Physiological factors affecting airway resistance in normal subjects and in patients with obstructive respiratory disease.

Although the airway conductance1 increases at larger lung volumes in normal subjects (1), it is reduced in patients with asthma or emphysema who are breathing with a large functional residual capacity (FRC) (2). We have therefore tried to assess the influence of other factors which might affect the airway conductance in both normal subjects and in patients with asthma, bronchitis and emphysema. These include the interrelationship of lung volume, lung elastic pressure and airway conductance, the effect of exercise, forced breathing and normal aging, and the composition of the respired gas and bronchomotor drugs. We investigated the balance of forces regulating the diameter of the airway lumen, i.e., the tone of the airway wall, directed inward, opposed by the traction of the lung tissues, directed outward. We studied whether, in normal subjects, this balance was altered by bronchmotor drugs or by a change in lung elastic pressure following chest strapping, and whether in patients with asthma, the airway conductance/lung volume relationship was altered after forced breathing or exercise. Patterns of disturbance leading to air trapping