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Evaluation of pulmonary function is important in many clinical situations, both when the patient has a history or symptoms suggestive of lung disease, and when risk factors for lung disease are present, such as cigarette smoking . An overview of pulmonary function testing will be presented here, summarizing the types of pulmonary function tests and their indications. Specific aspects of pulmonary function testing also are discussed elsewhere. (See "Office spirometry", see "Reference values for pulmonary function testing", and see "Diffusing capacity for carbon monoxide").
PULMONARY FUNCTION TESTS The major types of pulmonary function tests include spirometry, measurement of lung volumes, and quantitation of diffusing capacity. Measurements of maximal respiratory pressures and forced inspiratory flow rates are also useful in specific clinical circumstances (show figure 1).
Spirometry Spirometry, which includes measurement of forced expiratory volume in one second (FEV1) and forced vital capacity (FVC), is the most readily available and most useful pulmonary function test. It takes ten to 15 minutes, uses a $2000 instrument, and carries no risk. Spirometry should be performed in all smokers over the age of 45 to screen for chronic obstructive pulmonary disease; over one-fourth will have abnormal results. (See "Office spirometry").
The slow vital capacity (SVC) can also be measured with spirometers which collect data for at least 30 seconds. The SVC may be a useful measurement when the forced vital capacity (FVC) is reduced and airways obstruction is present. Slow exhalation results in a lesser degree of airway narrowing, and frequently the patient can exhale a larger, even normal, volume. In contrast, the vital capacity with restrictive disease is reduced during both slow and fast maneuvers. Thus, if the slow or forced vital capacity is within the normal range, a significant restrictive disorder is virtually excluded, and it is generally unnecessary to measure static lung volumes (residual volume and total lung capacity) .
Post-bronchodilator spirometry Administration of albuterol by metered-dose inhaler (MDI) is indicated during an initial workup if baseline spirometry demonstrates airway obstruction or if one suspects asthma. Spirometry should be repeated ten minutes after administration of a bronchodilator, and proper MDI technique is important to prevent false negative results. (See "Metered dose inhaler techniques").
In a patient with airway obstruction, an increase in the FEV1 of more than 12 percent and greater than 0.2 L suggests acute bronchodilator responsiveness . However, the lack of an acute bronchodilator response should not preclude a six to eight week therapeutic trial of bronchodilators and/or inhaled corticosteroids, with reassessment of clinical status and change in FEV1 at the end of that time.
Lung volumes Measurement of the total lung capacity (TLC) may be helpful when the vital capacity is decreased. For example, in the setting of chronic obstructive pulmonary disease (COPD) with a low vital capacity, measurement of the TLC can help determine if there is a superimposed restrictive disorder.
There are four methods of measuring TLC:
The first two methods are used extensively in hospital pulmonary function laboratories, but they may underestimate the TLC in patients with moderate to severe COPD. The gold standard for measurement of TLC, particularly in the setting of significant airflow obstruction, is body plethysmography.
Measurements of TLC using the chest x-ray correlate within 15 percent of those obtained by body plethysmography . These measurements can be made in the office in about five minutes from a standard PA and lateral chest x-ray, using a $300 planimeter. Since the TLC is equivalent to the amount of air seen in the lungs on a chest x-ray taken at maximal inspiration, it is important that the subject inhales maximally during the chest x-ray.
Diffusing capacity Measurement of the single-breath diffusing capacity for carbon monoxide (DLCO) is quick, safe, and useful in the evaluation of both restrictive and obstructive disease. It requires use of a piece of equipment that costs $20,000. In the setting of restrictive disease, the diffusing capacity helps distinguish between intrinsic lung disease, in which DLCO is usually reduced, from other causes of restriction, in which DLCO is usually normal. In the setting of obstructive disease, the DLCO helps distinguish between emphysema and other causes of chronic airway obstruction. (See "Diffusing capacity for carbon monoxide").
Maximal respiratory pressures Measurement of maximal inspiratory and expiratory pressures is indicated whenever there is an unexplained decrease in vital capacity or respiratory muscle weakness is suspected clinically. Maximal inspiratory pressure (MIP) is the maximal pressure that can be produced by the patient trying to inhale through a blocked mouthpiece. Maximal expiratory pressure (MEP) is the maximal pressure measured during forced expiration (with cheeks bulging) through a blocked mouthpiece after a full inhalation. Repeated measurements of MIP and MEP are useful in following the course of patients with neuromuscular disorders. The slow vital capacity may also be followed, but it is less specific and usually less sensitive.
Maximal inspiratory and expiratory pressures are easily measured using a simple mechanical pressure gauge connected to a mouthpiece. MIP measures the ability of the diaphragm and the other respiratory muscles to generate inspiratory force, reflected by a negative airway pressure. The average MIP and MEP for adult men are -100 cmH2O and +170 cmH2O, respectively, while the corresponding values for adult women are about -70 cmH2O and +110 cmH2O, respectively [5,6]. The lower limit of the normal range is about two-thirds of these values .
