A 52-year-old man presents to your office for an acute visit because of coughing and shortness of breath. He is well known to you because of multiple office visits in the past few years for similar reasons. He has a chronic "smoker’s cough;’ but reports that in the past 2 days his cough has increased, his sputum has changed from white to green in color, and he has had to increase the frequency with which he uses his albuterol inhaler. He denies having a fever, chest pain, peripheral edema, or other symptoms. His medical history is significant for hypertension, peripheral vascular disease, and two hospitalizations for pneumonia in the past 5 years. He has a 60-pack-year history of smoking and continues to smoke two packs of cigarettes a day.
On examination, he is in moderate respiratory distress. His temperature is 98.4°F, his blood pressure is 152/95 mm Hg, his pulse is 98 beats/min, his respiratory rate is 24 breaths/min, and he has an oxygen saturation of 94% on room air. His lung examination is significant for diffuse expiratory wheezing and a prolonged expiratory phase of respiration. There are no signs of cyanosis. The remainder of his examination is normal. A chest x-ray done in your office shows an increased anteroposterior (AP) diameter and a flattened diaphragm, but otherwise he has clear lung fields.
What is the most likely cause of this patient’s dyspnea?