Work of breathing

The work of breathing is closely related to the mechanical properties of the respiratory system and to the ventilatory pattern chosen by the patient. Classically, the work of breathing is divided into resistive and elastic works. Elevated airways resistance is the main component of the increased resistive work of breathing in stable COPD patients. When acute respiratory failure develops in these patients, the resistive work is further increased due to the impairment in airway resistance. Nevertheless, as hyperinflation appears and patients breathe in the less compliant part of the pressure-volume curve, the elastic work also increases. More importantly, dynamic hyperinflation maintains positive intrapulmonary pressure at the end of expiration (intrinsic PEEP or auto-PEEP). This forces the inspiratory muscles to perform excess work in early inspiration to overcome this threshold pressure. In some COPD patients this extra work induced by hyperinflation can exceed the resistive work.

Expiratory work is commonly not taken into account in normal subjects. However, in COPD patients expiration is active rather than passive. In this scenario, the contraction of the expiratory muscles helps to increase expiratory flow, but also increases the work of breathing. Moreover, hyperinflation also assists in developing expiratory work because the thorax becomes expanded and tries to return to the resting position.

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