Measurements: HME and HH were randomly compared during 2 NIV periods of 30 minutes separated by a 20 to 30 minutes period of spontaneous breathing with oxygen. ABG were collected at baseline and at the end of each period. As well as respiratory parameters including volumes, pressures, P0.1, a comfort score, capnometry and oxygen saturation. Two sets of patients were successively studied whether they have a flex tube added to the ventilatory circuit or not.