Discriminating Measures and Normal Values for Expiratory Obstruction
Since Hutchinson introduced spirometry in 1846, a multitude of measurements, including volumes, flows, time constants, and ratios, have evolved to assess normalcy and disease. Five decades ago Lueallan and Fowler2 added maximal midexpiratory flow, later labeled as forced expiratory flow, midexpiratory phase (FEF25-75), to assess expiratory airway obstruction. In 1967, Macklem and Mead divided airway resistance between central and peripheral components. Following the morphologic characterization of small airways disease, many publications reported reference values not only for FVC, FEV1, and FEV1/VC, but also for FEF25-75.
In 1972, a publication entitled, “A Reduction in Maximum Mid-Expiratory Flow Rate: A Spirometric Manifestation of Small-Airways Disease,” without giving FEV1/FVC data, described the conditions of 53 symptomatic smokers as abnormal because their FEF25 75 values were < 80% of the mean predicted values. The common practice of reporting spiromet-ric values as a percent of predicted values with highlighting of values < 80% of predicted added confusion. Despite evidence of high variability of FEF25 75 values and expert opinion recommending the use of statistically derived 95% confidence limits for the lower limit of normal (LLN)” small airways disease continued to be diagnosed if FEF25 75 values were < 75 to 80% of mean percentage of predicted values, and FEV1 or FEV1/FVC were > 75 to 80% of the mean percentage of predicted values. natural asthma inhaler
In 1981 and 1985, respectively, Crapo et al and Miller et al published reference equations for white adults that included forced expiratory volume in 3 s (FEV3) and FEV3/FVC ratio values. Despite this, a recent search of PubMed found 695 citations for “FEF25-75,” far exceeding the 22 citations found for “FEV3/FVC.” In the Third National Health and Nutrition Examination Survey (NHANES III), spirometric values for FVC, peak flow, forced expiratory volume in 0.5 s, FEV1, FEV3, forced expiratory volume in 6 s (FEV6), FEF25-75, flow after 75% of the FVC has been exhaled (FEF75), and duration of FVC in > 20,000 US residents were collected, using American Thoracic Society (ATS) standards. Using this database, Hankinson et al analyzed and reported the mean and LLN formulas for FEV1, FVC, FEV1/ FVC, FEV1/FEV6, and FEF25-75, but not for FEV3 or FEV3/FVC in healthy never-smoking white, African-American, and Mexican-American male and female patients from childhood through age 80 years.