Improved spirometric detection of small airway narrowing: concavity in the expiratory flow–volume curve in people aged over 40 years
2017
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 postbronchodilator
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%M , 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.
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