A 68 year old man with a history of very severe COPD (FEV1~1.0L, <25% predicted) presents to the ER complaining of worsening dyspnea and an increase in the frequency and purulence of his sputum production over the past 2 days. His oxygen saturation is 78% on room air. Before he is placed on supplemental oxygen, a room air aerial blood gas is drawn and reveals: pH 7.25, PCO2 - 68, PO2 - 48, HCO3 -31. What is the diagnosis?