Deviation of end-expiratory lung volume from the elastic equilibrium v
olume of the respiratory system is recognized as a cardinal feature in
mechanically ventilated patients with severe chronic obstructive pulm
onary disease (COPD) and acute asthma. The presence of dynamic hyperin
flation implies that alveolar pressure remains positive throughout exp
iration. At the end of the expiration, this positive difference betwee
n alveolar and external airway pressures is termed auto-positive end-e
xpiratory pressure (PEEP) or intrinsic-PEEP. The aims of this article
are (1) to review the physiologic mechanisms of auto-PEEP and the use
of PEEP in counterbalancing auto-PEEP and (2) to examine the clinical
criteria for application of PEEP in patients with COPD.