Using simple, accurate spirometers can assist physicians in diagnosing patients in early stages of lung diseases so they can enter into treatment programs, which can alter the course and prognosis of their disorders.
The value of spirometry is still not appreciated by many physicians, allied health care workers, or life insurance companies. This is so ironic when one considers the fact that spirometric tests are fundamental to medicine for the diagnosis of common lung diseases and for monitoring responses to therapy, as well as for prognostic purposes.
The spirometer was invented and introduced into medicine in 1846 by John Hutchinson, a surgeon.1 Hutchinson coined the term vital capacity because he recognized that this simple measurement of the volume of air that can be exhaled from fully inflated lungs correlated with survival. When reduced, it predicted premature mortality.
This fact has been known for many years. The Framingham Study revealed that a reduced vital capacity was predictive of premature death from heart attack.2 Other population studies have confirmed this observation.3
Tiffeneau added the concept of time to spirometric measurements.4 This was the origin of the forced expiratory volume in 1 second (FEV1), popularized in the United States by Gaensler, another surgeon, in the 1950s.5 Thus, we have two basic spirometric tests: the vital capacity (volume test) and the FEV1 (flow test).
Other Spirometric Measurements
When flow transducers became available, well-meaning engineers and some zealous pulmonologists delighted in adding more indices to the expiratory flow volume curve. But none of these measurements have any special meaning above and beyond the FEV1 and forced vital capacity (FVC) and the ratio between them. The forced expiratory flow (FEF) during the middle portion of the expiratory flow curve, FEF25%-75%, became popular. It was presented as a test of small airways disease.6 But the FEF25%-75% does not indicate small airways disease any better than the FEV1 or FEV1/FVC. In addition, various flow points on the expiratory flow volume curve were also added. These additional parameters also have no special meaning. Accordingly, it is time to get the junk and other nonsense removed from spirometry. Simply stated, we must focus on two-parameter spirometry.
Normal Spirometric Values
Normal spirometric values are a function of age, height, sex, and race (due to variations in habitus). Weight is not a factor except in extremes of morbid obesity or malnutrition, where thoracic muscle function may be impaired. The Large Population Study, NHANES III, provides the best predictive normal values.7 Normal values are available from computer sources, nomograms, and slide rulers, and are programmed into the software of modern spirometers including simple office devices.
Determinants of Expiratory Airflow
Spirometry is a simple expression of a complex process. Expanded alveoli empty into small airways and small airways empty into large airways and the forced expiratory airflow is measured by a displacement device or a flow transducer.
|Figure 1. The equalization of forces between the inward retraction of the lungs and the outward recoil of the thorax.
|Figure 2. Alveolar attachments to small airways.
Expiratory airflow is a function of the elastic recoil of the inflated lungs and thorax, and the resistance of the conducting air passages. Figure 1 is a diagrammatic representation of the balance of forces between the inward retraction of the lungs and the outward recoil of the thorax. The interrelationship of the smallest conducting air passages and surrounding alveoli attachments is illustrated in Figure 2, page 25.
Expressing Expiratory Airflow
The expiratory volume over time curve is the easiest expression of the expiratory spirogram. This is because the FEV1, FVC, and expiratory time can be directly visualized from the curve (see Figures 3-6). In these figures, normal values are in parentheses (LLN refers to the lower limit of normal). Exactly the same information is available from the expansion of expiratory flow over the vital capacity (the flow volume curves [Figures 3-6]). But the FEV1 and expiratory time cannot be directly visualized. The computer does this and uses an arrow to point to the FEV1. Some experts believe that there are some quantitative advantages to being able to view the contours of the expiratory flow volume curve (progressive concavity associated with loss of elastic recoil). My own view is that this is very much like the Rorschach test of inkblot interpretation used in psychoanalysis.
|Figure 3. Normal flowvolume (A) and timevolume (B) expiratory curves.
|Figure 4. Mild airflow obstruction seen in flowvolume (A) and timevolume (B) curves.
|Figure 5. Moderate airflow obstruction seen in flowvolume (A) and timevolume (B) curves.
|Figure 6. Severe obstruction seen in flowvolume (A) and timevolume curves.
In any case, the primary care practitioner should be comfortable with either expression of expiratory airflow. Figure 3 is a normal expiratory spirogram. Figures 4-6 indicate progressive stages of airflow obstruction.
Use of a flow transducer also allows the reading and measurement of inspiratory airflow. In states of upper airway obstruction (vocal cord paralysis, tracheal stenosis, or tracheal tumors), the inspiratory airflow is truncated; however, the conditions that are characterized by upper airway obstruction are relatively rare and not of concern to primary care practitioners.
Spirometric interpretations should be easy. This is why the two basic parameters of FVC and FEV1 are used. The normal ratio between FEV1 and FVC is 70% to 75%. Since the FEV1 is an index of flow, a reduced FEV1 indicates a flow or obstructive abnormality. Since the FVC is an index of volume, a reduced FVC indicates a volume or restrictive ventilatory abnormality.
Thus, an obstructive defect is present when the FEV1/FVC ratio is <.70. A restrictive defect is usually present when the FEV1/FVC is >80%.
