Mechanical factors determining maximum bronchoconstriction.

The principal feature which distinguishes asthmatics from normal subjects is that their airways constrict excessively to stimuli, to the extent that airway narrowing exceeds the maximum degree of bronchoconstriction which occurs in normal subjects. The load against which bronchial smooth muscle contracts is a very important determinant of the degree of airway narrowing. The elastic properties of the airway wall itself and the interdependence between the airways and the surrounding parenchyma are the elastic load which bronchial smooth muscle has to overcome to narrow the airway. The most likely explanation of a decreased load in asthma is oedema of the airway wall, because it may unlink the interdependence between the airway and the surrounding parenchyma.