[Physiopathology of COPD (steady-state)]

Chronic obstructive pulmonary disease (COPD) have several pathophysiological characteristics in common, the main one being an increased airways resistance (raw). It is the result of bronchial abnormalities and reduced parenchymal elasticity, and is influenced by lung volume. Raw decreases with increasing lung volume, and increases with decreasing lung volume. Such expiratory events are generally compensated on the inspiratory side. Inspiration is shortened to prolong expiration, and breathing takes place at higher lung volume to take benefit of the higher lung recoil. This inspiratory load is associated to an increased inspiratory drive, and contributes to put inspiratory muscles at disadvantage. However, with time, adaptative changes take place that restore their force at a shorter length. Chronic fatigue, often suspected in this setting, is therefore not currently demonstrated. Bronchial and parenchymal abnormalities lead to ventilation-perfusion mismatch, that contribute to hypoxemia and hypercapnia through deadspace and shunt effects. Hypercapnia can also correspond in part to protective mechanisms, if the energy requirements for its maintenance are too high.