Forced inspiratory maneuvers Flow-volume loops, which include forced inspiratory maneuvers, should be performed whenever stridor is heard over the neck during forced breathing or for evaluation of unexplained dyspnea. (See "Flow-volume loops"). Airway obstruction located in the pharynx, larynx, or trachea (upper airways) is usually impossible to detect from standard FVC maneuvers. Reproducible forced inspiratory vital capacity (FIVC) maneuvers detect variable upper airway obstruction, as can be seen with vocal cord paralysis or dysfunction, which causes a characteristic limitation of flow (plateau) during forced inhalation but little if any obstruction during exhalation (show figure 2).
Less commonly, a fixed upper airway obstruction (UAO), eg, from tracheal stenosis, causes flow limitation during both forced inhalation and forced exhalation maneuvers (show figure 2). However, the flow-volume loop is not sensitive for detecting a fixed UAO, since the tracheal lumen is often reduced to less than one cm before a plateau is recognized. Poor effort mimics the flow-volume loop shapes of upper airway obstruction, but can be excluded when three or more maneuvers are seen to be reproducible.
Pulse oximetry Instruments which measure arterial oxygen saturation continuously and noninvasively are ubiquitous in hospitals and clinics. Indications for oximetry in the pulmonary function laboratory include screening for oxygen desaturation in patients with exercise limitation and determining the adequacy of supplemental oxygen therapy. A fall of more than four percent (ending at a saturation below 93 percent) suggests significant desaturation, and confirmation with arterial blood gas (ABG) measurements may be indicated. (See "Pulse oximetry").
Use of oximeters to guide oxygen therapy in severe COPD or interstitial fibrosis is limited, since the saturation obtained from the oximeter may be in error by up to four percent. Thus, direct measurement of the arterial PO2 from the standard ABG measurement is still the best index of the need for oxygen therapy. (See "Guidelines for long-term supplemental oxygen therapy")
INDICATIONS Pulmonary function testing is useful for evaluation of a variety of forms of lung disease or for assessing the presence of disease in a patient with known risk factors, such as smoking. Other indications for pulmonary function tests include:
Evaluation of chronic dyspnea Many lung diseases begin slowly and insidiously and finally manifest themselves with the nonspecific symptom of dyspnea on exertion. Pulmonary function tests are an essential part of the workup of such patients. In the outpatient setting, in which several days to weeks are available to make the diagnosis, a cost efficient method of ordering pulmonary function tests is to start with spirometry and then order further tests in a stepwise fashion to refine the diagnosis (show figure 3). (See "Approach to the patient with dyspnea").
When a patient is hospitalized and a diagnosis is needed within a day or two, a battery of pulmonary function tests may be ordered, often including spirometry before and after (pre- and post-) bronchodilator therapy, static lung volumes, and diffusing capacity. If the cause of dyspnea on exertion remains uncertain after these tests have been performed, cardiopulmonary exercise testing should be considered.
Evaluation of asthma Spirometry before and after a bronchodilator is indicated during the initial workup of patients suspected of having asthma. Spirometry is also indicated during most follow-up office visits to provide an objective measure of the therapeutic response . (See "Diagnosis of asthma", see "Use of pulmonary function testing in the diagnosis of asthma").
Cough or chest tightness with exercise or exposure to cold air, dusts, or fumes suggests bronchial hyperresponsiveness (BHR). However, BHR may not be detected by pre- and post-bronchodilator spirometry if the patient is asymptomatic at the time of evaluation. Commonly, the patient is asked to return for retesting when symptoms occur; however, this delays the diagnosis and may be impractical. Inhalation challenge testing will usually confirm or exclude the diagnosis of asthma in less than an hour. (See "Bronchoprovocation testing").
An alternative to inhalation challenge testing for the detection of airway hyperreactivity is measurement of airway lability for two weeks in the patient's own environment, using ambulatory monitoring of peak flow or FEV1. Children with asthma (not controlled by medication) typically demonstrate peak flow lability (amplitude/mean) in excess of 30 percent, while adults with active asthma have PEF lability greater than 20 percent. (See "Peak expiratory flow rate monitoring in asthma").
A forced inspiratory maneuver performed as part of a flow-volume loop may be useful in detecting "vocal cord dysfunction" in atypical patients with a diagnosis of asthma who do not respond appropriately to therapy. (See "Diagnosis of wheezing illnesses other than asthma", and see "Paradoxical vocal cord motion").
