[The analysis of bronchial obstruction and its reversibility after bronchodilators using a study of "density-effect"...

The change in maximal expiratory flows after inhalation of gases of various physical properties is studied since 1963. The Montreal team proposed in 1972 the helium-to-air difference in maximal expiratory flows at low lung volumes (delta He) and the volume at which the flows breathing air and the helium-oxygen mixture (the "volume isoflow" - Viso V) as "sensitive tests" for the detection of peripheral airway obstruction. "Density-dependence" (DD) has been widely used subsequently in the study of the site of bronchial obstruction in asthmatics in smokers, in subjects with early chronic bronchitis, subjects with occupational exposures to respiratory irritants, etc. The site of action of bronchodilators according to their nature or route of administration (atropine derivates acting on "central airways", "global" bronchodilation obtained by parenteral administration) was studied using DD. However, the interpretation of these studies was always based on Mead's "equal pressure point" concept minimizing the role played by bronchial wall compliance as put forward by Pride and coll. (1967). The results of certain experimental or clinical studies was found not to agree with the original explanations. Finally, the model of flow limitation recently described by Dawson and Elliott (1977) takes into account gas density and bronchial wall elasticity which determine the speed of waves propagating against flow in elastic tubes, the site of "choke points" and the cross-sectional airway area at this level. Besides methodological problems, the complex nature of mechanisms controlling bronchial flow with gases of different densities and the unavoidable influence of bronchodilators or bronchial wall compliance are major limiting factors for the use of DD in the study of the site of airflow obstruction, and effect of bronchodilator action.