Positive end-expiratory pressure (PEEP) is, as the name implies, positive pressure applied at the end of expiration. With PEEP the pressure in the circuit will never return to the baseline atmospheric pressure; it will always stay "positive," or above atmospheric pressure. This can benefit oxygenation in several ways: one mechanism is to keep alveoli open and thus participating in gas exchange, preventing intrapulmonary shunt.

5-8 cm H₂0 is a common range for normal lungs. Patients with ARDS will often require higher PEEP; 15 cm H₂0 is sometimes used as a starting value, with titration done rapidly based on pulse oximetry (with a goal of >88% SaO2).