Pahophysiology of respiratory failure during exacerbation of COPD

摘要

The COPD patient during tidal breathing has to overcome an increased load: besides the "threshold" load of PEEPi and the resistive load of stenotic airways he has an increased elastic work since the overinflated lung is less distensible, being on the steeper portion of the pressure/volume curve. With respect to force, hyperinflation flattens the diaphragm and shortens the apposition zone fibers, with a consequent reduction of the diaphragm force capacity, due to a disadvantageous force/length relation. During exacerbationsm, the increase in respiratory rate shortens the time available for lung emptying with consequent worsening of dynamic hyperinflation (DH) and this places the diaphragm in a more disadvantageous position, reducing its force generating capacity. Therefore indirect indexes of DH, like inspiratory capacity (IC), and of reduced diaphragmatic force, like maximal inspiratory pressure (Pi max) can better describe the pathophysiologic changes during exacerbations. Hypoxia is caused by worsening of ventilation/perfusion ratio, while hypercapnia is the consequence of ventilatory pump failure and alveolar hypoventilation.