A 67-year-old man presents to your office with worsening cough, sputum production, and shortness of breath. He has been a cigarette smoker for the past 50 years, smoking approximately 1 pack a day. He has a chronic AM cough productive of some yellow sputum but generally feels okay during the day. He was in his usual state of health until two weeks ago when he developed a cold. Since then, he has had a hacking cough and increased thick sputum production. He also has had difficulty walking more than a block without stopping due to shortness of breath. Physical examination reveals prolonged expiration, audible wheezing, and diffuse rhonchi throughout both lung fields. Chest x-ray shows hyperinflation of both lungs with a flattened diaphragm. His most likely diagnosis is: