A 72-year-old woman with a history of chronic obstructive pulmonary disease (COPD) on home oxygen (2 L/min), cor pulmonale, and chronic atrial fibrillation (on warfarin) presents for elective total abdominal hysterectomy. She reports dyspnea on exertion (climbing one flight of stairs). Preoperative examination reveals cyanosis, clubbing, and a loud P2. Which investigation is most appropriate to assess her functional cardiopulmonary reserve before surgery?