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 [FEF25%-75%]; r=-0.70 to -0.79). The additional number of individuals detected as abnormal using the concavity indices was substantial, especially compared with FEF25%-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.