A 55-year-old man is evaluated in follow-up for severe COPD, which was diagnosed 2 years ago. He has had two exacerbations in the past year requiring hospitalization, and his baseline exercise tolerance is low. He completed pulmonary rehabilitation 3 months ago without much improvement in exercise capacity. He quit smoking 1 year ago. His medications are tiotropium, fluticasone/salmeterol, daily roflumilast, and albuterol as needed.

On physical examination, vital signs are normal; BMI is 22. Oxygen saturation is 92% breathing ambient air. Scattered wheezing is noted bilaterally.

Chest radiograph and CT scan both show emphysematous changes in the upper lobes. Spirometry shows an FEV1 of 40% of predicted and a DLCO of 25% of predicted. His 6-minute walking distance is 240 meters (787 feet), consistent with decreased exercise tolerance.

Which of the following is most likely to benefit this patient?