Adult respiratory distress syndrome is characterised by fulminant interstitial and alveolar oedema which usually develops 12 to 48 hours after an initial trauma. It results from increased alveolar capillary permeability and is not cardiogenic in origin. Injury may occur directly or as part of a generalised systemic acute inflammatory process.
Progressive pulmonary insufficiency ensues which may be self-limiting if the patient is adequately supported without exacerbating injury to the lung. Approximately a third of patients have a residual pulmonary disability due to pulmonary fibrosis.
Some advocate that ARDS, as opposed to acute lung injury, occurs only when there is severe abnormalities in pulmonary gas exchange - defined by a PaO2:FiO2 ratio < 20 KPa.