Positive end-expiratory pressure
Positive end-expiratory pressure (PEEP) is the pressure in the lungs (alveolar pressure) above atmospheric pressure (the pressure outside of the body) that exists at the end of expiration. The two types of PEEP are extrinsic PEEP (PEEP applied by a ventilator) and intrinsic PEEP (PEEP caused by a non-complete exhalation). Pressure that is applied or increased during an inspiration is termed pressure support.
Intrinsic PEEP (auto)
Auto (intrinsic) PEEP — Incomplete expiration prior to the initiation of the next breath causes progressive air trapping (hyperinflation). This accumulation of air increases alveolar pressure at the end of expiration, which is referred to as auto-PEEP.
Auto-PEEP develops commonly in high minute ventilation (hyperventilation), expiratory flow limitation (obstructed airway) and expiratory resistance (narrow airway).
Once auto-PEEP is identified, steps should be taken to stop or reduce the pressure build-up. When auto-PEEP persists despite management of its underlying cause, applied PEEP may be helpful if the patient has an expiratory flow limitation (obstruction).
Extrinsic PEEP (applied)
A small amount of applied PEEP (3 to 5 cmH2O) is used in most mechanically ventilated patients to mitigate end-expiratory alveolar collapse. A higher level of applied PEEP (>5 cmH2O) is sometimes used to improve hypoxemia or reduce ventilator-associated lung injury in patients with acute lung injury, acute respiratory distress syndrome, or other types of hypoxemic respiratory failure.
Positive end-expiratory pressure can contribute to:
- Decrease in systemic venous return
- Pulmonary barotrauma can be caused. Pulmonary barotrauma is lung injury that results from the hyperinflation of alveoli past the rupture point.
- The effects of PEEP on intracranial pressure(ICP)have been studied. Although PEEP is hypothesized to increase ICP due to to impedance of cerebral blood flow, it has been shown that high PEEP does not increase ICP. 
- Renal functions and electrolyte imbalances, due to decreased venous return metabolism of certain drugs are altered and acid-base balance is impeded.
- thefreedictionary.com > positive end-expiratory pressure (PEEP) Citing: Saunders Comprehensive Veterinary Dictionary, 2007
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