Pathophysiology and main forms
The development of pulmonary edema is associated with a disruption of fluid balance between the compartments of the lung tissue. Two main types are distinguished:
- Hydrostatic (cardiogenic) edema. Occurs when fluid shifts from the vascular bed due to increased venous or capillary pressure. The most frequent cause is left heart failure. A similar effect is produced by a decrease in plasma osmotic pressure (e.g., in patients with hypoalbuminemia).
- Non-cardiogenic pulmonary edema. Develops against the background of increased permeability of the alveolar-capillary barrier. A classic example of such a condition is acute respiratory distress syndrome (ARDS, ARDS).
Regardless of the cause, the pathological sequence is the same: fluid first transudates into the interstitium and then fills the alveoli.
Radiography: stages and symptoms
Chest radiography (CXR) is more sensitive than clinical examination in the early stages. Radiologic signs are divided into three groups:
- Vascular changes. Pulmonary venous pressure rises, causing a redistribution of blood flow from the basal regions to the apices (cephalization). To assess dilatation, the diameters of the adjacent pulmonary artery and bronchus are compared.
- Interstitial edema. Manifests as thickening of the interlobular septa — prominent septal lines (Kerley B lines) are visualized on the images.
- Intra-alveolar edema. Fluid fills the acini. The "bat's wing" sign (Bat's Wing) is observed — bilateral opacification of the central regions, while the apices, bases, and peripheral zones remain intact. Confluence of shadows to the pattern of "white-out" (white-out) with an air bronchogram is possible.
Important: In bedridden patients, fluid accumulates in dependent (lowermost) regions, which can produce a pattern of unilateral involvement.
Computed tomography in edema
Thin-section CT reveals the smallest changes, including subclinical ("silent") edema in ICU patients. Normally, CT visualizes interlobular septa (containing venules and lymphatic vessels) and the centrilobular arteriole, whereas the wall of the terminal bronchiole is not visualized due to its small thickness.
Early CT signs of edema:
- Increase in the background density of lung tissue (due to increased intravascular volume).
- Vascular dilatation in the perihilar zones (and in the anterior regions — as an equivalent of cephalization).
- Scant smooth septal thickening and focal ground-glass opacification (ground-glass opacification).
With progression, peribronchovascular cuffs, consolidation, and pleural effusion appear.
Additional finding: In congestive heart failure, ~50% of patients show reversible enlargement of mediastinal lymph nodes and "hazy" attenuation of the mediastinal fat.
Differential diagnosis
It is radiologically impossible to reliably differentiate hydrostatic edema from non-cardiogenic edema. Blood flow cephalization is typical for the former, and peripheral distribution for ARDS, but in practice, the forms often overlap. Assessing the width of the vascular pedicle lacks sufficient diagnostic value.
A pattern of rapidly changing airspace opacities similar to edema is produced by diffuse pulmonary hemorrhage (DPH), for example, in idiopathic hemosiderosis or anti-basement membrane antibody disease (Goodpasture syndrome). Blood in the alveoli causes consolidation and ground-glass opacity, but like edema fluid, it is rapidly (over a few days) cleared by macrophages. The absence of distortion of the parenchymal architecture and smooth septal thickening help differentiate cardiogenic edema from carcinomatous lymphangitis and sarcoidosis.
Briefly: answers to topical questions
What types of Kerley lines exist and where are they visualized on a radiograph?
Two main types of Kerley lines are distinguished on a radiograph, corresponding to thickened pulmonary septa.
- Kerley A lines (Kerley A lines) correspond to thickened deep septa. They are longer (up to 80–100 mm), sometimes curved, and cross the inner two-thirds of the lung, heading medially toward the lung root (hilum).
- Kerley B lines (Kerley B lines) correspond to thickened interlobular septa. They have a width of 1–2 mm and a length of 30–60 mm. They are well visualized in the subpleural regions of the lungs perpendicularly to the pleural surface, and are most pronounced in the basal regions of the right lower lobe.
What diseases should cardiogenic pulmonary edema be differentiated from?
Cardiogenic pulmonary edema should be differentiated from non-cardiogenic pulmonary edema, as well as from other causes of interlobular septal thickening and airspace opacities:
- Non-cardiogenic pulmonary edema — occurs when the permeability of the alveolar-capillary barrier increases; a classic example is acute respiratory distress syndrome (ARDS, ARDS).
- Carcinomatous lymphangitis — a malignant infiltration that can also lead to the visualization of thickened interlobular septa.
- Sarcoidosis.
- Fibrosis.
- Infection — one of the frequent causes of airspace opacities.
Speed helps differentiate pulmonary edema from tumor or infection: look for ultra-fast dynamics. Edema can dramatically change its localization or density in just hours, especially with a change in the patient's position.
Class and exam questions
Can an imaging scan tell for sure whether the edema is cardiogenic or ARDS?
No, radiological differentiation of these forms is considered unreliable, as they often coexist in practice. However, blood redistribution to the upper lobes is more typical for hydrostatic (cardiogenic) edema.
What is the "bat's wing" sign?
This is a classic radiological pattern of intra-alveolar edema, in which the central (perihilar) regions of the lungs are symmetrically opacified, while the periphery, apices, and bases remain air-filled.
Why do lymph nodes enlarge in heart failure?
This is a reactive change associated with fluid overload of the lymphatic system. The enlargement is reversible: against the background of adequate therapy, the lymph nodes rapidly decrease in volume.
What else to study on this topic
- Anatomy of the secondary pulmonary lobule on thin-section CT
- Diffuse pulmonary hemorrhage: CT imaging and clinical presentation
- Anti-basement membrane antibody disease (Goodpasture syndrome)
- Idiopathic pulmonary hemosiderosis in children
- Radiologic semiotics of interstitial lung diseases
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