Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 22

In asthma (see Chapter 10), airway narrowing occurs, leading to a greater resistance between point A (the alveolus) and point B. The pressure drop, A to B, for any given flow rate will therefore be greater than in the healthy lung, and the critical (maximal) flow rate (when the pressure difference between A and B is just enough to overcome the retractile force of the lung) will be lower. You may have known for some time that peak expiratory flow is reduced in asthma, but now you understand why.

In COPD (see Chapter 11), the loss of alveolar walls (emphysema) reduces the elastic recoil of the lung. There is therefore less protective retractile force on the airway wall and the critical pressure drop along the airway required to cause airway collapse will occur at a lower flow rate. Thus, maximum expiratory flow is also reduced in COPD.