A patient with chronic bronchitis becomes confused and drowsy after several days of increased sputum production. What is the most likely cause?

Chronic bronchitis is characterized by excessive mucus production, airway inflammation, and impaired gas exchange. Patients with chronic bronchitis, particularly those with advanced disease, rely on an altered ventilatory drive and are prone to carbon dioxide retention. After several days of increased sputum production, worsening airway obstruction leads to progressive hypoventilation and rising arterial carbon dioxide levels — a state called hypercapnia. CO2 readily crosses the blood-brain barrier where it lowers cerebral pH; the resulting respiratory acidosis causes central nervous system depression manifesting as confusion and drowsiness, which are classic signs of CO2 narcosis. Hypoglycemia can cause confusion but is not directly linked to increased sputum production in this clinical context, and there is no indication the patient has diabetes or is on insulin. Dehydration can cause confusion but typically presents with additional signs such as dry mucous membranes, reduced urine output, and concentrated urine rather than the respiratory pattern described. Anxiety may present with confusion but generally produces agitation and tachycardia rather than drowsiness, and it does not mechanistically result from increased sputum production in chronic bronchitis.