A 64-year-old man presents to the emergency department with increased cough, sputum purulence, and dyspnoea over 3 days. He has moderate COPD (FEV1 55 percent predicted) and uses a long-acting bronchodilator and inhaled corticosteroid. He is afebrile, with oxygen saturation 93 percent on room air. Which of the following acute treatments is most clearly indicated?