Spirometry: A valuable but overlooked tool
Recent studies show that many patients are being treated for COPD without confirmatory spirometry. In-office testing is easy and available.
A recent study showed that only 32% of patients admitted to a hospital for chronic obstructive pulmonary disease (COPD) have ever had spirometry.1 Many of these patients had been treated for years of respiratory symptoms with a growing number of expensive drugs, particularly beta agonists and corticosteroid combinations. Some were receiving long-term home oxygen without any documentation of the diagnosis.
No clinician would treat hypertension or diabetes without the guidance of BP and blood sugar measurements. You wouldn’t consider giving powerful cholesterol-lowering drugs without measuring lipid levels. So why treat COPD patients without doing spirometry? Just as a sphygmomanometer and an ECG machine are fundamental to the practice of primary-care medicine, so, too, is a spirometer.
What does spirometry tell us?
Spirometry is the measure of airflow from fully inflated lungs, i.e., the amount of air in a breath deep enough to cause maximum inflation of the lungs. It represents the balance of elastic forces involved in breathing—an outward pulling on the lungs by the thorax and an inward suction created by the lung’s elasticity. Expiratory airflow is thus the result of muscular effort forcing all the air out of the lungs combined with elastic recoil (which originates in the alveoli) and the patency of small and large airways.
Explained in those terms, spirometry is a simple expression of a complex process, similar to BP, blood urea nitrogen/creatinine, lipid levels, and other determinations.
Only two of the commonly listed parameters of expiratory airflow are of clinical importance. The forced expiratory volume in one second (FEV1) is the flow index that indicates what is going on in asthma, COPD, and rare obstructive disorders of ventilation.
The whole breath is the forced vital capacity (FVC). Since normal lungs empty in less than six seconds, this duration of measurement is enough. In fact, the forced expiratory volume in six seconds (FEV6) is an excellent surrogate for the FVC, which may take 12-20 seconds to complete in advanced COPD.2 There is no point in exhausting patients to get the last 50-200 cc of air from inelastic lungs. The FVC is an index of restricted ventilatory processes that impair the filling of the lungs and thorax. A reduced FVC is a hallmark of congestive heart failure.
While it has been historically common to display many more measurements from the expiratory flow curve than the FEV1 and FVC, the others are not necessary in clinical practice and may be confusing. They should be removed from simple office spirometer printouts and, in my opinion, probably eliminated altogether from the more complex spirometers as well.
The spirometric records utilize two conventions, i.e., airflow over volume and airflow over time, which express expiratory airflow in different ways. Different-appearing curves are generated, but they provide the same information (flow/volume or flow/time). Most spirometers demonstrate both methods (Figure 1).
Interpretation of spirometric tests is easy. There are only four possibilities: normal or obstructive, restrictive, or mixed defects. Normal values represent a range for the individual’s age, sex, height, and race. Greater than 80% of predicted is a simple cutoff point for normal. An FEV1 <80% of predicted indicates the patient has either an obstructive disease (Table 1) or a restrictive disease (Table 2). The ratio of FEV1/FVC can be used to distinguish the two (Figure 2). Mixed values include reductions in both FEV1 and FVC.
Normally, the FEV1 is 70% of the FVC. When the ratio is low, an obstructive disease is suspected. When it is very high, a restrictive disease is likely. Further evaluation requires measurement of the total lung capacity and the residual volume.
Benefits of in-office spirometry
The National Lung Health Education Program (NLHEP) advises the early detection of COPD and related disorders,3 and there is evidence that office spirometry improves early diagnosis.4 The Global Initiative for COPD offers a severity classification based on the degree of airflow normality.5
Accurate diagnosis: The pulmonary literature is full of debate on screening and the overuse and underuse of spirometry.6-8 Doctors may end up with incorrect diagnoses if they use only the FEV1/FVC ratio without other considerations. Some pulmonologists argue that a low ratio indicates chronic airflow obstruction if it is not fully reversible after inhaling a bronchodilator.
This is an oversimplification, particularly in elderly persons, whose FEV1/FVC tends to drop slightly with age. It is preferable to compare your patient’s values with those predicted for his age, height, and sex. All of these comparisons are provided on the computer or visual printout of modern spirometers.
Appropriate therapy: Reversible obstructive airflow indicates asthma, for which the cornerstone of treatment is bronchodilators and, in many cases, inhaled corticosteroids alone or in combination. Not so with COPD, where no study shows an alteration of mild-to-moderate disease with those medications, often used in an expensive drug-dispensing device. Present studies of newer drugs may draw different conclusions regarding whether their use yields sustained improvement or delays loss of lung function.
Risk management: Whatever the researchers’ conclusions, it is clear that treatment for asthma and COPD is not identical, and an accurate diagnosis is essential to good patient care. I have testified in at least three malpractice cases in which a pulmonologist gave large doses of corticosteroids to a patient believed to have asthma, with no spirometry evidence of disease. The jury found negligence in the most recent of these cases (Shinley v Sosenko, Joliet, Ill., 2003). Better to avoid the courtroom by the appropriate use of spirometry.
Ease of use: Spirometry is simple and relatively inexpensive. I know of no spirometry service that is analogous to blood-drawing services where patients could be referred for respiratory testing, so it makes sense to add a spirometer to your office. The new machines are very easy to use, and they have built-in quality-control features. They display the results for FEV1 and FVC in terms of percent predicted and offer an interpretation.
Established reimbursement codes: Reimbursement is also established (Table 3), so you can generate some revenue from spirometry done in your own office. The technician who does ECGs can be taught to do spirometry.
Certification is offered by the American Association for Respiratory Care, but most spirometry technicians are not certified. Patients with restrictive disease, which represents a growing number of deaths per year, i.e., >55,000, should be referred to a pulmonologist for a definitive diagnosis of disease type and possible treatment. Today there is no established treatment of idiopathic pulmonary fibrosis. Hypersensitive reactions, sarcoidosis, and drug reactions in the lung have a much better prognosis.
Other patients who warrant referral include those who are not responding to therapy or who require oxygen, pulmonary rehabilitation, or lung surgery. With use of in-office spirometry, you can be confident that you have given your patient the best care possible before offering referral.
Dr. Petty is professor of medicine, University of Colorado Health Sciences Center, in Denver, and professor of medicine, Rush-Presbyterian-St. Luke’s Medical Center, in Chicago.
1. Han MK, Kim MG, Mardon R, et al. Spirometry utilization for COPD: how do we measure up? Chest. 2007;132:403-409.
2. Swanney MP, Jensen RL, Crichton DA, et al. FEV6 is an acceptable surrogate for FVC in the spirometric diagnosis of airway obstruction and restriction. Am J Respir Crit Care Med. 2000;162(3 Pt 1):917-919.
3. 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. Respir Care. 2000;45:513-530.
4. Buffels J, Degryse J, Heyrman J, Decramer M; DIDASCO Study. Office spirometry significantly improves early detection of COPD in general practice: the DIDASCO Study. Chest. 2004;125:1394-1399.
5. Buist AS, Anzueto A, Calverley P, et al; GOLD Scientific Committee. Global strategy for the diagnosis, management, and prevention of chronic obstructive pulmonary disease. Available at: www.goldcopd.comL1=2&L2=1&intId=989. Accessed March 5, 2008.
6. Enright P. Does screening for COPD by primary care physicians have the potential to cause more harm than good? Chest. 2006;129:833-835.
7. Enright P, Quanjer P. Spirometry for COPD is both underutilized and overutilized. Chest. 2007;132:368-370.
8. Petty TL. Harm from spirometry? Chest. 2006;130:1629-1630.