Commentary: Quality of Spirometry Testing
Thomas L. Petty, MD
Spirometry is necessary for the diagnosis and monitoring of both obstructive and restrictive lung diseases. Today, the most rapidly growing health problem in the United States is chronic obstructive pulmonary disease (COPD). COPD is now the fourth most common cause of death and the only cause of death that is rising among the top ten causes. There is also an increasing prevalence of interstitial lung diseases. Both obstructive and interstitial lung diseases are identified and monitored by Spirometry.
Key words: Quality, smoking cessation, Spirometry.
THE ESSENCE OF SPIROMETRY
Spirometry measures air flow from fully inflated lungs. Effort is required to fully fill the lungs to the maximum. A forced expiratory breath of a full 6 seconds is then required to fully empty the lungs. This is the forced vital capacity (FVC). Normal lungs empty in 6 seconds or less. It is now known that forced expiratory volume in 6 seconds (FEV6) is a good surrogate for the full FVC (1, 2). The other important test is the measure of flow, or forced expiratory volume in 1 second (FEV1). Other so-called tests of small airways function, such as FEF25%-75%, are not recommended (3). They do not measure small airways disease any more accurately than the FEV1 does. The FEV1/FVC ratio is an important derived number. Normally, the FEV1/FVC (or FEV1/FEV6), is 70-75%, or more. Lower ratios indicate an obstructive ventilatory disorder. A high ratio (ie, >85%) indicates very rapid lung emptying of a small vital capacity and is indicative of a restrictive ventilatory defect.
Spirometry measures muscular effort, elastic recoil, small airway function, large airway function, and interdependence between alveoli and small airways. The common obstructive and restrictive ventilatory defects are listed in Table 1. Figure 1 presents a simple algorithm that helps to differentiate between obstructive and restrictive ventilatory defects.
QUALITY CONTROL ISSUES
The calibration of all spirometers should be tested using a 3-L syringe. A technician or doctor with known spirometric values can also be used to check on Spirometry accuracy (4).
Spirometric tests are effort dependent. Thus, the patient must cooperate to fully inflate the lungs. Initiating expiratory airflow with a blast so that peak flow is achieved within one tenth of 1 second (0.1 second) and a full forced expiration for 6 seconds without in- terruption is required. Software on modem flow-sensing spirometers monitors the quality of the expiratory effort, the time from initiation of air flow to peak flow, and a complete tracking of the expiratory time-volume or flow-volume curve. Poor efforts or incomplete efforts are detected, and coaching instructions are offered by visible cues from the readout of the spirometer. Re- peatability within 3% is required on most devices. Comparison with predicted normal values, based on age, sex, height, and race, offers an interpretation for the clinician. The new handheld devices are accurate when compared with a standard laboratory instrument (5). One such device is illustrated in Figure 2. Previous concerns about inadequate or improper testing, particularly with poor end expiratory efforts, can be minimized by application of modem clinical spirometers (6).
Figure 2. A handheld spirometer
THE NATIONAL LUNG HEALTH EDUCATION PROGRAM
The National Lung Health Education Program (NLHEP) is a new national health care initiative that aims to encourage the use of spirometry in all primary care physicians' and other specialists' offices (7,8). The foundation for the NLHEP can be found in the Lung Health Study (9), which showed that smoking cessation leads to an improvement in ventilatory function, as judged by FEV1 values compared with continued smoking (Figure 3). The NLHEP recommends using the FEV6 as a surrogate for FVC (8). Normal lungs empty in 6 seconds or less. Using a shorter expiratory time creates less stress on the subject and is easier for the spirometry technician. The FEV6 bears a close relation to the FVC and is suitable for diagnosing obstructive and restrictive ventilatory abnormalities (1, 2). The third National Health and Nutrition Examina tion Survey (NHANES III) (10), was an extensive study of a large, noninstitutionalized and nonmilitary population. Most of the subjects in this study underwent spirometry. This is the largest database for both subjects with normal and subjects with abnormal lung function from the US population. NHANES III found a huge underdiagnosis of COPD, even in symptomatic patients (10). Thus, a challenge exists to find early-stage disease and to treat it with smoking cessation and bronchoactive drugs before it evolves into late-stage COPD.
The early identification of inflammatory fibrotic restrictive abnormalities will often detect patients in early stages of disease. A more specific diagnosis made through consultation with a pulmonologist may lead to anti-inflammatory therapy, which can reverse abnormal lung function in subsets of this population. These patients commonly have desquamative interstitial pneumonia (DIP), bronchiolitis obliterans with orga- nizing pneumonia (BOOP), or nonspecific interstitial pneumonitis (NSIP) (11, 12). Most patients with idiopathic usual interstitial pneumonitis (UIP) have a poor prognosis, with essentially no response to immunosuppressive drugs (12). These patients should be enrolled in controlled clinical trials designed to study the basic inflammatory processes involved.
Spirometry, like blood pressure measurement, is a measurement of 2 values which represents a simple expression of a complex process. Abnormal spirometry predicts an increased mortality from all causes includ- ing heart attack, lung cancer, stroke, and COPD. Primary care physicians and specialists should perform spirometry testing in all smokers over age 45 and in any person with shortness of breath, chronic cough, excess mucus production, or wheeze.
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Dr Petty is with the National Lung Health Education Program, Denver, Colo. Corresponding author: Thomas L. Petty, MD, National Lung Health Education Program, HealthOne Center, 1850 High St, Denver, CO 80218 (e-mail: firstname.lastname@example.org).