The acute respiratory distress syndrome (ARDS) is an ínflammatory process of the lungs in response to pulmonary and systemic insults to the alveolar-capillary membrane resulting in alveolar protein-rich edema.Clinically, ARDS is characterized by severe hypoxemia, bilateral radiographic pulmonary infiltrates, and no clinical evidence of cardiogenic pulmonary edema.These criteria allow the inclusion of a highly heterogeneous group of patients.From a research and therapeutic perspective, a precise definition is crucial since the effects on outcome of ventilator and adjunctive techniques depend on the degree of lung injury.