A 70-year-old man is admitted with an acute exacerbation of chronic obstructive pulmonary disease (COPD). He is treated with nebulised bronchodilators, oral corticosteroids, and antibiotics. His arterial blood gas on 28% Venturi oxygen shows: pH 7.32, PaCO2 8.5 kPa, PaO2 7.5 kPa, HCO3- 34 mmol/L, BE +5. What is the interpretation of this blood gas?