Clinical question
Is there a need for a new spirometry interpretation algorithm that contains decision-making criteria consistent with current guidelines on asthma1 and chronic obstructive pulmonary disease (COPD)2 diagnosis?
Using spirometry to distinguish between COPD and asthma
Office spirometry provides valuable information about the relationship between flow and volume in relation to lung function and can be useful for diagnosing common conditions such as asthma and COPD.1,2 Mechanical abnormalities of the respiratory system can be classified as either obstructive (flow-related) or restrictive (volume-related) ventilatory defects; obstructive defects are much more common in clinical practice. The relationship between flow and volume is described well by the ratio of the forced expiratory volume in 1 second (FEV1) to the forced vital capacity (FVC). These measurements can be easily obtained with a simple office spirometer during a forced expiratory maneuver. The ratio of FEV1 to FVC can be useful to identify obstructive, restrictive, and combined (obstructive-restrictive) defects, but it is important to recognize that total lung capacity, a more sophisticated measurement (and not the FVC), is the best measurement to confirm a diagnosis of pulmonary restriction.3 Traditionally an FEV1-FVC ratio below 0.70 has been used to define a pure obstructive defect if the FVC is within normal limits. Measurement of postbronchodilator FEV1-FVC ratio is necessary to differentiate between an acute or persistent obstructive defect and to evaluate whether reductions in the FVC are the result of hyperinflation (air trapping, where the FVC improves after bronchodilator challenge) or are related to problems in pulmonary compliance; in the latter case, the FEV1-FVC ratio is often normal or elevated and spirometric indices often change very little after bronchodilator challenge.3
Current guidelines2 indicate that a spirometric diagnosis of COPD must include an FEV1-FVC ratio that is reduced consistently below 0.70 after bronchodilation. An interpretation algorithm currently endorsed by the Ontario Thoracic Society4 lacks these diagnostic criteria in its decision tree; consequently, a spirometric diagnosis of COPD cannot be confirmed. Criteria for a spirometric diagnosis of asthma include an improvement in FEV1 of 12% (preferably 15%) and 200 mL after bronchodilator challenge.1 Although the FEV1-FVC ratio might be normal in many patients with asthma (on the basis of a normal FVC value), this does not exclude the possibility that FEV1 will improve substantially with bronchodilator challenge (see case 1 below). In a currently available algorithm,4 the finding of a normal FEV1-FVC result does not prompt further testing after bronchodilator challenge. Differences between asthma and COPD and how the FEV1-FVC ratio can change after bronchodilator challenge are heavily influenced by different pathophysiologic mechanisms; in asthma the FEV1-FVC ratio results can be normal or can return to normal after bronchodilator challenge at any given time. In COPD, FEV1 is influenced by permanent architectural changes, such as loss of alveolar attachments, that predispose airways to collapse more readily.2 Further, reductions in lung elastic recoil often reduce FEV1 in COPD.2 These latter changes in COPD result in a persistent reduction in FEV1. By contrast, asthmatic airway obstruction is determined to a great extent by factors related to bronchospasm, airway inflammation, and mucous plugs; these changes can improve either spontaneously or in response to therapy.1 It is common for the FEV1 value to be normal in many patients with asthma, particularly when the disease is well controlled.
Spirometric overlap between asthma and COPD can cause confusion
Traditionally, COPD has been described as a disease characterized by fixed airflow obstruction because in many patients FEV1 values improve little after bronchodilator challenge. Current guidelines2 on diagnosis and management describe COPD as a condition that is partially reversible because some patients exhibit substantial improvements in FEV1 (despite an FEV1-FVC ratio that remains below 0.70) that compare in magnitude to what is observed in some asthma patients. In fact, Tashkin et al5 have shown that about 54% of a large COPD cohort (N = 5756) exhibited an improvement in FEV1 values > 12% and 200 mL, while about 65% of patients had FEV1 increases > 15%. This substantial overlap in FEV1 reversibility between asthma and COPD underscores an important limitation of using this measurement to distinguish between asthma and COPD; instead we need to formulate a clinical diagnosis based on physical examination, history, and spirometric data. An algorithm currently promoted in primary care4 is limited by its focus on using changes in FEV1 to distinguish asthma from COPD.
