Intro

Many of the diseases that affect the lungs and chest wall increase the resistance or decrease the compliance of the respiratory system. Consequently, the work of breathing is increased. Patients with COPD have increased airway resistance and, if they are also hyperinflated, have decreased compliance of the respiratory system because both the lungs and chest wall are stiffer at higher volumes. The increased work associated with hyperinflation is described as an “elastic load.” An elastic load reflects the difficulty moving a system because of its stiffness. As the chest/lungs become hyperinflated, inspiration must be performed on a steeper portion of the pressure-volume curve of the respiratory system; this is another way of saying the system is less compliant. The experimental model often used to mimic the sensation of breathing against an elastic load is that of breathing out of a rigid drum – as one inspires, the air is decompressed in the drum and one has to generate greater inspiratory force as the inspiration continues.