Effects of Positive End Expiratory Pressure (PEEP) Decrease in Patient With Acute Respiratory Distress Syndrome (ARDS) Ventilated According to High PEEP Strategy (ExPress Trial).

The perfusion of the deeply different areas coexisting in ARDS lung generates two type of venus-admixure : true shunt and shunt effect. Briefly, true shunt and shunt effect are related to non-aerated and poorly-ventilated lung areas perfusion respectively. Practically, it is not possible to quantify the true shunt and the shunt-effect directly, but we can measure the "total-shunt", i.e. the sum of two kind of venus-admixured. As it is known from classical physiology, the effects of FiO2 variations on PaO2 are deeply different depending on the type of venous admixures. In the case of a patient with virtually only true shunt, any FiO2 variation will not modify the PaO2. On the other hand, in a patient with virtually only a shunt effect, increasing the FiO2 the PaO2 will progressively increase. Indeed, ventilating a patient presenting only a shunt effect (without true shunt) with pure oxygen will generate a "normal" P/F ratio. In patients ventilated with high PEEP levels it is difficult to predict based on the P/F ratio the relative amount of true shunt and shunt effect. However, patients presenting a significant shunt effect when ventilated with higher PEEP level would likely have "unstable" poorly aerated lung areas that need PEEP to remain opened. In those patients, a PEEP-weaning protocol based on the P/F ratio could induce significant alveolar de-recruitment and clinical deterioration. On the contrary, patients with less shunt effect would be likely less prone to alveolar de-recruitment and would benefice from PEEP decrease.