A 66-year-old man with severe COPD (FEV1 38% predicted) presents with increased dyspnea, wheeze, and purulent sputum for 2 days. He uses tiotropium and albuterol inhalers at baseline. Vitals: T 99.5 F, HR 96, BP 130/78, RR 24, SpO2 89% on room air. After oxygen and nebulized bronchodilators, his work of breathing improves. Which additional therapy is most appropriate to treat this COPD exacerbation?