The elastic component of the respiratory system is affected during acute respiratory failure only in cases of lung consolidation. Nevertheless, because the elastance in COPD patients is usually high, a slight reduction in elastance is unable to counterbalance the increase in resistance in order to maintain airflow. As a result, these patients are unable to expire to functional residual capacity and become hyperinflated. This volume excess helps to increase expiratory flow but represents an additional load for the inspiratory muscles, as will be discussed below. Because total lung capacity is fixed, the increase in end-expiratory volume leads to a reduction in inspiratory capacity. The only strategy left for increasing minute ventilation is not to increase tidal volume, but to increase respiratory rate. This pattern results in a further reduction in expiratory time and further hyperinflation.
Additionally, hyperinflation forces patients to breathe in a less compliant part of their pressure-volume curve. This impairment in the elastic properties of the lung further burdens the inspiratory muscles, but enhances expiratory flow by increasing pressure available for flow. In this way the problem of expiratory flow limitation must be compensated by inspiratory muscles.
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