inclusion criteria :  patients aged over 18 years and admitted in intensive care invasive ventilation with assisted mode (assist-control or spontaneous with pressure support) for more than 48 hours, stabilization of the cause of admission defined as: positive end-expiratory pressure level lower than or equal to 8 cm of water, a arterial oxygen saturation higher than 90 percent with a fraction of inspired oxygen lower than or equal to 50 percent for acute respiratory failure; infusions of norepinephrine less than or equal to 0,2 gamma/kg/min, no epinephrine or dobutamine for cardiogenic or septic shock; body temperature 36-38,5°C.  exclusion criteria :  schedulded extubation within 24 hours after the inclusion, schedulded surgery within week after the inclusion, no available capnometer, end stage chronic disease (cardiac, pulmonary, or renal), "do not resuscitate order", prior ventilation greater than 24 hours during the same hospitalization, tracheostomy, known or suspected severe myopathy or neuropathy, severe chronic obstructive pulmonary disease, pregnancy, patients without medial care assistance. conditions and/or circumstances that might result in difficulty to understand the informed consent