All patients have three measurement periods. 1) Pressure support during 20 min, NAVA equilibration period of 30 min, followed by 2) NAVA ventilation for 20 min, pressure support equilibration period of 30 min and 3) pressure support for 20 min. Blood gases and cardiac output measurements is performed before and after each measurement period. Regional ventilation is measured by Electric Impedance Tomography (EIT).