COPD patients are characterized by an increase in airway resistance which induces a reduction in the ability to generate airflow. The main feature of acute respiratory failure in COPD patients is a further reduction in airflow caused by an additional increase in airways resistance.
The interaction between inspiratory airflow and resistance is more complex. The resistance of the airways increases linearly with the flow, while parenchymal lung resistance increases at low flows. Thus the inspiratory flow pattern chosen by COPD patients during acute respiratory failure is a delicate compromise between airways and lung resistances. Moreover, lung volume also influences respiratory resistances. At high lung inflation volumes, lung resistance increases while airways resistance reduces, leading to a rise in total resistance with increasing inflation volume ( Tantucci...ef.a/ 1.991).
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