This common medical emergency is precipitated by a variety of acute processes that directly or indirectly injure the lung, e.g., primary bacterial or viral pneumonias, aspiration of gastric contents, direct chest trauma, prolonged or profound shock, burns, fat embolism, near-drowning, massive blood transfusion, cardiopulmonary bypass, oxygen toxicity, or acute hemorrhagic pancreatitis. The incidence of ARDS is estimated to be over 30% following the "sepsis syndrome" characterized by leukocytosis or leukopenia, fever, hypotension, and a known potential source of systemic infection, whether or not blood cultures are positive for a bacterial pathogen. Patients usually have not had previous lung disease. It usually develops within 24-48 hours after the initial injury or illness. Dyspnea occurs first, usually accompanied by rapid, shallow respiration. Intercostal and suprasternal retraction may be present on inspiration. The skin may appear cyanotic or mottled,, and may not improve with oxygen administration. This CXR shows diffuse bilateral alveolar infiltrates similar to acute pulmonary edema of cardiac origin, except that the cardiac silhouette is usually normal. However, CXR changes often lag many hours behind functional changes and the hypoxemia may seem disproportionately severe compared to the edema observed by CXR.