Under normal conditions, the end-expiratory elastic recoil pressure of the respiratory system is zero (case A in Fig 1). In this instance, as soon as the inspiratory muscles contract, the alveolar pressure becomes subatmospheric and gas flows into the lungs. When breathing takes place at lung volumes higher than Vr, the end-expiratory elastic recoil pressure is positive (15 cmH 2O in case B of Fig 1). The elastic recoil pressure present at end-expiration has been termed auto-PEEP or intrinsic PEEP. When intrinsic PEEP is present, onset of inspiratory muscle activity and inspiratory flow are not synchronous; inspiratory flow starts only when the pressure developed by the inspiratory muscles exceeds intrinsic PEEP because only then does alveolar pressure becomes subatmospheric. In this respect, intrinsic PEEP acts as an inspiratory threshold load which increases the elastic work of breathing. As indicated above, this places a significant burden on the inspiratory muscles, which in hyperinflated COPD patients are operating under disadvantageous force-length conditions and abnormal thoracic geometry. Thus the presence of expiratory flow limitation during resting breathing is associated with hypercapnia and dyspnea.
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