Interstitial pulmonary edema
Initial elevation of pulmonary venous pressure leads to fluid transudation into the interstitial space. On radiographs and computed tomography scans, this process is characterized by a number of classic signs.
- Kerley B lines (Kerley B lines): smooth thickening of the interlobular septa that does not follow the hydrostatic gradient. Most prominent in the basal regions of the right lower lobe.
- Bronchial cuffing (cuffing): visual thickening of the bronchial walls due to the accumulation of interstitial fluid around them.
- General dilatation of the venous bed.
- Pleural changes: a small pleural effusion (pleural effusion) is frequently observed. It is important to differentiate true fluid accumulation in the interlobar fissures from their thickening due to subpleural interstitial edema.
- Overall parenchymal density can range from a ground-glass pattern (ground-glass) to dense consolidation (dense consolidation). The peribronchovascular interstitium in the perihilar regions is predominantly affected.
Alveolar pulmonary edema (Grade 3)
With a further increase in pressure, fluid enters the alveolar spaces. Nodular opacities appear against the background of Kerley B lines and eventually merge into bilateral symmetric consolidation in the middle and lower zones. With dense consolidation, air bronchograms (air bronchograms) are well visualized.
On CT, fluid distribution is often focal with a pronounced ventrodorsal gradient, while subpleural areas may remain spared. The distribution of edema may indicate its etiology:
- Bat-wing sign (perihilar bat's wing): perihilar airspace consolidation. Most commonly accompanies left ventricular or renal failure.
- Right upper lobe edema: pathognomonic for severe mitral regurgitation, as the blood jet is directed straight into the right upper lobe pulmonary vein.
- Upper lobe edema: characteristic of neurogenic edema (neurogenic oedema) associated with severe traumatic brain injury.
- Gravitational effect: in supine patients, fluid accumulation depends on body position. Prolonged positioning on one side causes asymmetric consolidation that can mimic pneumonia.
Cardiomegaly and atypical patterns
Most cases of pulmonary venous hypertension present with cardiomegaly due to myocardial or valvular dysfunction. However, pulmonary edema with a normal cardiac silhouette size also occurs. This is observed in acute myocardial infarction (within the first 24–48 hours due to a sharp decrease in myocardial compliance), as well as in non-cardiogenic factors such as acute hypervolemia or drug toxicity (aspirin, heroin, nitrofurantoin).
Pre-existing pulmonary diseases can significantly alter the radiographic appearance. For example, patients with extensive emphysema do not develop homogeneous consolidation. Fluid is retained within thickened septa, preserving an interstitial distribution pattern. As a result, the imaging findings appear less alarming, which may lead to an underestimation of the condition's severity.
Chronic pulmonary venous hypertension, in turn, can lead to pulmonary arterial hypertension (PAH / PAH). A specific sign of prolonged congestion (e.g., in severe mitral stenosis) is a fine nodular interstitial pattern formed by hemosiderin deposits.
Briefly: answers to topical questions
What types of Kerley lines exist other than type B, and where are they located?
In addition to type B, diagnostic radiology identifies Kerley A lines.
- Kerley A lines are longer (up to 80–100 mm), sometimes angulated (angulated), and course through the inner two-thirds of the lung in various directions.
By comparison, Kerley B lines are 30–60 mm long and 1–2 mm wide. They are well visualized exclusively in the subpleural regions of the lungs (most prominent in the basal aspects of the right lower lobe), lying strictly perpendicular to the pleural surface.
What are the radiographic stages of cardiogenic pulmonary edema development?
Three radiographic stages of cardiogenic pulmonary edema development are distinguished based on the pulmonary venous pressure level.
- Grade 1 (Grade 1) — blood volume redistribution (occurs at a pressure of 12–20 mm Hg).
- Grade 2 (Grade 2) — interstitial edema (develops at a pressure of 20–25 mm Hg).
- Grade 3 (Grade 3) — alveolar edema (occurs at a pressure exceeding 25–30 mm Hg).
In the pathological sequence, fluid first transudes into the interstitium and then accumulates in the alveolar spaces; therefore, signs of interstitial edema precede intra-alveolar opacification.
The causes of localized edema are easy to remember: Right Upper lobe — Valvular mitral disease (jet directed upward and rightward); Upper lobes symmetrically — Head trauma (neurogenic).
Class and exam questions
Why do bronchi appear thickened on chest X-rays in pulmonary edema?
This is the so-called bronchial cuffing sign. Due to increased pressure, fluid transudes into the interstitial space and accumulates around the bronchial walls, visually increasing their thickness.
How to differentiate pulmonary edema from pneumonia in a bedridden patient?
In bedridden patients, alveolar fluid obeys gravity. If a patient lies on one side for a prolonged period, asymmetric consolidation develops, which outwardly mimics unilateral pneumonia.
Can cardiogenic pulmonary edema occur without cardiomegaly?
Yes, this occurs in the first 24–48 hours of acute myocardial infarction. Myocardial compliance drops sharply, and edema develops before the heart has time to undergo compensatory enlargement.
What else to study on this topic
- Criteria for pulmonary arterial hypertension (PAH) on right heart catheterization
- The role of CT angiography (CTPA) in the differential diagnosis of PAH
- Mechanism of hemosiderin deposit formation in mitral stenosis
- Differential diagnosis between fluid accumulation in the interlobar fissure and subpleural interstitial thickening
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