Predominant site of airway resistance in chronic obstructive pulmonary disease.

The purpose of the present clinical research was to establish the major site of resistance and obstruction in patients with severe, symptomatic chronic obstructive pulmonary disease. We used a noninvasive technique by measuring airflow in a plethysmograph after patients breathed air and after breathing a mixture of 80 percent helium and 20 percent oxygen. In group 1 were 14 patients (mean age, 63 +/- 7 years [+/- 1 SD]) with bronchitis. The ratio of their forced expiratory volume in one second over the vital capacity (FEV1/VC) was 43 +/- 10 percent (mean +/- 1 SD), and their single-breath carbon monoxide diffusing capacity (Dsb) was normal (125 +/- 41 percent of the predicted value). In group 2 were 13 patients with emphysema (mean age, 60 +/- 10 years), whose FEV1/VC was 33 +/- 11 percent and Dsb was 55 +/- 14 percent of predicted. All patients had abnormally low increases in the maximum expiratory flow at midvital capacity after breathing the helium-oxygen mixture. The range in group 1 was -11 percent to +14 percent and in group 2 was -10 percent to +15 percent. Normal subjects in our laboratory show increases of at least +24 percent. This indicates that the limitation of airflow occurs primarily in the small (less than 2 mm in internal diameter) and not the large airways.