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

David P Johns,1 Aruneema Das,1 Brett G Toelle,2,3 Michael J Abramson,4 Guy B Marks,2,5 Richard Wood-Baker,1 E Haydn Walters1,6 1Faculty of Health, NHMRC Centre of Research Excellence for Chronic Respiratory Disease, School of Medicine, University of Tasmania, Hobart, Tasmania, 2Woolcock Emphysema Centre, Woolcock Institute of Medical Research, University of Sydney, 3Sydney Local Health District, Sydney, New South Wales, 4Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, Victoria, 5South Western Sydney Clinical School, University of New South Wales, Sydney, New South Wales, 6Allergy and Lung Health Unit, Centre for Epidemiology and Biostatistics, School of Population and Global Health, University of Melbourne, Melbourne, Victoria, Australia 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. Keywords: early airway disease, airflow obstruction, COP