An approach to interpreting spirometry - Cover article: office procedures

Author: Timothy J. Barreiro, Irene Perillo
Date: March 1, 2004

Chronic obstructive pulmonary disease (COPD) is the most common respiratory disease and the fourth leading cause of death in the United States. (1) Despite preventive efforts, the number of new patients with COPD has doubled in the past decade, and this trend is likely to continue. (2,3) Evidence indicates that patient's history and physical examination are inadequate for diagnosing mild and moderate obstructive ventilatory impairments. (4) Although a complete pulmonary function test provides the most accurate objective assessment of lung impairment, spirometry is the preferred test for the diagnosis of COPD because it can obtain adequate information in cost-effective manner.

A great deal of information can be obtained from a spirometry test; however, the results must be correlated carefully with clinical and roentgenographic data for optimal clinical application. This article reviews the indications for use of spirometry, provides stepwise approach to its interpretation, and indicates when additional tests are warranted.

Background

The National Health Survey of 1988 to 1994 found high rates of undiagnosed and untreated COPD in current and former smokers. (5) Population-based studies have identified vital capacity (VC) as a powerful prognostic indicator in patients with COPD. The Framingham study identified a low forced vital capacity (FVC) as a risk factor for premature death. (6) The Third National Health and Nutritional Examination Survey and the multicenter Lung Health Study showed potential benefits for patients with early identification, intervention, and treatment of COPD. (7,8) The Lung Health Study was the first study to show that early identification and intervention in smokers could affect the natural history of COPD. (7) These surveys also showed that simple spirometry could detect mild airflow obstruction, even in asymptomatic patients.

Increased public awareness of COPD led to the formation of the National Lung Health Education Program (NLHEP) as part of a national strategy to combat chronic lung disease. (9) The World Health Organization and the U.S. National Heart, Lung, and Blood Institute recently published the Global Initiative for Chronic Obstructive Lung Disease to increase awareness of the global burden of COPD and to provide comprehensive treatment guidelines aimed at decreasing COPD-related morbidity and mortality. (10)

Spirometry Measurements and Terminology

Spirometry measures the rate at which the lung changes volume during forced breathing maneuvers. Spirometry begins with a full inhalation, followed by a forced expiration that rapidly empties the lungs. Expiration is continued for as long as possible or until a plateau in exhaled volume is reached. These efforts are recorded and graphed. (A glossary of terms used in this article can be found in Table 1.)

Lung function is physiologically divided into four volumes: expiratory reserve volume, inspiratory reserve volume, residual volume, and tidal volume. Together, the four lung volumes equal the total lung capacity (TLC). Lung volumes and their combinations measure various lung capacities such as functional residual capacity (FRC), inspiratory capacity, and VC. Figure 1 (11) shows the different volumes and capacities of the lung.

The most important spirometric maneuver is the FVC. To measure FVC, the patient inhales maximally, then exhales as rapidly and as completely as possible. Normal lungs generally can empty more than 80 percent of their volume in six seconds or less. The forced expiratory volume in one second (FE[V.sub.1]) is the volume of air exhaled in the first second of the FVC maneuver. The FE[V.sub.1]/FVC ratio is expressed as a percentage (e.g., FE[V.sub.1] of 0.5 L divided by FVC of 2.0 L gives an FE[V.sub.1]/FVC ratio of 25 percent). The absolute ratio is the value used in interpretation, not the percent predicted.

Some portable office spirometers replace the FVC with the FE[V.sub.6] for greater patient and technician ease. The parameter is based on a six-second maneuver, which incorporates a standard time frame to decrease patient variability and the risk of complications. One of the pitfalls of using this type of spirometer is that it must be calibrated for temperature and water vapor. It should be used with caution in patients with advanced COPD because of its inability to detect very low volumes or flows. However, the FE[V.sub.1]/FE[V.sub.6] ratio provides accurate surrogate measure for the FE[V.sub.1]/FVC ratio. (12) The reported FE[V.sub.1] and FE[V.sub.6] values should be rounded to the nearest 0.1 L and the percent predicted and the FE[V.sub.1]/FE[V.sub.6] ratio to the nearest integer. (13)

Different spirographic and flow volume curves are shown in Figure 2. (11) It is important to understand that the amount exhaled during the first second is a constant fraction of the FVC, regardless of lung size. The significance of the FE[V.sub.1]/FVC ratio is twofold. It quickly identifies patients with airway obstruction in whom the FVC is reduced, and it identifies the cause of a low FE[V.sub.1]. Normal spirometric parameters are shown in Table 2. (14)

Indications for Office Spirometry

Spirometry is designed to identify and quantify functional abnormalities of the respiratory system. The NLHEP recommends that primary care physicians perform spirometry in patients 45 years of age or older who are current or former smokers; in patients who have a prolonged or progressive cough or sputum production; or in patients who have a history of exposure to lung irritants. (9) Other indications for spirometry are to determine the strength and function of the chest, follow disease progression, (15,16) assess response to treatment, (17,18) and obtain baseline measurements before prescribing drugs that are potentially toxic to the lungs, such as amiodarone (Cordarone) and bleomycin (Blenoxane). (19) Spirometry also is helpful in preoperative risk assessment for many surgeries (20-23) and often is used in workers' compensation and disability claims to assess occupational exposure to inhalation hazards. (24) Tables 3 and 4 list indications and contraindications for spirometry.

