A 62-year-old male smoker with a 40 pack-year history presents with progressive dyspnea, chronic cough, and sputum production for the past 5 years. Clinical examination reveals barrel chest, pursed-lip breathing, and diminished breath sounds bilaterally. Chest X-ray shows hyperinflation with flattened diaphragms. Which investigation is most appropriate to confirm the diagnosis and assess the severity of airflow obstruction?