Evaluating the shape of the expiratory flow volume curve in asymptomatic ex-smokers

Spirometry is central to measuring pulmonary function and diagnosing respiratory disease. From spirometry, a flow-volume curve is generated and evaluating the shape of this curve may provide additional insight into pulmonary dysfunction compared to spirometry alone. We sought to determine if the shape of the flow-volume curve could detect signs of abnormal lung emptying in asymptomatic ex-smokers compared to never-smokers. A retrospective analysis of the shape of the flow-volume curves generated from routine spirometry from 61 ex-smokers (46% female) with normal spirometry and no diagnosis of respiratory disease and 42 never-smokers (58% female) was performed. The shape of the flow-volume curve was quantified via the slope ratio method. A slope ratio of 1 would indicate a linear decrease in flow with decreasing lung volume, whereas a slope ratio > 1 indicates a scooped pattern to the flow-volume curve, as seen in obstructive lung disease. Both ex-smokers and never-smokers had normal pulmonary function (Forced Expiratory Volume in 1 s (FEV₁): 97 ± 20 vs. 97 ± 18%predicted; forced vital capacity (FVC): 99 ± 21 vs. 98 ± 18%predicted; and FEV₁/FVC ratio: 97 ± 05 vs. 99 ± 6%predicted, all p > 0.05). The average slope ratio was higher in ex-smokers compared to never-smokers (1.90 ± 0.93 vs. 1.50 ± 0.78, p < 0.001). These findings suggest that slope ratio analyses may supplement pulmonary function testing to identify subclinical patterns of abnormal lung emptying dynamics in ostensibly healthy ex-smokers. This pattern is distinct from normal healthy ageing and support the inclusion of slope ratio as a tool to supplement evaluation of pulmonary function.