Pulmonary edema is a broad descriptive term and is usually defined as an abnormal accumulation of fluid in the extravascular compartments of the lung 1.
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Clinical presentation
The clinical presentation of pulmonary edema includes:
acute breathlessness
orthopnea
paroxysmal nocturnal dyspnea (PND)
foaming at the mouth
distress
Pathology
One method of classifying pulmonary edema is as four main categories on the basis of pathophysiology which include:
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increased hydrostatic pressure edema
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two pathophysiological and radiological phases are recognized in the development of pressure edema
these phases are virtually identical for left heart failure and fluid overload
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permeability edema with diffuse alveolar damage (DAD)
permeability edema without diffuse alveolar damage (DAD)
mixed edema due to simultaneous increased hydrostatic pressure and permeability changes
Etiology
Broadly causes can be classified as cardiogenic and non-cardiogenic:
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arrhythmias
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myocardial pathology
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non-cardiogenic pulmonary edema:
postobstructive pulmonary edema / postintubation pulmonary edema/negative pressure pulmonary edema
near-drowning pulmonary edema / asphyxiation pulmonary edema
ARDS: pulmonary edema with diffuse alveolar damage
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activity-related
The causes of non-cardiogenic pulmonary edema 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 edema 3,4.
Features useful for broadly assessing pulmonary edema on a plain chest radiograph include:
increased cardiothoracic ratio/cardiac silhouette size: useful for assessing for an underlying cardiogenic cause or association
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features of pulmonary interstitial edema:
peribronchial cuffing and perihilar haze
thickening of interlobar fissures
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features of pulmonary alveolar edema:
air space opacification classically in a batwing distribution
may have air bronchograms
pleural effusions and fluid in interlobar fissures (including 'vanishing' pulmonary pseudotumor)
A useful mnemonic is ABCDE.
There is a general progression of signs on a plain radiograph that occurs as the pulmonary capillary wedge pressure (PCWP) increases (see pulmonary edema grading). Whether all or only some of these features can be appreciated on the plain chest radiograph, depend on the specific etiology 1. Furthermore, pulmonary edema is usually a bilateral process, but it may uncommonly appear to be unilateral in certain situations and pathologies (see unilateral pulmonary edema).
CT
Interstitial pulmonary edema is most commonly demonstrated by the following CT signs 7:
bronchovascular bundle thickening (due to increased vascular diameter and/or peribronchovascular thickening)
Alveolar edema is demonstrated by airspace consolidation in addition to the above findings.
Pleural effusions are a frequent accompanying finding in cardiogenic/hydrostatic pulmonary edema.
Mediastinal and/or hilar lymph node enlargement can occur with congestive cardiac failure 12.
Ultrasound
The appearance of pulmonary edema is defined as a function of the perturbation of the air-fluid level in the lung, a spectrum of appearances coined the alveolar-interstitial syndromes.
As subpleural interlobular septa thicken among air-filled alveoli, they create a medium in which incident ultrasound waves will reverberate within, creating a short path reverberation artifact. Referred to as B-lines, these are pathological when more than three appear, garnering the title lung rockets, and consistent with thickened interlobular septa. When spaced 7 mm apart they correlate with radiographic interstitial edema and when 3 mm apart with ground glass opacification. When surrounding alveoli become fluid-filled, the resultant interface assumes a tissue-like pattern. The tissue-like sign and shred sign are pathognomonic 10.
Differential diagnosis
General imaging differential considerations include other causes of diffuse airspace opacification:
diffuse pulmonary hemorrhage: has no dependent gradient and usually no pleural effusion
diffuse pneumonia 6: usually no dependent gradient
pulmonary alveolar proteinosis: usually no pleural effusion
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