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3 clinical and pathological phases of ARDS

Initial phase

  • lasts 3-5 days
  • severe defect in oxygenations
  • ¯ lung compliance
  • bilateral pulmonary infiltrates
  • endothelial and epithelial cell injury ̃ ­ permeability of endothelial and epithelial barriers ̃ accumulation of protein-rich oedema fluid in interstitium and air spaces. Contains RBCs, WBCs and hyaline membranes.
  • abnormal surfactant/inactivation of surfactant
  • neutrophil sequestration and migration in lung are histological hallmarks of ARDS. Result from both chemotactic stimuli released within lung and activation of neutrophils by circulating mediators
  • followed either by recovery or progression to sub-acute phase


  • starts 5-7 days after onset
  • increased alveolar dead space
  • persistently decreased lung compliance
  • persistent oxygenation defect
  • CXR: persistent, largely unchanging bilateral infiltrates
  • Histology: interstitial fibrosis with proliferation of alveolar type II cells. Obstruction and destruction of portions of microcirculation of lung.
  • Followed by recovery or progression to chronic phase


  • After 14 days there is a gradual transition to chronic phase
  • Persistent low compliance
  • Markedly increased VD/VT (>0.6)
  • Extensive pulmonary fibrosis with obliteration of normal alveolar architecture and progressive development of emphysematous regions of lung including development of discrete bullae.

Pulmonary vascular changes

  • early in ARDS pulmonary vasoconstriction, thromboembolism and interstitial oedema, all of which are potentially reversible, can increase pulmonary vascular resistance resulting in increased pulmonary artery pressure
  • after several weeks, more permanent structural changes, such as fibrous obliteration of the microcirculation and increased arterial muscularization, contribute to pulmonary hypertension.

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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