Expiratory flow limitation (EFL) is defined as a dynamic condition that expiratory flow cannot be further increased with higher expiratory driving pressure. Under mechanical ventilation, it can cause intrinsic positive end-expiratory pressure (PEEP) and dynamic hyperinflation, and be associated with worse clinical outcome. The detection of EFL however needs special maneuvers and offline analysis of flow-volume curves, which are infeasible in routine practice and cannot be used during real-time monitoring. The investigators propose a new and simple approach using flow derived parameters to detect EFL in real time without needing any intervention.