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 concavityin the descending limb of the maximum expiratory flow–volume curve enhanced spirometricdetection 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 peripheralconcavity indices were developed from forced expired flows at 50% and 75% of the forced vitalcapacity, respectively, using an ideal line joining peak flow to zero flow.Results: From the 268 subjects classified as normal never smokers, mean values for postbronchodilatorcentral concavity were 18.6% in males and 9.1% in females and those forperipheral concavity were 50.5% in males and 52.4% in females. There were moderately strongcorrelations 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 theFVC [FEF25%–75%]; r=-0.70 to -0.79). The additional number of individuals detected as abnormalusing the concavity indices was substantial, especially compared with FEF25%–75%M, where it wasapproximately doubled. Concavity was more specific for symptoms.Conclusion: The inclusion of these concavity measures in the routine reports of spirometrywould add information on small airway obstruction at no extra cost, time, or effort