Pulmonary Oedema

  • Excessive fluid in the lung
  • Normally, mechanisms are in place to protect the lung from the entry of circulatory fluid into alveolar spaces (See functional anatomy)
  • Occurs when exudation of fluid from vessels into interstitium or alveoli exceeds the rate of alveolar or lymph removal
  • Generally a sequel to or part of congestion or inflammatory process
  • Generally begins as interstitial oedema characterised by expansion of perivascular and peribronchial and peribronchiolar fascia and distension of interstitial lymphatics
  • Only when this interstitial compartment is overwhelmed does fluid flood the airspaces causing alveolar oedema
  • Gross pathology:
    • Heavy wet lungs which do not properly collapse
    • Subpleural and interstitial tissue distended with fluid
    • Foamy fluid oozing from the cut surface and airways
  • Micro pathology:
    • Pinkish fluid in alveoli and airways in association with air bubbles, and also in dilated lymphatics of the interstitium
    • Colour of the fluid enhanced in cases where the endothelium is damaged - more protein present
    • In slowly developing cases, macrophages contain haemosiderin
  • The major causes of pulmonary oedema are:
    • Increased capillary or type I epithelial permeability caused by
      • Systemic toxins
      • Shock
      • Inhaled caustic gases
    • Increased capillary hydrostatic pressure (cardiogenic oedema - left-sided or biventricular heart failure, sympathetic stimulation in acute brain damage)
    • Decreased plasma oncotic pressure (hypoalbuminaemia)
    • Overloading in excessive fluid therapy
    • As part of inflammatory process