A 70-year-old male with a history of cough and sputum production for 30 years, shortness of breath on exertion for 10 years, and bilateral lower extremity edema for the past 1 year. Pulmonary function tests show obstructive ventilatory impairment with FEV<sub>1</sub>%:50%. The obstructive emphysema caused by chronic bronchitis is most commonly: