Sensitivity of volume of isoflow in the detection of mild airway obstruction.

The relative sensitivity of volume of isoflow test for detecting obstruction was compared to that of other tests, including flow at 60 per cent of total lung capacity, closing volumes, and frequency dependence of dynamic lung compliance. The volume of isoflow was measured in a waterless spirometer after 3 vital capacity inspirations of a mixture of 80 per cent oxygen. We studied 22 asymptomatic, healthy smokers (18 men and 4 women, 32.5 +/- 7.2 years of age, who smoked 5 to 20 pack-years). Thirteen smokers had an abnormal volume of isoflow, yet only 4 had an abnormal flow at 60 per cent of total lung capacity and 2 had an abnormal closing volume. Abnormal frequency dependence of dynamic lung compliance was demonstrated in 3 of 7 smokers tested with an abnormal volume of isoflow; none was detected in 7 smokers tested with normal volume of isoflow. Static pressure-volume curves and diffusing capacity were normal in all smokers with an abnormal volume of isoflow, and after bronchodilator inhalation, volume of isoflow improved in one half of the subjects. These results suggest reversible, intrinsic airway obstruction in the presence of normal flow at 60 per cent of total lung capacity and closing volume. Furthermore, volume of isoflow was a more sensitive test that flow at 60 per cent total lung capacity, closing volume, or frequency dependence of dynamic lung compliance.