The heart casts a homogeneous shadow on the chest film. No internal detail can be seen within its contours because the radiodensities of blood, myocardium, and other cardiac tissues are so similar that one cannot be distinguished from the others. Only two borders of the heart, where it contacts the radiolucent, air-containing lung, can be discerned in any one projection.
[...] Air-containing lung interposed between this portion of the heart and the anterior chest wall forms the "retrosternal clear space." The posterior border of the heart extends from the level of the pulmonary carina to the diaphragm. Its upper half is formed by the back of the left atrium, and the lower half represents the posterior wall of the left ventricle. The shadow of the inferior vena cava is usually seen in the lateral projection extending obliquely upward and anteriorly from the diaphragm to enter the posterior aspect of the right atrium. [...]
[...] They can be separated by fluoroscopy as the aortic valve tends to move in a vertical direction as the heart beats, while the motion of the mitral valve approximates the horizontal. This distinction can also be accurately made from the lateral chest film. If a line is drawn from the left main bronchus, seen as a dark circular shadow over the lower extreme of the trachea, to the anterior costophrenic angle, then the mitral valve lies below the line and the aortic valve is above it. [...]
[...] This redistribution of the pulmonary vasculature is a reliable sign of pulmonary venous hypertension, although it is often difficult to recognize unless quite marked. With a sufficient further increase in venous pressure, pulmonary edema develops. PULMONARY EDEMA Normally, extravascular circulation of fluid in the lungs from the capillaries through the interstitium and back to the blood stream by way of the lymphatics is constant. When pulmonary venous pressure increases, more and more fluid leaks from the capillary bed, the capacity of the lymphatics to remove the fluid is exceeded, and the interstitium becomes waterlogged. [...]
[...] Although pulmonary arteries and veins also become abnormally prominent in heart failure, the vessels are not usually sharply outlined, and additional signs of pulmonary venous hypertension or interstitial edema are present. The vessels to the lower lobes carry about 60 to 70% of the pulmonary blood flow and are normally of greater caliber than the vessels to the upper lobes. As pulmonary venous pressure increases, the lower lobe vessels become constricted, so more blood is distributed to the upper lobes, which makes their vessels more prominent. [...]
[...] It forms the part of the left heart border between the pulmonary artery segment and the left ventricular segment. Normally, the border of the appendage is flat or slightly concave. Any convexity is abnormal and usually indicates left atrial enlargement. LEFT VENTRICLE The shape of the dilated left ventricle depends to a large extent on the underlying cause. When it is due to insufficiency of the aortic or mitral valve, the ventricle elongates and its apex is displaced downward, to the left, and posteriorly. [...]
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