The FEV3/FVC ratio is not discussed in the American Thoracic Society/European Respiratory Society (ATS/ERS) guidelines for lung function interpretation in spite of narrow confidence limits of normal and its association with smoking. We sought to determine whether a reduction in only the FEV3/FVC ratio was associated with physiologic changes compared with subjects with normal FEV1/FVC and FEV3/FVC ratios.
Lung volumes and diffusion were studied in individuals with concomitant spirometry. Patients with restriction on total lung capacity (TLC) were excluded, as were repeat tests on the same patient. A total of 13,302 subjects were divided into three groups: (1) normal FEV1/FVC and FEV3/FVC (n = 7,937); (2) only a reduced FEV3/FVC (n = 840); and (3) reduced FEV1/FVC (n = 4,525).
Subjects with only a reduced FEV3/FVC compared with those with normal FEV1/FVC and FEV3/FVC ratios had higher mean % predicted TLC (99.1% vs 97.1%, P < .001), residual volume (RV) (109.4% vs 102.3%, P < .001), and RV/TLC ratio (110.1% vs 105.4%, P < .001). They had lower mean % predicted FEV1 (82.6% vs 90.2%, P < .001), inspiratory capacity (94.5% vs 98.2%, P < .001), and diffusing capacity of lung for carbon monoxide (Dlco) (78.3% vs 81.9%, P < .001). Their mean BMI was lower (30.8 vs 31.5, P < .005), they were older (61.2 vs 57.2, P < .001), and more likely male (52.0% vs 40.4%, P < .001), with no racial differences. Comparing this group to those with a reduced FEV1/FVC, similar but greater differences were noted in all of the previous measurements, though mean age and sex were not significantly different.
The FEV3/FVC ratio should be routinely reported on spirometry. An isolated reduction may indicate an early injury pattern of hyperinflation, air trapping, and loss of Dlco.