New spirometry interpretation algorithm
Given the limitations of the currently available algorithm,4 members of the Primary Care Respiratory Alliance of Canada have proposed a new algorithm (Figure 1) where spirometric diagnostic criteria for both asthma and COPD are included and consistent with current guidelines.1,2 The new algorithm focuses on the FEV1-FVC ratio before and after bronchodilator challenge as a means of identifying acute or persistent airflow obstruction. This approach helps to exclude a diagnosis of COPD quickly if the FEV1-FVC ratio returns to normal after bronchodilator challenge. The new algorithm also includes bronchodilator challenge in patients with a normal baseline FEV1-FVC ratio, recognizing the variable nature of asthma and the possibility that a normal FEV1 result could improve greatly in response to bronchodilator challenge. The new algorithm also addresses the subject of reversibility as it is described in both asthma and COPD guidelines.1,2 Current COPD guidelines underscore that airflow limitation is only partially reversible because the FEV1-FVC ratio does not return to normal despite improvements in airway calibre (airflow) in many COPD patients after bronchodilator challenge; improvements in FEV1 can also be observed in asthma patients. The new algorithm does not focus on changes in FEV1 after bronchodilator challenge as a means of separating asthma from COPD because of the substantial spirometric overlap between these 2 conditions. Because a clinical diagnosis of asthma and COPD cannot be confirmed with spirometric data alone, Table 16 highlights historical and physical examination data that can help differentiate asthma from COPD. This table is included because one of the decision nodes in the new algorithm leads the reader to consider asthma versus COPD. It is important to consider conditions other than asthma and COPD in patients who present with respiratory complaints, including wheezing. It should be noted that this algorithm can be used for both adults and children, although some school-aged children might not meet international criteria for spirometry.7
Spirometry interpretation algorithm from the Primary Care Respiratory Alliance of Canada
COPD—chronic obstructive pulmonary disease, FEV1—maximal volume of air exhaled after a maximal inhalation in the first second of a forced exhalation, FVC—maximal volume of air exhaled after inhalation during forced exhalation, LLN—lower limit of normal.
*FVC <80% predicted—perform full pulmonary function tests to rule out hyperinflation vs combined obstructive and restrictive defect.
†FVC ≥80% predicted.
‡FEV1 and FVC < 80% predicted.
§The % change is calculated as
; FEV1 might not improve after β2-agonist challenge.
||Lack of change in FEV1 is not diagnostic; referral for methacholine challenge recommended.
Differences between asthma and COPD
Application to clinical practice
Four brief spirometry cases, all meeting American Thoracic Society8 criteria for acceptability and reproducibility, highlight how the new algorithm could be used as a stand-alone document to interpret spirometric data.
Case 1
The prebronchodilator and postbronchodilator FEV1-FVC ratios are 0.79 and 0.82, respectively (Figure 2), while the FEV1 improves from 2.92 to 3.29 L after bronchodilation (increase of 370 mL and 13%). The new algorithm indicates that these data are consistent with asthma given the normal FEV1-FVC ratio and improvements in FEV1 after bronchodilation. The patient in this case was a 45-year-old man who had never been a smoker. He had intermittent bouts of shortness of breath and chest tightness and normal results from cardiovascular workup. His response to asthma therapy was favourable.
Spirometric data for case examples
FEV1—maximal volume of air exhaled after a maximal inhalation in the first second of a forced exhalation, FVC—maximal volume of air exhaled after inhalation during forced exhalation, % Pred—percent of predicted normal value, Pre—prebronchodilator value, Post—postbronchodilator value.
*
.
Case 2
The prebronchodilator and postbronchodilator FEV1-FVC ratios are 0.48 and 0.50, respectively (Figure 2). The prebronchodilator and postbronchodilator FEV1 results are 1.52 and 1.88 L, respectively (increase of 360 mL and 24%). The new algorithm recognizes the reduction in FEV1-FVC ratio before bronchodilator use, but the postbronchodilator FEV1-FVC ratio is evaluated to determine whether there is a combined defect of obstruction and restriction or hyperinflation. Given that the FVC increased to more than 80% of the predicted value with bronchodilation, it becomes clear that hyperinflation contributed to the reduced prebronchodilator FVC measurement. Because the postbronchodilator FEV1-FVC ratio remains below 0.70 and the FEV1 reversibility criterion is met,1 the clinician is led to differentiate asthma from COPD using historical data (Table 1).6 The patient in this case is a 73-year-old man with a 40-pack-year smoking history, no allergies to environmental factors, and a history of progressive shortness of breath over the past 10 years. The medical history and family history were otherwise unremarkable for asthma risk factors. The historical and spirometric data in this case are consistent with a clinical diagnosis of COPD.
