Inclusion Criteria:  i.aged ≥ 40 years (ii) hospitalization for acute exacerbation of COPD; (iii) smoker or ex-smoker (iv) FEV1 <80% of predicted and FEV1/FVC <70% postbronchodilator within 12 months prior to admission (v) have no other lung disease (including asthma) as a principal cause of pulmonary function limitation (vi) baseline PaCO2 ≥ 45 mm Hg at study entry. (vii) willing to give informed consent (viii) willing to participate in measurement of ABGs (ix) will to provide a brief history and physical examination and answer questionnaires.  Exclusion Criteria:  Adults unable to consent Individuals who are not yet adults (infants, children, teenagers) Women who are known to be pregnant at the time of hospital admission Prisoners Individuals who do not understand English