In the case of mixed obstructive and restrictive spirometry abnormalities, measurements of so-called lung volume or lung capacities are also useful. The vital capacity is the air that can be exhaled from fully inflated lungs and the residual volume (RV) is the air that remains (cannot be exhaled due to closure of the small conducting air passages). Thus, the total lung capacity (TLC) is the sum of the VC (also FVC) and the RV. In obstructive disease, the RV and TLC are usually increased. In restrictive disease, they are decreased.
One other important pulmonary function indicator is the gas transfer test, known as the diffusion test. This test involves the use of a tiny amount of an indicator gas, normally carbon monoxide (CO), that transverses the air blood interface and is measured in cubic centimeters of CO taken up per minute. The diffusion test is often reduced in emphysema due to loss of alveolar walls. In the scarring restrictive disease, the diffusion test is also reduced due to thickening of alveolar walls at the air-blood interface.
Assessment of Dyspnea
Spirometry is to dyspnea as the electrocardiogram (ECG) machine is to chest pain. It never ceases to amaze me how many physicians continue to use potent bronchodilators and even corticosteroids without any documentation of spirometric abnormalities or responses to therapy. Any patient with dyspnea should have spirometry as part of the evaluation for this cardinal symptom of both respiratory and cardiac disease. Why dont the 25,000 board-certified cardiologists in this country have and use spirometry in their offices? Pulmonologists and primary care physicians have ECG machines!
Spirometry is critical to evaluating the effectiveness of bronchoactive drugs, including bronchodilators (anticholinergics and beta agonists), theophyllines, and corticosteroids. Finding the maximum lung improvement in both obstructive and restrictive ventilatory diseases, which require different therapeutic strategies, is extremely important and frequently overlooked by primary care physicians. Monitoring responses to therapy in the idiopathic interstitial pneumonitis/fibrotic states where corticosteroids and other immunosuppressive drugs are employed is of critical importance. All lung transplantation patients are taught to monitor their spirometry at home for the early identification of rejection or incipient lung infections.
Spirometry is reimbursed by CPT codes. Basic spirometry is Code #94010 ($30). Interestingly, a respiratory flow volume loop, Code #94375, is reimbursed at $37it is exactly the same thing as spirometry. Already one state insurance program requires spirometric documentation for reimbursement of care for chronic obstructive pulmonary disease (COPD) and asthma and for the prescription of bronchodilators and corticosteroids.
I was recently contacted by a defense attorney in a Midwestern state who was trying to organize a defense for a board-certified pulmonologist who had used corticosteroids appropriately for advanced and uncontrolled lifelong asthma in a 41-year-old man. This patient had also smoked cigarettes and so the issue of COPD was also in the equation.
When the patient developed bilateral femoral head necrosis following an unrelated inflammatory renal disease, a lawsuit was brought on. The plaintiff argued that since no spirometric tests had ever been done, that the doctors care was substandard. Peak flow measurements had been done showing improvements in airflow in response to therapy, but peak flow is only a snapshot of flow and does not correlate well with FEV1 or FVC in states of chronic airflow obstruction. The sad fact of failure to do spirometry was indeed substandard, although the care received by the defendant was appropriate. Thus, even if physicians feel that they can get away with treating patients without spirometry, the use of spirometry may save them a trip to the courthouse.
Spirometry is as fundamental to medicine as the blood pressure cuff, ophthalmoscope, ECG machine, chest x-ray, clinical thermometer, weight scale, and tape measure. It is a sad fact that only approximately 30% to 40% of primary care physicians, both family practitioners and general internists, have and use spirometry regularly in their daily practice. The availability of simple, accurate spirometers to evaluate abnormalities in lung mechanics must change in the practice of medicine as we emerge into the new millennium.
The National Lung Health Education Program (NLHEP) is a new health care initiative aimed at the early identification and intervention in COPD and related disorders.8 The NLHEP stresses the importance of testing all smokers age 45 or older and anyone with chronic cough, expectoration, dyspnea on exertion, mucus hypersecretion, or wheeze at any age.9 Both the FEV1 and FEV6 (as a surrogate for FVC)10 are measured, then the results are compared with NHANES III normal values. Finding patients in early stages of diseases can be the first step in smoking cessation and can enter patients into treatment strategies that can alter the course and prognosis, not only of COPD, but also of other less common pulmonary disorders.
Thomas L. Petty, MD, is chairman of the National Lung Health Education Program and professor of medicine, University of Colorado Health Sciences Center, Denver.
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2. Kannel WB, Lew EA, Hubert HB, Castelli WP. The value of measuring vital capacity for prognostic purposes. Trans Assoc Life Insur Med Dir Am. 1980;64:66-83.
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6. Stanescu DC, Rodenstein DO, Hoeven C, Robert A. Sensitive tests are poor predictors of the decline in forced expiratory volume in one second in middle-aged smokers. Am Rev Respir Dis. 1987;135:585-590.
7. Hankinson JL, Odencrantz JR, Fedan KB. Spirometric reference values from a sample of the general US population. Am J Respir Crit Care Med. 1999;159:179-187.
8. Petty TL. Strategies in preserving lung health and preventing COPD and associated diseases. The National Lung Health Education Program (NLHEP). Chest. 1998;113:S123-S163.
9. Ferguson GT, Enright PL, Buist AS, Higgins MW. Office spirometry for lung health assessment in adults. A consensus statement from the National Lung Health Education Program. Chest. 2000;117:1146-1161.