Chronic obstructive pulmonary disease Primary care physicians should obtain spirometry in patients 45 years of age or older who currently smoke (or have quit only in the preceding year) and in patients with symptoms of COPD. Spirometry is the best method to detect the borderline to mild airways obstruction which occurs early in the course of the disease . (See "Diagnosis of chronic obstructive pulmonary disease"). In these cases, the FEV1/FVC ratio is often decreased. Measurement of lung volumes is rarely useful in patients with obstructive spirometric parameters because it does not allow reliable differentiation of asthma and chronic obstructive pulmonary disease, and a concomitant restrictive ventilatory defect is detected in <10 percent of patients with a reduced FVC .
Once the diagnosis of COPD is established, the course and response to therapy are best followed by observing changes in the FEV1, as was done in the multicenter Lung Health Study . Continued smoking in a patient with airways obstruction often results in an abnormally rapid decline in FEV1 (90 to 150 mL/yr). On the other hand, smoking cessation often results in an increase in FEV1 during the first year, followed by a nearly normal rate of FEV1 decline (30 mL/yr). Both a low FEV1 and chronic mucus hypersecretion are predictors of hospitalization due to COPD .
Once the airways obstruction due to COPD has become very severe, with an FEV1 of 0.7 L or less, changes from visit to visit are usually within the error of the measurement (0.2 liters). In this circumstance, measurements of oxygen saturation during exercise and distance walked during six minutes may be more clinically meaningful for evaluating disease progression or therapeutic response than are changes in spirometry values [12,13].
Measurement of the diffusing capacity for carbon monoxide helps to distinguish between emphysema and other causes of chronic airway obstruction. As an example, emphysema lowers the DLCO, obstructive chronic bronchitis does not affect the DLCO, and asthma frequently increases the DLCO. (See "Diffusing capacity for carbon monoxide"). Changes in the DLCO in patients with established, smoking-related COPD are probably not clinically useful during follow-up visits, unless dyspnea suddenly worsens without an obvious cause.
Restrictive lung disease The many disorders which cause reduction of lung volumes (restriction) may be divided into three groups:
The history, physical examination, and chest x-ray are often helpful in distinguishing among these disorders. Spirometry is useful in detecting restriction (reduction) of lung volumes, but it rarely helps in establishing the cause. The DLCO is useful for differentiating intrinsic lung diseases, in which DLCO is generally reduced, from other causes of lung volume restriction, including neuromuscular disease or musculoskeletal deformity, in which DLCO is generally normal. (See "Diffusing capacity for carbon monoxide").
Changes in the DLCO are also useful for following the course of or response to therapy in patients with interstitial lung disease. Pulse oximetry during exercise is another test that can be useful in this setting, since oxygen saturation often falls during mild exercise in patients with interstitial lung disease . (See "Approach to the adult with interstitial lung disease").
Preoperative testing Spirometry is useful for determining the risk of postoperative pulmonary complications in certain high-risk situations, including patients known to have COPD or asthma, current smokers, and those scheduled for thoracic or upper abdominal surgery . The degree of airways obstruction (or an elevated PCO2 for patients with COPD) predicts the risk of postoperative pulmonary complications, such as atelectasis, pneumonia, and the need for prolonged mechanical ventilation. If spirometry demonstrates moderate to severe obstruction and the surgery can be delayed, a prophylactic program of pulmonary hygiene, including smoking cessation, inhaled bronchodilators or steroids, and antibiotics for bronchitis, will reduce the risk. However, the results of spirometry should not be used to deny surgery. Combining the results of spirometry with radioisotope or CT lung scans is also useful for predicting the remaining lung function following a lobectomy or pneumonectomy.
A number of studies indicate that the maximum oxygen uptake (as a percent of predicted), determined by cardiopulmonary exercise testing, is better than spirometry for predicting postsurgical complications , but the cost:benefit ratio is unknown. (See "Preoperative evaluation for lung resection").
Evaluation of impairment or disability Most schemes for evaluation of respiratory impairment use pulmonary function tests, but the results in studies performed at rest are only a rough indication of an individual's ability to perform a given job. It is ideal to measure maximal oxygen consumption (VO2max), but this test is often not available to the primary care physicians who perform "disability" testing, or the expense is not reimbursed [17,18].
The American Medical Association provides guidelines for the classification of respiratory impairment based upon the results of spirometry or maximal oxygen consumption . Severe impairment (AMA class 4 with estimated 50 to 100 percent impairment) is defined as any one of the following:
Of course, the results must be of good quality.
The Social Security Administration defines total respiratory disability using either height-corrected FEV1 (1.1 to 1.4 L) or a DLCO less than 30 percent predicted . In one study, approximately 33 percent of patients who met the above criteria were dead after four years, compared with seven percent of those who applied for disability but did not meet these criteria. (See "Evaluation of pulmonary disability").
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