Flow limitation: an overview.

Abstract

Expiratory flow limitation (EFL) refers to a functional condition in which expiratory flow cannot increase and, hence, is maximal under the prevailing conditions. Many factors, alone or combined, may cause EFL. Among them, airway obstruction, expiratory flow rate and body posture are the most important. EFL normally occurs during the forced expiratory manoeuvre after peak expiratory flow. However, its presence during tidal breathing either throughout exercise or at rest, initially in the supine and then in the seated position, is an abnormal finding which reflects progressively greater mechanical impairment. EFL promotes dynamic pulmonary hyperinflation (DH) by prolonging the time required for the respiratory system to reach its relaxation volume during expiration. Moreover, in the presence of EFL, any increase in expiratory flow can be accomplished only by raising the end-expiratory lung volume allowing tidal breathing to occur at a higher absolute lung volume. This mechanism, however, besides being physically limited, leads to an increment in DH and intrinsic end-expiratory alveolar pressure, adding an increasing threshold load on the inspiratory muscles, which become functionally weaker, and eliciting dyspnoea. In advanced chronic obstructive pulmonary disease, bronchodilators and lung volume reduction surgery do not usually reverse expiratory flow limitation, but they appear to be useful because they often allow expiratory flow limitation to occur at a lower absolute lung volume, thus reducing dynamic pulmonary hyperinflation and limiting exertional dyspnoea.