Pathophysiology of evolution of small airways disease to overt COPD

The slope of phase III (single breath nitrogen test), an index of ventilation inhomogeneity, has been used for early detection of COPD. Tidal airway closure (cyclic opening and closure of the peripheral airways during tidal breathing; AC(T)) and expiratory flow limitation (attainment of maximal expiratory flow during tidal expiration; EFL(T)) cause small airways disease (SAD). The relationships of these indices with COPD severity may reflect the progress from SAD to overt COPD.

In this cross-sectional study we have assessed for the first time the phase III slope, AC(T) and EFL(T) in 10 smokers with normal spirometry (group O) and 40 COPD patients with GOLD scores from I to IV.

In most group O smokers the phase III slope was increased, and further increased with GOLD severity (up to 800%pred in GOLD IV). A close correlation was found of slope with GOLD (r = 0.77). AC(T) was absent in smokers with normal spirometry and in most patients with mild COPD. EFL(T) first appeared in GOLD II patients and its prevalence progressively increased in GOLD III and IV patients.

Most group O smokers exhibit increased phase III. With overt COPD there is a progressive increase in phase III and reduction of FEV(1)/FVC ratio from GOLD I to IV. A reduction of FEV(1) occurs from GOLD stage II. As the disease progresses from moderate to severe, there is an increasing presence of AC(T). Tidal EFL, with dynamic hyperinflation and severe dyspnea is present only in GOLD III and IV.