This study evaluates the difference between Non Invasive Ventilation with Neurally Adjusted Ventilatory Assist (NIV-NAVA) and Conventional Non Invasive Ventilation with Pressure Support (PS) or Pressure Control (PC). All the patients are ventilated in each mode for 4 hours. Afterwards they will be subjected to a semi-structured interview where they will be asked to compare the two modes. The hypothesis is that NIV-NAVA will correct patients power of Hydrogen (pH), PaCO2 og PaO2 more quickly than NIV-PS and NIV-NAVA is more comfortable for the patients.