Lung units that participate in gas exchange are known as 'recruited' lung. Patients with lung injury suffer from a proportion of units that do not participate in gas exchange (i.e. the derecruited lung), which results in impaired gas exchange and induces an inflammatory cascade. The level of PEEP is often coupled to indices of oxygenation such as PaO2, PaO2 to FIO2 ratio, or oxygen index. Currently, two strategies are widely accepted and considered equivocal, one strategy using a lower PEEP level coupled to a certain oxygen requirement, the other using a higher PEEP level.