Causes may include sepsis, pancreatitis, trauma, pneumonia, and aspiration.[1] The underlying mechanism involves diffuse injury to cells which form the barrier of the microscopic air sacs of the lungs, surfactant dysfunction, activation of the immune system, and dysfunction of the body's regulation of blood clotting.[4] In effect, ARDS impairs the lungs' ability to exchange oxygen and carbon dioxide.[1] Adult diagnosis is based on a PaO2/FiO2 ratio (ratio of partial pressure arterial oxygen and fraction of inspired oxygen) of less than 300 mm Hg despite a positive end-expiratory pressure (PEEP) of more than 5 cm H2O.[1] Heart-related pulmonary edema, as the cause, must be excluded.[3]