Pulmonary oedema

Pulmonary oedema is a broad descriptive term and is usually defined as an abnormal accumulation of fluid in the extravascular compartments of the lung ¹.

Pathology

One method of classifying pulmonary oedema is as four main categories on the basis of pathophysiology which include:

• increased hydrostatic pressure oedema
  • two pathophysiological and radiological phases are recognised in the development of pressure oedema 
    • interstitial oedema
    • alveolar flooding or alveolar oedema
  • these phases are virtually identical for left-sided heart failure and fluid overload
• permeability oedema with diffuse alveolar damage
• permeability oedema without diffuse alveolar damage  
• mixed oedema due to simultaneous increased hydrostatic pressure and permeability changes

It can arise from a range of cardiogenic and non-cardiogenic causes.

Causes

• cardiogenic pulmonary oedema:
  • left heart failure
    • congestive cardiac failure
  • mitral regurgitation ^5,7
• non-cardiogenic pulmonary oedema: 
  • fluid overload
  • pulmonary oedema with acute asthma
  • post-obstructive pulmonary oedema/post intubation pulmonary oedema / negative pressure pulmonary oedema
  • pulmonary oedema in pulmonary thromboembolism
  • pulmonary oedema due to air embolism
  • pulmonary veno-occlusive disease
  • near drowning pulmonary oedema / asphyxiation pulmonary oedema
  • ARDS - pulmonary oedema with diffuse alveolar damage
  • heroin-induced pulmonary oedema
  • pulmonary oedema following administration of cytokines
  • transfusion-related acute lung injury
  • high-altitude pulmonary oedema
  • neurogenic pulmonary oedema
  • reperfusion pulmonary oedema
  • pulmonary oedema following lung transplantation
  • re-expansion pulmonary oedema
  • post-pneumonectomy pulmonary oedema
  • post lung volume reduction pulmonary oedema
  • pulmonary oedema from anti-snake venom administration ⁸

The causes of non-cardiogenic pulmonary oedema can be recalled with the following mnemonic: NOTCARDIAC.

Radiographic features

Plain radiograph

The chest radiograph remains the most practical and useful method of radiologically assessing and quantifying pulmonary oedema ^3-4.

Features useful for broadly assessing pulmonary oedema on a plain chest radiograph include:

• upper lobe pulmonary venous diversion / pulmonary venous engorgement / stag's antler sign
• increased cardio-thoracic ratio / cardiac silhouette size: useful for assessing for an underlying cardiogenic cause or association
• features of pulmonary interstitial oedema:
  • peri-bronchial cuffing and perihilar haze
  • septal lines / Kerley lines
  • thickening of interlobar fissures
• features of pulmonary alveolar oedema:
  • air space opacification classically in a batwing distribution 
  • may have air bronchograms
• pleural effusions and fluid in interlobar fissures (including 'vanishing' pulmonary pseudotumour)

There is a general progression of signs on a plain radiograph that occurs as the pulmonary capillary wedge pressure (PCWP) increases (see pulmonary oedema grading). Whether all or only some of these features can be appreciated on the plain chest radiograph, depend on the specific aetiology ¹. Furthermore, pulmonary oedema is usually a bilateral process, but it may uncommonly appear to be unilateral in certain situations and pathologies (see unilateral pulmonary oedema). 

Differential diagnosis

General imaging differential considerations include other causes of diffuse airspace opacification:

• diffuse pulmonary haemorrhage: has no dependent gradient and usually no pleural effusion
• diffuse pneumonia ⁶: usually no dependent gradient
• pulmonary alveolar proteinosis: usually no pleural effusion

See also

• pulmonary oedema grading
• unilateral pulmonary oedema