Mechanisms of maximal expiratory flow limitation in clinically unsuspected emphysema and obstruction of the peripheral airways.
We studied the determinants of maximal expiratory flow in 7 patients in whom surgical lung specimens showed evidence of mild emphysema or obstruction of the peripheral airways. All the patients showed frequency dependence of dynamic lung compliance and low maximal expiratory flow. Lung elastic recoil pressures were reduced at all lung volumes in 4 of 5 patients with emphysema but were normal in 2 patients with obstruction of the peripheral airways. Maximal flow-static recoil curves showed a high intercept in the pressure axis when the slope between 50 per cent and 30 per cent of vital capacity was extrapolated to zero flow (transmural pressure in the flow-limiting segment) in all patients. The slope of these curve represents the conductance of the segment that extends from the alveoli to the flow-limiting segment and includes the latter. Conductance of this segment was higher than normal in 4 patients with emphysema and borderline low in one, and low in one patient with obstruction of the peripheral airways and borderline low in the other. Increased transmural pressure in the flow-limiting segment was interpreted as being due to peripheral location of collapsible flow-limiting segment and, in part, limiting maximal expiratory flow. The results also suggested that flow limitation in emphysema may occur without physiologic evidence of increased resistance of peripheral airways when, in fact, bronchiolitis is present on histologic examination.