Detection of expiratory flow limitation during mechanical ventilation: a simulation study

Expiratory flow limitation (EFL) is frequent in mechanically ventilated patients with obstructive pulmonary diseases and its prompt detection is important to choice the right therapy. Aim of this work is to compare by simulation two methods for detection of flow limitation: the negative expiratory pressure (NEP) method and the external resistance (ΔR) method. To this purpose, a non linear dynamic morphometric model of breathing mechanics, derived from the Weibel symmetrical description of lungs, was used to simulate a normal and an obstructive respiratory condition during artificial ventilation. Both the examined methods reveled the presence of EFL only in the pathological case. The NEP method seems to promote the collapse of the higher and intermediate airways, so producing an overestimation of the pathology. At the contrary, during the ΔR maneuver the same airways increase their radius and, therefore, EFL appears underestimated. The ΔR method seems less practical to use with respect to the NEP method, because of the procedure necessary to choose the right resistance degree, moreover the flow limited portion estimated with the ΔR technique sounds rather dependent to the choice of the external resistance.