Small airway disease. An assessment of the tests of peripheral airway function.

Abstract We measured specific conductance (SGa), 1 second forced expiratory volumes (FEV 1 ), maximum mid-expiratory flow rates (MMF) lung volumes, closing volumes (CV), static (Cst) and dynamic compliance (Cdyn), and resting gas exchange in 53 smokers in order to determine the relative yields and the interrelationships of the various tests for peripheral airway dysfunction. Small airway disease was considered to be present if values for Cst, FEV 1 and SGa were normal at the same time that values for CV, MMF, Cdyn or residual volume (RV) were abnormal. We found that Cdyn was abnormal more often (100 per cent) than either MMF (72 per cent), CV (55 per cent) or RV (47 per cent). Abnormalities of RV and CV were highly correlated. When we compared the over-all pulmonary function of the patients with depressed MMF with those with high CV, we found that the mechanism producing the former also affected SGa, FEV 1 , Cst and perhaps RV, but not CV. Conversely, the mechanism that caused an increase in CV tended to influence RV, gas exchange and the lower part of the static pressure volume curve, but not the other variables. To account for these differences we postulate that the combination of frequency dependent Cdyn with a low MMF or a high CV may each represent the end results of different pathologic processes within the lung or, alternatively, the effect of a similar process working at different sites.