Radiographic signs of left-sided heart failure include left atrial enlargement, pulmonary venous engorgement, peribronchial pattern, and air bronchograms. The constellation of these findings provides compelling evidence of pulmonary edema.
Left-sided heart failure occurs when the heart loses its ability to pump oxygenated blood to the rest of the body efficiently. This can result in the "backing up" of blood pressure from the left ventricle, to the left atrium, to the pulmonary veins and finally the pulmonary capillary beds. This elevated hydrostatic pressure in the pulmonary capillaries disturbs Starling Forces across the capillary membrane causing fluid to "leak" into the pulmonary interstitium resulting in edema. Radiographic signs of left sided heart failure therefore begin with evidence of pulmonary venous engorgement. As fluid "leaks" into the pulmonary interstitium it collects along the triad of artery, bronchus, and vein causing the so called "peribronchial pattern", and silhouetting of the margins of the pulmonary arteries and veins. And finally, as fluid continues to "leak" into the interstitium it fills the alveoli causing the radiographic finding of "air bronchograms". Patients often present with a history of exercise intolerance due to the reduced amount of oxygenated blood being pumped to the rest of the body by the left ventricle. Since fluid will be leaking out of the vasculature due to increased hydrostatic pressure, the patient may also have trouble breathing due to fluid in the lungs.
The increase in hydrostatic pressure results in the passage of fluid from the capillaries, through their loose junctions, into the surrounding interstitium. At this stage, peribronchial patterns would likely be detectable on radiographs.
Copyright (C) 1998, Lynne Larson .All rights reserved.
As the pressure increases, more fluid will pour out into the interstitium and the surrounding alveoli. At this stage, air bronchograms would likely be detectable on radiographs.
Copyright (C) 1998, Lynne Larson .All rights reserved.
As the left side of the heart fails, that is less blood is ejected and venous return continues, then preload (fluid volume) to the left side of the heart becomes elevated. The elevated volume in the pulmonary venous system results in dilation or pulmonary venous engorgement. As the volume continues to increase, pressures in the pulmonary venous system become elevated and also in the pulmonary capillary bed resulting in a net efflux of fluid into the pulmonary interstitium. Venous engorgement is the first sign of increased volume in the left heart, making their detection on radiographs extremely important.
Radiographic signs of pulmonary venous distension:
See also Pulmonary Venous Distension in the Clinical Evaluation of the Heart.
As mentioned previously the elevated capillary hydrostatic pressure causes fluid to "leak" into the pulmonary interstitium where it collects along the triad of artery, bronchus, and vein causing the so-called "peribronchial pattern", and silhouetting of the margins of the pulmonary arteries and veins. With respect to the development of a peribronchial pattern, the fluid weeps down both the inside and outside of an airway. The walls of the tertiary bronchioles are not detectable radiographically due to the fact that they no longer have cartilaginous rings. However, once the inner and outer wall of the bronchiole is covered in fluid, it becomes more radiopaque and therefore contrasts very well against the radiolucent lumen (filled with air). As a result, on cross section the bronchiole appears as a radiopaque circle with a radiolucent center ("donut"), and if cut longitudinally it appears as two parallel radiopaque lines ("railroad track").Hint: Where there are "donuts", there are likely "railroad tracks". The bronchioles have a branching arrangement and therefore many branches of them will be located in the same area, but cut in different planes.
Peribronchial Pattern - example 1
Peribronchial Pattern - example 2
Peribronchial Pattern - example 3
Note: Realize that a peribronchial pattern is a term given to a radiographic sign that it caused by fluid lining either the inside or outside of an airway resulting in a "donut" (on cross-section) or a "railroad track" (if cut longitudinally). The fluid that is responsible for the peribronchial pattern can be of almost any nature: fluid of edema, hemorrhage, excessive production of mucous etc. Therefore, the presence of peribronchial patterns alone does not conclude that a patient has pulmonary edema. The suspicion does increase however when there are signs of left atrial enlargement, pulmonary venous engorgement, peribronchial pattern, and/or air bronchograms.
The final stage in the development of pulmonary edema and the most advanced stage is represented by the radiographic finding of air bronchograms. Air bronchograms are formed when continued fluid accumulation occurs into the alveoli themselves, resulting in alveolar edema. When the alveoli accumulate fluid, they appear radiopaque and only the air filled bronchioles remain visible. As you can see from the picture (side), the air bronchograms are easily detectable against such an opaque, fluid-filled lung. It is important to note that usually peribronchial patterns will be seen before air bronchograms during the stages of pulmonary edema development. Some areas of the lung however, will be at various stages and may show a mix of both peribronchial patterns and air bronchograms
Air Bronchogram - example 1
Air Bronchogram - example 2
Note: Realize that air bronchograms occur when fluid accumulates in the alveoli of the lung causing alveolar edema. Alveolar edema causes a sharp contrast between the fluid filled alveoli and the patent airway. Much like peribronchial patterns, air bronchograms will result regardless of the origin of the fluid and therefore can't be automatically credited to pulmonary edema.
Students frequently have difficulty distinguishing between peribronchial patterns and air bronchograms
The difference between these two radiographic signs is the ability to visualize the outer membrane of the airway wall. With a peribroncial pattern the outside and the inner airway walls are visualized (because fluid collects only along the airway wall and not into the adjacent alveoli and so there is radiographic contrast between the airway wall (in white) and the adjacent alveoli (in black)). With an air bronchogram only the inner airway wall, and hence lumen, is visible. The outer airway wall is obscured/silhouetted because the adjacent alveoli are flooded and thus the radiographic density of the immediate region around the airway is of the same density (radiographic color) as the airway wall itself. The absence of radiographic contrast between the airway wall and the adjacent region results in the loss of the visualization of the outer airway wall.
A lobar sign is present when a sharp line of demarcation is noted between a lung lobe that is opaque (consolidated) and adjacent to one that is either normally aerated or near normally aerated. Hence in a lobar sign, the margin of the consolidating process must extend to the margin of the lung lobe to provide the sharp radiographic contrast between the non-aerated and aerated lung lobes. Just like with fissure lines, one must ensure that this line of demarcation occurs where lung lobe fissures actually exist.
Lobar signs are seen when a patient has fluid accumulation in the alveoli and parenchyma and as a result, the affected lung lobe is radiopaque compared to its radiolucent adjacent lobes. The presence of a lobar sign indicates alveolar disease due to the presence of exudates, edema, and/or hemorrhage in that area.
Lobar Signs - example 1
Lobar Signs - example 2
As a general rule lobar signs are not observed in the setting of cardiogenic pulmonary edema. In example 2, one can see the lobar sign associated with the right cranial lung lobe. If the lobar sign was due to increased pressures within the vasculature, all lobes of the lung should be affected equally. Lobar signs suggest a focal process whereas pulmonary edema is a more diffuse process.