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Dr. Ren is Associate Professor of Pediatrics, and Chief, Division of Pediatric Pulmonology and Allergy, University of Rochester, Rochester, NY.
Correspondence to: Clement L. Ren, MD, FCCP, University of Rochester Medical Center, 601 Elmwood Ave, Box 667, Rochester, NY 14642
Pulmonologists routinely use pulmonary function tests (PFTs) to help guide their care of patients with respiratory disease. Spirometry is the most commonly used PFT because it is simple and reproducible. The introduction of computerized spirometers has led to PFT reports containing multiple parameters. Because small airway obstruction is an early feature of many obstructive lung diseases, such as asthma, cystic fibrosis, and bronchiolitis obliterans, it is desirable to identify parameters that reflect flows in this area of the respiratory system. In this regard, both the forced expiratory flow at 50% of vital capacity (FEF50) and forced expiratory flow at 25 to 75% of vital capacity (FEF25–75) have been considered to more closely reflect small airway flow.1 2 How should a clinician decide which of these parameters is most helpful in assessing lung function? In this issue of CHEST (see page 731), Bar-Yishay et al compare FEF50 and FEF25–75 to provide some answers to this question by reviewing the spirograms of 1,350 children followed up at their center. They found that the two measures correlated very well, and that the FEF50/FEF25–75 ratio remained constant even in the setting of severe obstruction. To decrease the number of unnecessary parameters in spirometry reports, they recommend reporting the FEF50 only, because it is directly measured rather than calculated using an algorithm and shows less variability.
Spirometric data can only be obtained from subjects who can comply with forced expiratory maneuvers. Although some centers have successfully obtained spirograms from young children,3 it is still difficult for this patient group to generate adequate studies. For these patients, other techniques of lung function measurement may be required to assess the presence of airflow obstruction. Forced oscillometry has been studied in young children and correlates well with spirometry.4 However, further studies are needed to determine its clinical utility.
Although the findings of Bar-Yishay et al are helpful in reducing the number of unnecessary parameters on a spirometry report, they still do not answer the question of what is the best way to measure lung function. Studies such as spirometry only measure selected mechanical properties of the respiratory system. In the case of a disease such as asthma, alterations in physiology are a result of underlying airway inflammation.5 Measurements of airflow correlate only very roughly with the presence of airway inflammation and the risk of asthma exacerbation. Daily measurement of peak expiratory flow rates has not been shown to consistently improve asthma control.6 7 8 9 Changes in the FEV1 are directly related to the risk of a subsequent asthma attack.10 However, even in children with normal FEV1, up to 25% may experience an asthma attack in the subsequent 12 months,10 indicating that this measure still fails to reflect underlying airway inflammation in a significant number of patients. There have been some studies indicating that FEF25–75 may be a more sensitive marker of airflow obstruction,11 12 but whether this results in improved asthma control remains to be determined. It may be that noninvasive measures of airway inflammation will be the best way to monitor asthma control.13
Despite the limitations discussed above, spirometry plays a key role in the assessment and management of patients with respiratory disease. Bar-Yishay et al have made a significant contribution to minimizing the number of unnecessary parameters on spirometry reports and making use of this study more efficient. Further work will be needed before we can answer the question of what is the best way to measure lung function.
References
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