Patients who will pass the SBT will be extubated or, in the presence of tracheostomy, mechanical ventilation will be discontinued. If a patient will be re-intubated or mechanically ventilated through a tracheostomy again within 48 hours, PSV or PSV+Sigh (to maintain study group assignment) will be restored. If a patient will remain extubated or separated from the ventilator for >48 hours data collection only will continue.