Peep in Mechanical Ventilation
PEEP refers to an adjunct to PPV in which the alveolar pressure is maintained above atmospheric pressure (usually by 5–15 cm H2O) at the conclusion of expiration. PEEP is almost universally applied in mechanically ventilated patients due to benefits in gas exchange, recruitment of alveolar units, counterbalance of hydrostatic forces leading to pulmonary oedema and maintenance of airway patency. However, there are exceptions to this general rule of using PEEP, particularly in a tenuous patient where there is concern that increased intrathoracic pressure will worsen a patient's haemodynamic stability or where the risk of pulmonary barotrauma is high.
The past two decades have produced a plethora of research clarifying the utility and safety of PEEP. Prospective randomised controlled trials have shown that high, yet controlled, levels of PEEP can be safely used in patients with acute respiratory distress syndrome (ARDS). The general consensus maintains that complications of PEEP, particularly barotrauma, dynamic hyperinflation and auto (intrinsic) PEEP, can be avoided by carefully monitoring the tidal volumes, peak and plateau pressures and transthoracic pressure gradients. Thus, maintaining at least a small degree of PEEP remains a consensus recommendation for most mechanically ventilated patients.
While a small amount of PEEP (3–5 cm H2O) is beneficial in the majority of cases, higher levels of PEEP may be necessary in specific subsets of mechanically ventilated patients, such as those with ARDS, in whom pulmonary compliance is reduced and, thus, recruitment of collapsed alveoli and reversal of atelectasis are critical. In addition, early studies on patients with pulmonary oedema established that PEEP can shift fluid from alveoli and the interstitial space back into the circulation, thereby reducing the degree of intrapulmonary shunting and improving oxygenation.