Inclusion Criteria:  age 18 years or older ability to give informed consent previously diagnosed COPD (either confirmed diagnosis at prior hospital - contact or from their general practitioner or confirmed diagnosis by the treating physician in the emergency department (verified by use of relevant medication)) admitted with acute exacerbation (acute and worsened shortness of breath) of COPD requiring oxygen treatment  Exclusion Criteria:  Instability at arrival requiring immediate lifesaving treatment, e.g. intubation or non-invasive ventilation, within the first 30 minutes Expected total length of stay in hospital < 12 hours Planned transfer to another hospital within 12 hours Unwilling to have repeated arterial blood gas analyses within the first 12 hours Patients judged terminal by treating physician in the emergency department Non-residents of the particular country Expected impossible follow-up Fertile women (<50 years of age) with positive urine human gonadotropin (hCG) or plasma-hCG Prior participation in the study