A 68-year-old woman with a history of advanced chronic obstructive pulmonary disease comes to the emergency department with COPD exacerbation. She is given additional supplemental oxygen, intravenous antibiotics, methylprednisolone, and 2 treatments of nebulized ipratropium and salbutamol. Following these measures, she remains dyspneic and uses accessory muscles. Results of ABG are pH 7.32, PCO2 60 mm Hg, and PO2 52 mm Hg. Which of the following is the best next step in management of this patient?