**1.1. Definition**

Expiratory flow limitation (EFL) is defined as an absence in increase in flow with application of negative expiratory pressure (NEP) during quiet breathing (Koulouris et al., 1994). The test is a simple, noninvasive, practical and accurate technique.

Application of the NEP technique provides a simple, rapid, non-invasive, and reliable test to detect tidal expiratory flow limitation; b) it does not require a body-box or cooperation on the patient's part; c) it can be applied in any posture, during mechanical ventilation, and during exercise; d) it provides new insights into the physiology and pathophysiology of sev‐ eral diseases and the symptom of dyspnea.
