A 65-year-old woman with a history of chronic obstructive pulmonary disease (COPD) comes to the clinic because of a 4-month history of worsening exertional dyspnea. Physical examination shows diffuse bilateral wheezes on lung auscultation and slight bilateral ankle edema. Further investigation confirms the diagnosis of pulmonary hypertension. An arterial gas analysis during exercise shows a partial pressure of oxygen of 57 mmHg (80-100 mmHg), partial pressure of carbon dioxide of 45 mmHg (35-45 mmHg), and saturation of oxygen in ambient air of 88% (95-100%). Which of the following is the most appropriate treatment for this patient?