Relationship between supramaximal flows and flow-limiting mechanisms

Abstract

The aim of this study was to evaluate the relationship between supramaximal flows (SF) and indices of airway mechanics. We studied 26 asymptomatic young subjects (13 smokers, mean ± SD 15.9 ± 6.6 pack-years). Subjects performed maximal expiratory flow-volume (MEFV) curves, according to the American Thoracic Society (ATS) criteria, on a rolling-seal spirometer and then repeated them through a specially devised valve, which occluded the mouthpiece either 3 or 6 times · s-1 with a ratio of open:closed time of 3:1. Envelope MEFV curves for 3 and 6 Hz occlusions were constructed passing through the SF obtained after each occlusion, and the increment in flow at 50% of vital capacity was measured with respect to the basal curve (ΔV'(max50)). We found that the ΔV'(max50) at 3 and 6 Hz correlated to the baseline forced midexpiratory flow (FEF25-75) in % of predicted value (r = -0.73 and r = -0.55, respectively). Our results suggest that inhomogeneities within the lung are an important mechanism in the occurrence of supramaximal flow in normal subjects.