgery, and type of anesthesia or neuromuscular blockage).¹⁵

Office Spirometry to Improve Early Detection of Chronic Obstructive Pulmonary Disease

The National Lung Health Education Program has recommended that office spirometry be used to screen for subclinical lung disease in adult smokers. For patients who are smokers or for any patients over 40 who have unexplained dyspnea, cough, wheezing, or excessive mucus, spirometric measurements can be considered another vital sign to measure during the routine physical examination, along with blood pressure and cholesterol levels.16,17

In a study of 35- to 70-year-old individuals visiting their general practitioners, patients were given a questionnaire on symptoms of obstructive lung disease.¹⁸ Spirometry was performed in patients with positive answers to the questions and in a random sample of 10% of the group. It was found that 42% of the newly diagnosed cases of obstructive disease would not have been detected without spirometry. The researchers concluded that office spirometry is essential in general practice and can be done by general practitioners who have training in the performance and interpretation of the pulmonary function tests. It is essential that there be good quality assurance and good training when these tests are performed in a general practitioner’s office. Studies have shown variability between the results of pulmonary function tests done in the office versus those done in a lab, so the results should not be considered interchangeable.^19,20

Respiration: Effect of Anatomical and Physiological Dead Space

The most important function of the lungs is to supply the body with oxygen and to the remove carbon dioxide (CO₂) that is produced as a waste product of metabolism. As this continuous gas exchange takes place, sufficient ventilation is needed to move the gases to the alveoli. A number of conducting airways in the lungs, from the trachea down to the terminal bronchi, do not participate in respiration but only move the gases to the alveoli. The volume of this series of airways is known as anatomical dead space. Generally, the anatomical dead space is proportional to the adult body weight. For example, in a 150-pound person, there is an anatomical dead space of approximately 150 mL. A normal tidal volume (TV), the breath normally taken, has to be large enough to reach the alveoli well past the anatomical dead space. In a normal adult, the TV is generally 450 to 600 mL. The anatomical dead space would thus represent about one-third of the TV. The rest of the breath would reach the alveoli and be considered “alveolar ventilation.” In many neurologically impaired patients who have limited TVs, it is important to note that little alveolar ventilation may be taking place when the patient is breathing in a rapid and shallow pattern. For example, if a patient’s TV is 200 mL, 150 mL would be anatomical dead space, and only 50 mL of each breath would be effective alveolar ventilation.

Many diseases and conditions can alter the volume of dead space that requires ventilation. In some cases the dead space decreases—for instance, in a pneumonectomy, when it is physically removed, or in asthma, when bronchospasm may narrow the airways. In other conditions such as pulmonary embolism, dead space increases when ventilated areas of lung cease to be perfused. The alveoli continue to receive fresh gas, but there is no blood available for gas exchange. This type of dead space is known as physiological dead space.

When dead space is increased, a larger percentage of the tidal volume is ventilating the dead space, leaving a smaller percentage for alveolar ventilation. The patient must work harder to get enough air to the functioning alveoli. This causes increased work of breathing and may result in patient fatigue. Work of breathing, or minute ventilation (MV), is defined as the TV times the respiratory rate (RR). The formula for minute ventilation is: MV = TV × RR. For example, an MV of 8 L per minute = 500 mL × 16 breaths per minute. When the ventilatory rate increases, the TV decreases; thus the effective alveolar ventilation decreases.

Neurological and neuromuscular weakness may result in the inability to reach a normal TV. Similarly, surgical procedures or pain caused by fractured ribs can also compromise a patient’s ability to take a breath. Patients given a thoracolumbosacral orthosis (TLSO), or a corset for low back pain, may find their pulmonary function compromised by the restriction of the rib cage and abdomen. As the TV drops, a large percentage of the breath is anatomical dead space. This results in increased work of breathing for the patient and may ultimately result in respiratory failure if the patient is unable to provide effective alveolar ventilation.

Lung Volumes

The lung has four volumes: TV, inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV) (Figure 9-1).

Fig. 9-1 A spirogram (pulmonary function testing).

TV is the normal breath.

IRV is the maximum amount of air that can be inhaled above a normal inspiration.

ERV is the maximum amount of air that can be expired after a normal exhalation.

RV is the volume of gas that remains in the lungs at the end of a maximum expiration.

Changes in RV can help in the diagnosis of certain medical conditions. An increase in RV means that even with maximum effort, the patient cannot exhale excess air from the lungs. This results in hyperinflated lungs and indicates that certain changes have occurred in the pulmonary tissue, which in time may cause mechanical changes in the chest wall (e.g., increased [AP] diameter, flattened diaphragm). These changes may be reversible in patients with partial bronchial obstruction, such as young patients with asthma, or irreversible, as in patients with advanced emphysema. Restrictive lung disease can cause a decrease in residual volume, as can cancer of the lung, microatelectasis, or musculoskeletal impairment.²¹

Lung Capacities

A lung capacity is two or more volumes added together (see Fig. 9-1). The capacities include total lung capacity (TLC), vital capacity (VC), inspiratory capacity (IC), and functional residual capacity (FRC).

