1. The changes in maximum expiratory flow rates after washing out lung air with a helium/oxygen mixture (He/O2, 80: 20) were measured in 24 patients with chronic irreversible airflow obstruction (FEV1 1.77 ± sd 0.39 litres; FVC 3.62 ± 0.59 litres), and in six normal subjects. The percentage increase in flow breathing He/O2 was variable; however, it was similar in normal subjects and in patients with airflow obstruction, and in both groups decreased at low lung volumes. Contrary to previous studies, only three patients with chronic airflow obstruction failed consistently to increase flow rates by 20% when breathing He/O2 at all lung volumes measured.

2. In six normal subjects and 12 patients with chronic airflow obstruction airway gas viscosity was increased by breathing a neon/oxygen mixture (Ne/O2, 80:20). The response to Ne/O2 was again variable (normal subjects δVmax.40 4 ± sd 14%; patients δVmax.40 18 ±8%). Only two normal subjects and one patient with airflow obstruction consistently reduced their flow rates when breathing Ne/O2.

3. These results indicate either that there is no difference in the distribution of airflow resistance in normal subjects and in patients with chronic airflow obstruction, or that density as well as viscosity is an important determinant of flow in very small airways. In either case, He/O2 breathing is not a good discriminator of the site of airflow obstruction.

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