What are the definitions of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS)?
ALI and ARDS both reflect permeability pulmonary edema secondary to an insult to the alveolar-capillary unit. The inciting insult is more typically at the level of the vascular endothelium and is mediated via a neutrophil-dependent inflammatory cascade. There is a long list of causes that includes both intrathoracic (e.g., pneumonia, inhalational injury) and extrathoracic (e.g., pancreatitis, sepsis) etiologies.
From a clinical standpoint, ALI is defined as an acute lung disease with bilateral pulmonary opacities on a chest radiograph, a pulmonary wedge pressure of 18 mm Hg or less, and a ratio of arterial oxygen to the fraction of inspired oxygen (PaO 2 /FiO 2 ) of 300 mm Hg or less. ARDS is a more severe form of ALI and is defined by a PaO 2 /FiO 2 ratio of 200 mm Hg or less.