A 70-year-old man is evaluated in follow-up for COPD, which was diagnosed 1 year ago. He has had two exacerbations in the last year, with the second exacerbation 1 month ago. He has also had baseline chronic cough with mucoid sputum consistently for the last 3 years. Sputum culture 6 months ago grew Haemophilus influenzae and Mycobacterium avium-intracellulare. He has to stop to catch his breath after walking 100 meters (328 feet). His inhaler technique is good. He quit smoking 2 years ago and completed a pulmonary rehabilitation program 4 months ago. His medications are tiotropium, fluticasone/salmeterol, and as-needed albuterol. Spirometry performed 3 months ago showed an FEV1 of 32% of predicted and an FEV1/FVC ratio of 50%. Chest radiograph performed last month showed no infiltrate, mass, or increased vascular congestion.

On physical examination, vital signs are normal. Oxygen saturation is 92% breathing ambient air. Pulmonary examination reveals diminished breath sounds.

Which of the following is the most appropriate long-term COPD treatment?