Relationship of flow and cross-sectional area to frictional stress in airway models of asthma.

Frictional stress from air flowing through narrowed airways may damage the airway mucosa and thereby increase airway inflammation and airway obstruction. To investigate the levels of frictional stress that might occur in the airway, we measured the frictional stress in three physical airway models (model 1: normal state; models 2 and 3: narrowed states with cross-sectional area half and one-fourth of model 1, respectively) at tracheal expiratory flow rates of 1, 2, 3, 4, 5, 6, 7, and 8 L/sec. Frictional stress measured at stations in the trachea (A), two each in the left (B and C) and right (D and E) major bronchi, and one in the right secondary bronchus (F) indicated that at higher flow rates, high values of the frictional stress seen in model 3 (highest value being 139.2 N/m2 at 8 L/sec at station) could well damage the airway wall, especially during episodes of cough, and particularly when the mucosa is inflamed and friable as it is in asthmatic patients. Conversely, control of cough may have anti-inflammatory benefits in asthmatic patients.