Positive end-expiratory pressure or PEEP is a measurement of air pressure in the lungs in excess of atmospheric pressure at the end of exhalation. It is used both as a measurement of the efficiency of breathing in a patient and for properly setting up a ventilator.
If a patient does not properly exhale and still has high positive pressure in their lungs before beginning inhalation, this leads to hyperinflation of the lungs and hyperventilation. This can be caused by an obstructed or narrowed airway.
On a ventilator, PEEP will be deliberately set to negative pressure to draw remaining air from the lungs before starting inflation to increase the efficiency of oxygen transfer. It can also be adjusted to reduce further damage to the lungs after a trauma.