TLC is the amount of gas the lung contains at the end of a maximum inspiration. It is made up of all four lung volumes. An increased TLC is seen with hyperinflation, such as in emphysema. A decrease in TLC may be seen in restrictive lung disease, such as pulmonary fibrosis, atelectasis, neoplasms, pleural effusions, and hemothorax, as well as in restrictive musculoskeletal problems, such as spinal cord injury and kyphoscoliosis. Decreased TLC can also be secondary to morbid obesity or pregnancy.

VC is the maximum amount of gas that can be expelled from the lungs by forceful effort after a maximum inspiration. It contains the IRV, TV, and ERV. A decrease in VC can occur as a result of absolute reduction in distensible lung tissue. This is seen in pneumonectomy, atelectasis, pneumonia, pulmonary congestion, occlusion of a major bronchus by a tumor or foreign object, and restrictive lung disease, due to either a primary cause such as pulmonary fibrosis or a secondary cause such as the application of a TLSO.

A decrease in VC may also be seen without primary lung disease or airway obstruction. In neuromuscular or musculoskeletal dysfunction, VC can be compromised (as in Guillain-Barré syndrome, spinal cord injury, drug overdose, motor vehicle accident with fractured ribs, severe scoliosis, pectus excavatum, and kyphoscoliosis). Additional contributing factors, such as morbid obesity, pregnancy, enlarged heart, and pulmonary effusion, may involve limitation of expansion of the lungs.

IC is the maximum amount of air that can be inspired when starting at the resting expiratory level. It contains the IRV and the TV.

FRC is the volume of air remaining in the lungs at the resting expiratory level. It contains the ERV and the RV. The FRC prevents large fluctuations in PaO₂ with each breath. An increase in FRC represents hyperinflation of the lungs. It causes the thorax to be larger than normal, which results in muscular inefficiency and some mechanical disadvantage. Patients on mechanical ventilators may increase their FRC with positive pressure and by additional modes such as positive end-expiratory pressure (PEEP). Spontaneously breathing patients can also be on continuous positive airway pressures (CPAP and BIPAP), which keep the lungs at a positive airway pressure to improve ventilation and oxygenation.

Air Flow Measurements

Forced Expiration

When a patient performs a VC maneuver, it can either be slow or fast. During exhalation, the amount of air exhaled over time can be measured. In a slow VC, a patient with emphysema can take a great deal of time to empty his or her lungs. In a forced VC, a normal individual can exhale 75% of the VC in the first second of exhalation (FEV₁). Patients with emphysema often have greatly decreased VCs, only 40% of which are predicted. Some pulmonary function laboratories also offer an FEV₆, which measures the flow at six seconds rather than just at 1 second.

Flow Volume Curve

The flow volume curve is helpful in diagnosing lung disease because it is independent of effort. The curve in Figure 9-2 demonstrates that flow rises to a high value and then declines over most of expiration.²² In restrictive lung disease, the maximum flow rate is reduced, as is the total volume exhaled. In obstructive lung disease, the flow rate is low in relation to lung volume, and a scooped-out appearance is often seen (see Figure 9-2).

Another diagnostic test that uses forced expiration is the flow volume loop. It is a graphic analysis of the flow generated during a forced expiratory volume maneuver followed by a forced inspiratory volume maneuver (Figure 9-3). This graph offers a pictorial representation of data from many individual tests (e.g., peak inspiratory and expiratory flow rates, FVC, and FEV). The shape of the graph may also be helpful in diagnosing disease, because again there is a more scooped-out appearance in cases of obstructive disease.

Fig. 9-3 Comparison of flow volume loops in the normal patient and in the patient with obstructive lung disease.

Closing Volume and Airway Closure

The assessment of closing volume is used to help diagnose small-airway disease. A test called the single-breath nitrogen (N₂) washout is used for assessing closing volume and closing capacity of the small airways. In this test, the patient takes a single VC breath of 100% oxygen. During complete exhalation, the N₂ concentration can be measured. The characteristic tracing of N₂ concentration can be measured. The characteristic tracing of N₂ concentration versus lung volume reflects the sequential emptying of differentially ventilated lung units, resulting in different expiratory concentrations of N₂. Four phases can be identified (Figure 9-4). Phase I contains pure dead space and virtually none of the potential N₂ from the RV. Phase II is associated with an increasing N₂ concentration of a mixture of gas from the dead space and alveoli. The plateau in N₂ concentration observed in phase III reflects pure alveolar gas emanating from the base and middle lung zones. Phase IV occurs toward the end of expiration and is characterized by an abrupt increase in N₂ concentration. This high N₂ concentration reflects the closure of airways at the base of the lungs and the expiration of gas from the upper lung zones, because in the single breath of 100% oxygen, less oxygen was initially directed to this area.

