Impact of Proportional Assisted Ventilation on Dyspnea and Asynchrony in Mechanically Ventilated Patients

The PAV mode will be delivered by Puritan Bennett 980 ventilator (Covidien, Boulder, USA). Levels of PEEP and FiO2 will be kept constant. The level of assistance in PAV, named %-assistance will be set in order to keep the patient in a respiratory effort zone corresponding to a respiratory muscles pressure time product (PTPmus) between 50 and 150 cm H2O • s / min. As it is not possible to calculate directly the PTPmus at bedside, the investigators will use as a substitute its main component, the pressure peak muscle of the airways according to the previous report from Carteaux et al. This setting has been described extensively and its use has been the subject of a feasibility study in 50 patients. After a 20-minutes stabilization period, a 30-minutes recording will be performed.