Improved spirometric detection of small airway narrowing: Concavity in the expiratory flow–volume curve in people aged over 40 years

Background and objective: We have explored whether assessing the degree of concavity in the descending limb of the maximum expiratory flow–volume curve enhanced spirometric detection of early small airway disease. Methods: We used spirometry records from 890 individuals aged ≥40 years (mean 59 years), recruited for the Burden of Obstructive Lung Disease Australia study. Central and peripheral concavity indices were developed from forced expired flows at 50% and 75% of the forced vital capacity, respectively, using an ideal line joining peak flow to zero flow. Results: From the 268 subjects classified as normal never smokers, mean values for post-bronchodilator central concavity were 18.6% in males and 9.1% in females and those for peripheral concavity were 50.5% in males and 52.4% in females. There were moderately strong correlations between concavity and forced expired ratio (forced expiratory volume in 1 second/forced vital capacity) and mid-flow rate (forced expiratory flow between 25% and 75% of the FVC [FEF_25%−75%]; r=−0.70 to −0.79). The additional number of individuals detected as abnormal using the concavity indices was substantial, especially compared with FEF_25%−75%, where it was approximately doubled. Concavity was more specific for symptoms. Conclusion: The inclusion of these concavity measures in the routine reports of spirometry would add information on small airway obstruction at no extra cost, time, or effort.