Interpreting Spirometry Results

Spirometry requires considerable patient effort and cooperation. Therefore, results must be assessed for validity before they can be interpreted. (17,25) Inadequate patient effort can lead to misdiagnosis and inappropriate treatment. An algorithm for interpreting spirometry results is given in Figure 3.

[FIGURE 3 OMITTED]

The clinical context of the test is important because parameters in patients with mild disease can overlap with values in healthy persons. (26) Normal spirometry values may vary, and interpretation of results relies on the parameters used. The normal ranges for spirometry values vary depending on the patient's height, weight, age, sex, and racial or ethnic background. (27,28) Predicted values for lung volumes may be inaccurate in very tall patients or patients with missing lower extremities. FE[V.sub.1] and FVC are greater in whites compared with blacks and Asians. FVC and VC values vary with the position of the patient. These variables can be 7 to 8 percent greater in patients who are sitting during the test compared with patients who are supine. FVC is about 2 percent greater in patients who are standing compared with patients who are supine.

To determine the validity of spirometric results, at least three acceptable spirograms must be obtained. In each test, patients should exhale for at least six seconds and stop when there is no volume change for one second. The test session is finished when the difference between the two largest FVC measurements and between the two largest FE[V.sub.1] measurements is within 0.2 L. If both criteria are not met after three maneuvers, the test should not be interpreted. Repeat testing should continue until the criteria are met or until eight tests have been performed. (26)

Figure 4 (25) shows normal flow-volume and time-volume curves. Notice that the lines of the flow-volume curve are free of glitches and irregularities. The volume-time curve extends longer than six seconds, and there are no signs of early termination or cutoff.

[FIGURE 4 OMITTED]

If the test is valid, the second step is to determine whether an obstructive or restrictive ventilatory pattern is present. When the FVC and FE[V.sub.1] are decreased, the distinction between an obstructive and restrictive ventilatory pattern depends on the absolute FE[V.sub.1]/FVC ratio. If the absolute FE[V.sub.1]/FVC ratio is normal or increased, a restrictive ventilatory impairment may be present. However, to make a definitive diagnosis of restrictive lung disease, the patient should be referred to a pulmonary laboratory for static lung volumes. If the TLC is less than 80 percent, the pattern is restrictive, and diseases such as pleural effusion, pneumonia, pulmonary fibrosis, and congestive heart failure should be considered.

(20.) Dunn WF, Scanlon PD. Preoperative pulmonary function testing for patients with lung cancer. Mayo Clin Proc 1993;68:371-7.

(21.) Celli BR. What is the value of preoperative pulmonary function testing? Med Clin North Am 1993;77:309-25.

(22.) Culver BH. Preoperative assessment of the thoracic surgery patient: pulmonary function testing. Semin Thorac Cardiovasc Surg 2001;13:92-104.

(23.) Powell CA, Caplan CE. Pulmonary function tests in preoperative pulmonary evaluation. Clin Chest Med 2001;22:703-14, viii.

(24.) Sood A, Redlich CA. Pulmonary function tests at work. Clin Chest Med 2001;22:783-93.

(25.) Crapo RO. Pulmonary-function testing. N Engl J Med 1994;331:25-30.

(26.) Crapo RO, Morris AH. Pulmonary function testing: sources of error in measurement and interpretation. South Med J 1989;82:875-9.

(27.) Petty T L. Simple office spirometry. Clin Chest Med 2001;22:845-59.

(28.) Margolis ML, Montoya FJ, Palma WR Jr. Pulmonary function tests: comparison of 95th percentile-based and conventional criteria of normality. South Med J 1997;90:1187-91.

(29.) Lung function testing: selection of reference values and interpretative strategies. American Thoracic Society. Am J Respir Crit Care Med 1991;144:1202-18.

TIMOTHY J. BARREIRO, D.O., is a second-year pulmonary disease and critical care medicine fellow at the University of Rochester (N.Y.) School of Medicine and Dentistry, Strong Memorial Hospital. Dr. Barreiro earned his medical degree from Ohio University College of Osteopathic Medicine, Athens, and completed an internal medicine residency at Allegheny General Hospital in Pittsburgh, Pa.

IRENE PERILLO, M.D., is assistant professor of medicine and director of the outpatient pulmonary clinic at the University of Rochester School of Medicine and Dentistry, Strong Memorial Hospital. Dr. Perillo earned her medical degree from State University of New York Upstate Medical University, Syracuse, and completed an internal medicine residency, and pulmonary and critical care fellowship at the University of Rochester School of Medicine and Dentistry.

Address correspondence to Timothy J. Barreiro, D.O., University of Rochester School of Medicine and Dentistry, 601 Elmwood Ave., Box 692, Rochester, NY 14642 (e-mail: Timothy_Barreiro@urmc.rochester.edu). Reprints are not available from the authors.

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