Fig. 9-4 The single-breath nitrogen washout test to assess airway closure.

Closing volume is the lung volume at which the inflection of phase IV, the marked increase in N₂ concentration after the plateau, is observed. Closing capacity refers to closing volume and RV. The same characteristic tracing of the single-breath nitrogen washout test can be obtained by the inhalation of a bolus of tracer gas (e.g., argon, helium, xenon 133).

The closing volume is 10% of the vital capacity in young, healthy individuals. It increases with age and is 40% of the vital capacity at age 65. Closing volume is used as an aid in the diagnosis of small-airway disease and as a means of evaluating treatment and drug response.

Maximum Voluntary Ventilation

Maximum voluntary ventilation measures the maximum breathing capacity of the patient. It reflects strength and endurance of the respiratory muscles. The patient is asked to pant for 15 seconds into the spirometer tubing. The results are commonly examined preoperatively with the other results to determine a patient’s prognosis for success after surgery, such as his or her ability to cough, to take deep breaths, and to enhance airway clearance.

Diagnosis of Restrictive and Obstructive Lung Disease

Physicians use the results of pulmonary function tests to diagnose lung disease or characteristic components of lung disease such as bronchospasm. A restrictive component describes conditions that limit the amount of volume coming into the lungs (restriction to inspiration). An obstructive component generally relates to problems in exhalation air flows and characteristic patterns of obstruction, such as in the first second of expiration FEV₁ measurement. Patients do not commonly have only one primary disease process but may have overlapping lung conditions.²³ A diagnosis may read, “PFTs consistent with moderate emphysema with bronchospastic component; good response to bronchodilators.” A patient with an abnormal PFT is given a bronchodilator and retested. If there is a 15% to 20% increase in the PFT after bronchodilators are administered, they will be a recommended part of the patient’s medications. However, some patients are given a trial of bronchodilators, even if there is not such a dramatic response on PFTs.

A pictorial demonstration of the differences in obstructive and restrictive lung disease is shown in Figure 9-5. Disease has a marked effect on pulmonary function; yet TV usually remains 10% of total lung capacity until the disease is relatively severe. Physiological pulmonary reserves in both disease processes are limited and generally affect a patient’s response to exercise. Exercise is limited by the ventilatory status rather than by a cardiac end point. As obstructive lung disease progresses, TLC, FRC, and RV are markedly increased. In severe COPD, the increased FRC can compromise the VC. More energy is expended to breathe than that expended by a normal individual. This effect can be disproportionally increased with minimal amounts of activity. In restrictive lung disease, restriction of the chest wall or lung tissue can produce a decrease in TLC. A VC of 80% or less of predicted values for a patient is considered a diagnostic feature. A residual decline in FRC potentiates airway closure.

Fig. 9-5 Examples of proportional changes in lung volumes and capacities characteristic of obstructive and restrictive lung diseases.

The phenomenon of closing volume in the lungs is particularly significant to physical therapists who prescribe breathing exercises and body positioning and can thereby alter pulmonary mechanics and gas exchange. These treatment interventions may have pronounced effects on lung volumes and airway closure.²⁴ At low lung volumes (e.g., breathing at FRC, in the Trendelenberg position, and in lung disease) intrapleural pressures are generally less negative, and the pressure of dependent lung regions may equal or exceed atmospheric pressure. Intrapleural pressure is less negative because the lungs are less expanded and elastic recoil is decreased. As a result, airway closure is potentiated. In young individuals, closure is evident at RV; however, in older individuals, closure is observed at higher lung volumes, such as at FRC. Premature closure of the small airways results in uneven ventilation and impaired gas exchange with a given lung unit. Airway closure occurs more readily in chronic smokers and in patients with lung disease.

Aging has a significant effect on airway closure. With aging, a loss of pulmonary elastic recoil results in a loss of intrapleural negative pressure. In older individuals, therefore, airway closure occurs at higher lung volumes. For example, closure has been reported to occur at the age of 65 in the upright lung during normal breathing. In the supine position, where FRC is reduced, closure occurs at a significantly younger age (about age 44). In addition to the often compounding effect of age, the lung volume at which airway closure occurs increases with chronic smoking and lung disease and is changed with alterations in body position.25,26

Summary

Pulmonary functions change as a patient’s condition gets better or worse.27,28 There are normal declines in volumes and flows with aging, as well as with disease processes. Basic bedside spirometry is often done to assess a patient’s breathing mechanical ability and screen preoperatively to predict possible postoperative complications, especially in high-risk patients. Bedside screening can detect improvement or decline in a clinical status. In a patient with Guillain-Barré syndrome, as breathing becomes labored, the VC is monitored to determine whether a ventilator is needed. On the other hand, in a patient with obstructed airways, such as with cystic fibrosis, the pulmonary function tests will improve.²⁹ In a patient with spinal cord injury or neuromuscular weakness, as strength improves, vital capacity increases. However, because of the lack of abdominal muscles, the flows may be reduced.