The utility of spirometry in measuring lung function. Spirometry is the most reproducible and objective measurement of airflow limitation,10 and it should precede any treatment decisions. This technique—in which the patient performs maximal inhalation followed by forced exhalation—measures airflow over time and determines the lung volume exhaled at any time point. Because this respiratory exercise is patient dependent, a well-trained technician is needed to ensure reproducibility and reliability of results based on technical standards.
Spirometry measures forced vital capacity (FVC) and forced expiratory volume in one second (FEV1), from which the FEV1/FVC ratio is calculated. FVC is the total amount of air from total lung volume that can be exhaled in one breath. FEV1 is the total amount of air exhaled in the first second after initiation of exhalation. Thus, the FEV1/FVC ratio is the percentage of the total amount of air in a single breath that is exhaled in the first second. On average, an individual with normal lungs can exhale approximately 80% of their FVC in the first second, thereby resulting in a FEV1/FVC ratio of 80%.
Spirometry findings with COPD. A post-bronchodilator FEV1/FVC ratio of less than 70% confirms airflow obstruction and is consistent with COPD according to GOLD criteria.10 Post-bronchodilator spirometry is performed after the patient has received a specified dose of an inhaled bronchodilator per lab protocols. In patients with COPD, the FEV1/FVC ratio is persistently low even after administration of a bronchodilator.
Another means of using spirometry to diagnose COPD is referring to age-dependent cutoff values below the lower fifth percentile of the FEV1/FVC ratio (ie, lower limit of normal [LLN]), which differs from the GOLD strategy but is consistent with the American Thoracic Society/European Respiratory Society guidelines.35 Because the FEV1/FVC ratio declines with age, older adults may have a normal post-bronchodilator ratio less than 70%. Admittedly, applying GOLD criteria to older adults could result in overdiagnosis, while using the LLN could lead to underdiagnosis. Although there is no consensus on which method to use, the best approach may be the one that most strongly correlates with pretest probability of disease. In a large Canadian study, the approach that most strongly predicted poor patient outcomes was using a FEV1/FVC based on fixed (70%) and/or LLN criteria, and a low FEV134
Spirometry findings with asthma. According to the American Thoracic Society, a post-bronchodilator response is defined as an increase in FEV1 (or FVC) of 12% if that volume is also ≥200 mL. In patients with suspected asthma, an increase in FEV1 ≥12% and 200 mL is consistent with variable airflow limitation2 and supports the diagnosis. Of note, lung function in patients with asthma may be normal when patients are not symptomatic or when they are receiving therapy. Spirometry is therefore ideally performed before initiating therapy and when maintenance therapy is being considered due to symptoms. If therapy is clinically indicated, a short-acting bronchodilator may be prescribed alone and then held 6 to 8 hours before conducting spirometry. If a trial of a maintenance medication is prescribed before spirometry, consider de-escalation of therapy once the patient is more stable and then perform spirometry to confirm the presence of airflow variability consistent with asthma. (In COPD, there can be a positive bronchodilator response; however, the post-bronchodilator FEV1/FVC ratio remains low.)
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