External positive end-expiratory pressure (PEEP) with auto-PEEP

The use of external positive end-expiratory pressure (PEEP) in the setting of auto-PEEP may be conceptualized by the "waterfall over a dam" analogy. In this analogy, the presence of dynamic hyperinflation and 10 cm H₂O of auto-PEEP is represented in the top panel by the reservoir of water trickling over the dam represented by the solid block. In the middle panel, as long as the external PEEP is less than or equal to the amount of auto-PEEP, the amount of water in the upstream reservoir, representing dynamic hyperinflation, does not increase. However, once the amount of water in the reservoir does increase (bottom panel), dynamic hyperinflation worsens. Additional experimental studies done by others suggest that this effect occurs once the external PEEP is greater than approximately 85% of the amount of auto-PEEP.