A Normal FEV1/VC Ratio Does Not Exclude Airway Obstruction

Abstract

Background: A decreased forced expiratory volume in 1 s/vital capacity (FEV₁/VC) ratio is the hallmark of the definition of airway obstruction. We recently suggested that a lung function pattern, we called small airways syndrome (SAOS), has a normal FEV₁/VC and total lung capacity (TLC) and reflects obstruction of small airways. Objectives: To substantiate our hypothesis we measured and compared lung function tests including maximal expiratory flow rates (MEFR), sensitive indicators of airway obstruction, in SAOS subjects and in matched controls. Methods: We selected 12 subjects with the pattern of SAOS, but without chronic lung or heart disease (average age: 40.7 ± 7.8 years) and 36 age-matched subjects with normal lung function (42.8 ± 6.3 years). We measured static and dynamic lung volumes, MEFR and lung diffusing capacity (DL_CO). Results: SAOS subjects were heavier smokers (p < 0.05) and body mass index was less than in control subjects (p < 0.01). Both FEV₁/VC ratio and TLC were comparable in the two groups. However, FEV₁, VC, DL_CO, and MEFR were lower and residual volume (RV) and RV/TLC ratio were higher (p < 0.05) in the SAOS group than in the control one. Furthermore, the MEFR curve of the SAOS group was displaced to the left without any change in slope, suggesting premature airway closure. Conclusion: Our results suggest that a normal FEV₁/VC ratio does not exclude airway obstruction. A decrease of FEV₁, provided TLC is normal, reflects small airway obstruction.