Expiratory flow limitation definition, mechanisms, methods, and significance

Abstract

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.

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 first patient (c), while eliciting greater expiratory flow in the second one (d).

Figure 2

Isovolume (low-lung volume) flow-pressure relationship in normal subjects, COPD without expiratory flow limitation (NFL) and COPD with expiratory flow limitation (FL). In any case, after P _crit, expiratory flow does not increase further on, and its driving pressure becomes P _el. In COPD patients with high airflow resistance and very low P _el, the P _crit occurs early, limiting expiratory flow in the tidal volume range.

Figure 3

Comparison of FEV₁, IC, DL_CO, and K_CO in COPD patients who exhibit tidal expiratory flow limitation (EFL) in the supine position (FL; n = 14) versus those who do not (NFL; n = 13). Both DL_CO and K_CO are significantly lower in FL patients (*P < 0.05), suggesting that emphysematous patients are more prone to develop recumbent EFL.