Figure 1 illustrates the pressure required to overcome the elastic recoil of the respiratory system for the same tidal volume (equal to 20 per cent of vital capacity (VC))
inhaled from Vr (equal to 34 per cent of VC) and from an end-expiratory lung volume increased to 67 per cent of VC. As shown by the hatched areas, the elastic work increases about fivefold when the breath is inhaled from 67 per cent VC (case B) relative to the breath taken from Vr (case A). Clearly, dynamic hyperinflation implies an increase of both elastic work and inspiratory muscle effort. Furthermore, as lung volume increases, there is a decreased effectiveness of the inspiratory muscles as pressure generators because the inspiratory muscle fibers become shorter (force-length relationship) and their geometrical arrangement changes. Thus, in COPD patients there is a vicious cycle: the inspiratory flow-resistive work is invariably increased due to airway obstruction but, more importantly, as a result of hyperinflation there is a concomitant increase in elastic work and impaired mechanical performance of the inspiratory muscles. With increased severity of COPD, a critical point is eventually reached at which the inspiratory muscles become fatigued.
Fig. 1 Volume-pressure diagram of the relaxed respiratory system showing the increase in elastic work caused by dynamic hyperinflation: area A, elastic work for a breath that starts from the relaxation volume Vr; area B, elastic work for a similar breath to A but starting from a volume 29 per cent VC higher than Vr. In case B, the intrinsic PEEP is 15 cmH2O, as indicated by the upper circle.
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