In normal subjects at rest, the end-expiratory lung volume (functional residual capacity (FRC)) corresponds to the relaxation volume ( Vr) of the respiratory system, i.e. the lung volume at which the elastic recoil pressure of the respiratory system is zero. Pulmonary hyperinflation is defined as an increase of FRC above the predicted normal value. This may be due to increased Vr as a result of loss of elastic recoil (e.g. emphysema) or to dynamic pulmonary hyperinflation, which is said to be present when the FRC exceeds Vr. Dynamic hyperinflation exists whenever the duration of expiration is insufficient to allow the lungs to deflate to Vr prior to the next inspiration. This tends to occur under conditions in which expiratory flow is impeded (e.g. increased airway resistance) or when the expiratory time is reduced (e.g. increased breathing frequency). Expiratory flow may also be retarded by other mechanisms such as persistent contraction of the inspiratory muscles during expiration and expiratory narrowing of the glottic aperture. In COPD patients with acute ventilatory failure, dynamic hyperinflation is invariably present and is due to severe expiratory flow limitation.
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