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.
Reduces air-trapping in susceptible patients Promotes increased mobilization and clearance of secretions from the airways Enhances collateral ventilation and opens airways behind mucus obstructions, improving pulm. mech & facilitating gas exchange http://www.respiratoryupdate.com/members/Bronchial_Hygiene_Therapy__Positive_Expiratory_Pressure_PEP_Therapy.cfm
- PEEP can cause a decrease in venous return
- PEEP can cause Barotrauma — Pulmonary barotrauma is lung injury that results from the hyperinflation of alveoli past the rupture point.
- PEEP is not indicated for patients with increased intracranial pressure — In people with normal lung compliance, PEEP may increase the intracranial pressure (ICP) due to an impedance of venous return from the head.
- PEEP may cause alterations of renal functions and electrolyte imbalances, due to decreased venous return metabloism of certain drugs are altered and acid-base balance is impeded. (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC471250/)
See also 
- thefreedictionary.com > positive end-expiratory pressure (PEEP) Citing: Saunders Comprehensive Veterinary Dictionary, 2007
- Caramez MP; Borges JB; Tucci MR; Okamoto VN; Carvalho CR; Kacmarek RM; Malhotra, A; Velasco, IT et al. (2005). "Paradoxical responses to positive end-expiratory pressure in patients with airway obstruction during controlled ventilation". Crit Care Med 33 (7): 1519–28. doi:10.1097/01.CCM.0000168044.98844.30. PMC 2287196. PMID 16003057.
- Smith TC, Marini JJ (1988). "Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction". J Appl Physiol 65 (4): 1488–99. PMID 3053583.
- Kondili E, Alexopoulou C, Prinianakis G, Xirouchaki N, Georgopoulos D (2004). "Pattern of lung emptying and expiratory resistance in mechanically ventilated patients with chronic obstructive pulmonary disease". Intensive Care Med 30 (7): 1311–8. doi:10.1007/s00134-004-2255-z. PMID 15054570.
- Manzano F, Fernández-Mondéjar E, Colmenero M, Poyatos ME, Rivera R, Machado J et al. (2008). "Positive-end expiratory pressure reduces incidence of ventilator-associated pneumonia in nonhypoxemic patients". Crit Care Med 36 (8): 2225–31. doi:10.1097/CCM.0b013e31817b8a92. PMID 18664777.
- Smith, RA. Physiologic PEEP. Respir Care 1988; 33:620.
- Frost EA (1977). "Effects of positive end-expiratory pressure on intracranial pressure and compliance in brain-injured patients". J Neurosurg 47 (2): 195–200. doi:10.3171/jns.1977.47.2.0195. PMID 327031.