Expiratory Flow Limitation

Expiratory flow limitation occurs when flow ceases to increase with increasing expiratory effort. The equal pressure point concept has been largely successful in providing intuitive understanding of the phenomenon, wherein maximal flows are determined by lung recoil and resistance upstream of the site where bronchial transmural pressure is zero (the EPP). Subsequent work on the fluid dynamical foundations led to the wave-speed theory of flow limitation, where flow is limited at a site when the local gas velocity is equal to speed of propagation of pressure waves. Each is a local theory; full predictions require knowledge of both density-dependent Bernoulli pressure drops and viscosity-dependent pressure losses due to dissipation. The former is dominant at mid to high lung volumes, whereas the latter is more important at low lung volumes as the flow-limiting site moves peripherally. The observation of relative effort independence of the maximal flow versus volume curves is important clinically insofar as such maneuvers, when carefully performed, offer a unique window into the mechanics of the lung itself, with little confounding effects. In particular, the important contributions of lung recoil and airways resistance can often be assessed, with implications and applications to diagnosis and management of pulmonary disease. (C) 2011 American Physiological Society. Compr Physiol 1: 1861-1882, 2011.