The ELVIS study is a prospective, randomized, multi-centre open label trial following a non-inferiority design to compare the nasal high-flow (NHF) to non-invasive ventilation (NIV) in treatment of acute acidotic hypercapnic exacerbation of chronic obstructive pulmonary disease. Accoring to the randomization the patient is treated with NHF or NIV until discharge. A change of device is possible, if switch criteria are fulfiled OR need for intubation criteria are met before 72h (timepoint for primary endpoint).