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