Discriminating Measures and Normal Values for Expiratory Obstruction: Discussion
Two prior studies have given reference values for FEV3/FVC. Each study used 200 to 300 nonsmoking white adults of northern European ethnicity. Over a broad age and height range, our mean FEV3/FVC values for white never-smokers are, on average, approximately 1.7% and 1.0% lower than those for men and women reported in the study by Crapo et al and < 1% lower than those reported in the study by Miller et al.> These small differences may relate to resident altitudes, socioeconomic factors, or other unknown factors. Although Miller et al did not emphasize the following information, their data from 359 current smokers showed that FEV3/FVC abnormalities exceeded those of FEV1/ FVC, FEF50, FEF25-75, FEV1, FEV3, FEF75, and flow between 75% and 85% of the FVC (FEF75-85). in detail
In a consensus statement from the National Lung Health Education Program, Ferguson et al reported that 9.6% of the adult (ie, 18 to 89 years of age) NHANES III smokers had an obstructive pattern, which they defined as FEV1/FEV6 and FEV1 values below the LLN. In our series of 3,570 current smokers, aged 20 to 80 years, a considerably larger percentage (Table 3) showed airflow limitation as manifested by significant decreases in FEV1/FVC and FEV3/FVC.
In the past, comparatively little attention has been paid to the FEV3/FVC or to the fraction of the FVC that had not been expired during the first 3 s of the FVC (ie, 1 — FEV3/FVC). Lower flow rates with aging or disease may be due to both intrinsic airway changes and the loss of lung elastic recoil, promoting increased compression of the airways with forced expiration. In contrast to FEV1/FVC, which reflects the reduction in short-time-constant lung units, an increase in 1 — FEV3/FVC assesses the increase in long-time-constant lung units and therefore should be sensitive in detecting developing expiratory flow limitation. With aging and injury, lung units with low elastic recoil and increased airway resistance may proportionally increase. These changes will affect expiratory flow after 3 s (eg, the 1 — FEV3/FVC measurement, which increases proportionately more than the decrease in FEV1/FVC (Fig 3). The very low variability in FEV3/FVC in healthy subjects makes for small deviations from the mean predicted values (Fig 1-3).