On the PA view of the chest, there are three visible cardiac segments (Fig 57:;1a). The lowermost segment, adjacent to the diaphragm, is the lateral and apical walls of the left ventricle. A small rounded density may be lateral to the cardiac apex; this represents the epicardial fat pad. Superior to the left ventricle is a rounded short segment that varies considerably in size. This is the pulmonary artery and its left main branch. Prominence of the pulmonary artery is normal in young women. Just below this level, the left atrial appendage forms a short portion of the lateral portion of the left heart border. Conditions that result in left atrial enlargement will cause a convex bulge in this portion of the cardiac silhouette. The third and most superior portion of the left edge of the cardiac silhouette is formed by the transverse aortic arch. On a properly exposed chest film, the descending aorta can be seen descending behind the heart shadow. Regarding the right side of the cardiovascular silhouette, the lower rounded portion is due to the lateral border of the right atrium. The upper segment is formed by the superior vena cava. In older adults in whom the ascending aorta becomes more rounded and dilated, the upper segment of the right heart border may become convex, representing the lateral border of the ascending aorta.
FIG. 57-1. Normal cardiac size and lung fields. A. Frontal view. The lateral wall of the left ventricle (large black arrow), the pulmonary artery (small black arrow), and the aortic arch (black arrowhead) form three visible cardiac segments on the left side. On the right side, the right atrium ( white arrow) and superior vena cava (white arrowhead) form lower and upper borders, respectively. B. Lateral view. The left ventricle (arrow) forms the lower posterior cardiac border, and the left atrium is superior to this (arrowhead.)
On the lateral view of the chest, the anterior and superiormost shadow of the cardiac silhouette is formed first by the ascending aorta, and then the pulmonary artery and the pulmonary artery outflow tract, and then the anterior border of the right ventricle ( Fig 5.7z.1.,b). Posteriorly, the superior shadow is due to the left atrium, with the left ventricle forming the cardiac silhouette inferiorly.
Early left ventricular failure is reflected on the chest x-ray by a "redistribution" pattern of blood flow. Normally, the most prominent pulmonary veins are at the lung bases. With the redistribution phase, the vascularity in the upper lung fields is increased, relative to that of the lower lung fields.
As pulmonary capillary wedge pressure elevates to 20 to 25 mmHg (Fig 57-2), fluid moves from the vascular bed into the interstitial spaces of the lung. The chest x-ray appears hazy at this stage, with short lines appearing perpendicular to the pleura (Kerley B lines). Finally, as pulmonary capillary wedge pressure further increases to 30 to 35 mmHg, frank alveolar edema ensues due to massive fluid movement from the vasculature to the alveolar spaces.
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