Case 3
The prebronchodilator and postbronchodilator FEV1-FVC ratios are 0.47 and 0.50, respectively (Figure 2). The prebronchodilator and postbronchodilator FEV1 values are 1.65 and 1.94 L, respectively (increase of 290 mL and 18%). The new algorithm recognizes the reduced FEV1-FVC ratio and the reversibility in FEV1 after bronchodilation and guides the clinician to differentiate asthma from COPD on the basis of historical factors as well (Table 1).6 The patient in this case is a 36-year-old woman who has never been a smoker. She has numerous environmental allergies and has severe asthma that is well controlled on maintenance therapy. Cases 2 and 3 highlight the spirometric overlap between asthma and COPD and the limitations of using FEV1 reversibility to help distinguish asthma from COPD.
Case 4
The prebronchodilator and postbronchodilator FEV1-FVC ratios are 0.64 and 0.78, respectively (Figure 2). The prebronchodilator and postbronchodilator FEV1 values are 2.17 and 2.74 L, respectively (increase of 570 mL and 26%). The new algorithm quickly excludes a spirometric diagnosis of COPD on the basis of the normal postbronchodilator FEV1-FVC value, and the increase in FEV1 would be consistent with a spirometric diagnosis of asthma. This case underscores the importance of using the postbronchodilator FEV1-FVC ratio to exclude COPD. The patient in this case is a 19-year-old boy with a history of childhood asthma and β2-agonist use increasing over several months. In the new algorithm, the central focus on postbronchodilator FEV1-FVC ratio will allow the person interpreting spirometric data to quickly exclude a spirometric diagnosis of COPD in many patients if the ratio returns to normal. In such patients, improvement in FEV1 (increase of 12% and 200 mL)1 is used to establish a spirometric diagnosis of asthma. This approach minimizes the risk of disease misclassification. Because spirometry can be used only to identify and differentiate between obstructive and restrictive ventilatory defects, historical and physical examination data are essential for establishing a clinical diagnosis (Table 1).6 This process can be quite rewarding for clinicians who are comfortable with interpretation of spirometry data.
Acknowledgments
We thank Deborah D’Urzo, Devra D’Urzo, and Vasant Solanki for their valuable assistance in preparing this manuscript.
Notes
BOTTOM LINE
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An algorithm commonly promoted in primary care is limited by its focus on using changes in forced expiratory volume in 1 second (FEV1) to distinguish asthma from chronic obstructive pulmonary disease (COPD). The new algorithm consolidates current spirometric concepts that are consistent with both asthma and COPD guidelines.
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The new algorithm facilitates spirometric diagnosis of COPD by focusing on postbronchodilator FEV1–forced vital capacity ratios, and does not use changes in FEV1 after bronchodilation to separate asthma from COPD.
POINTS SAILLANTS
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Un algorithme communément recommandé en soins de première ligne est limité parce qu’il est axé sur l’utilisation des changements dans le volume expiratoire maximal en 1 seconde (VEMS) pour distinguer l’asthme de la maladie pulmonaire obstructive chronique (MPOC). Le nouvel algorithme intègre les concepts spirométriques actuels qui concordent avec les guides de pratique concernant l’asthme ainsi que les MPOC.
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Le nouvel algorithme facilite le diagnostic spirométrique de la MPOC en ciblant les ratios VEMS par rapport à la capacité vitale forcée après usage d’un bronchodilatateur et n’utilise pas les changements dans le VEMS après bronchodilatation pour distinguer l’asthme de la MPOC.
Footnotes
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This article has been peer reviewed.
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Cet article a fait l’objet d’une révision par des pairs.
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Competing interests
None declared
- Copyright© the College of Family Physicians of Canada
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