When expiratory flow is maximal during tidal breathing and cannot be increased unless operative lung volumes move towards total lung capacity, tidal expiratory flow limitation (EFL) is said to occur. EFL represents a severe mechanical constraint caused by different mechanisms and observed in different conditions, but it is more relevant in terms of prevalence and negative consequences in obstructive lung diseases and particularly in chronic obstructive pulmonary disease (COPD). Although in COPD patients EFL more commonly develops during exercise, in more advanced disorder it can be present at rest, before in supine position, and then in seated-sitting position. In any circumstances EFL predisposes to pulmonary dynamic hyperinflation and its unfavorable effects such as increased elastic work of breathing, inspiratory muscles dysfunction, and progressive neuroventilatory dissociation, leading to reduced exercise tolerance, marked breathlessness during effort, and severe chronic dyspnea. 1. Definition Expiratory (air) flow limitation (EFL) during tidal breathing is a well-defined, mechanical pathophysiological condition occurring, either during physical exercise or at rest, before in supine and later on in sitting-standing position, when expiratory flow cannot be further increased by increasing expiratory muscles effort (i.e., by increasing pleural and alveolar pressure) because it is maximum at that tidal volume [1]. In other words, under the prevailing conditions, the respiratory system is globally limited as flow generator even during tidal expiration, and greater expiratory flow rates may be achieved just by increasing operating lung volumes, (i.e., moving progressively the end-expiratory lung volume (EELV) towards total lung capacity). In fact, the volume-related decrease of airway resistance and increase of elastic recoil are the only effective mechanisms to obtain higher expiratory flows in case of EFL [2]. As a consequence, the term airflow limitation widely used to indicate the abnormal decrease of maximal expiratory flow rates at a given lung volume, as compared to predicted (i.e., airflow reduction or airflow obstruction), is inappropriate and should not be adopted unless the condition previously described is present (Figure 1). Figure 1: Maximal and tidal flow-volume curve in two representative COPD patients: one with airflow reduction and tidal expiratory flow limitation (EFL) at rest (a), the other only with airflow reduction at rest and potential EFL during exercise (b). The NEP application at rest does not increase expiratory flow in the
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