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

Expiratory flow limitation (EFL) is frequent in mechanically ventilated patients with obstructive pulmonary disease and its prompt detection is important to optimize respiratory assistance. The present study aims to compare by simulation two methods for the detection of flow limitation in intensive care unit: 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 methods revealed the presence of EFL in the pathological case. The NEP method seems to promote the collapse of the upper and intermediate airways, so producing an overestimation of the pathology result. On the contrary, during the ΔR maneuver the same airways increase their radius and, therefore, EFL appears underestimated. The ΔR method appears less practical with respect to the NEP method, because of the procedure required to select the appropriate resistance degree. Moreover the flow limited portion of expiration estimated by the ΔR technique sounds rather dependent on the choice of the external resistance level.