New spirometry interpretation algorithm

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

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Figure 1.

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.

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.

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Figure 2.

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.

* .