Causes of Lung Injury in ARDS
the alveolar-capillary membrane is injured, which is normally only permeable to small molecules. Lung tissue normally remains relatively dry, but in patients with ARDS lung fluid increases and contains high levels of proteins. Surfactant (which maintains elasticity of lung tissue and prevents alveolar collapse) activity is decreased in ARDS. The alveoli become unstable, and tend to collapse unless they are filled with fluid, either way they cannot exchange gases. Transfusion related acute lung injury (TRALI) associated with the activation of the inflammatory response due to a recent transfusion of plasma containing products (PRBCs, platelets, FFP) are associated with ARDS in critically ill patients. When a patient is being transfused they are being exposed to plasma that contains foreign proteins and antibodies. This exposure activates WBCs and causes agglutination (clumping) of the foreign proteins and WBCs, which then travel to the lung. The clumped material injures the pulmonary capillaries causing capillary leak and additional inflammation. Common causes of acute lung injury: Shock, trauma, serious nervous system injury, pancreatitis, fat and amniotic fluid emboli, pulmonary infections, sepsis, inhalation of noxious gases, pulmonary aspiration, drug ingestion, hemolytic disorders, multiple blood transfusions, cardiopulmonary bypass, near drowning (esp. in fresh water). In many cases of ARDS, especially after trauma clot production is increased, and fibrinolysis (clot breakdown) is reduced. As a result small emboli remain in the lung.