The chest x-ray is often the first imaging test obtained for patients with suspected cardiovascular disease. Ideally, lateral and posteroanterior (PA) views of the chest are obtained, with film exposure at a distance of 6 ft (1.8 m). Patients should be upright and have a moderately deep inspiration. For patients who are too ill to stand, semierect x-rays are obtained with an anteroposterior (AP) film exposure. The AP film has two disadvantages: the heart demonstrates magnification, and the pulmonary vessels, which show information regarding the extent of congestive failure, are somewhat blurred because of longer exposure times.
Cardiac size can be measured from the chest x-ray with an error of ± 10 percent. Unfortunately, borderline cases require accurate measurements, and factors such as depth of inspiration, conformation of the chest wall, and pulmonary diseases all may affect the apparent heart size on a chest x-ray. The cardiothoracic ratio is the ratio between the transverse diameter of the heart on a PA chest x-ray and the greatest internal diameter of the thorax. Although there are other formulas for measuring cardiac size, this method is the quickest and easiest. The normal adult heart measures 50 percent or less of the transverse diameter of the chest. Cardiothoracic ratios greater than 50 percent are considered to be cardiomegaly. In children, the normal cardiothoracic ratio is 65 percent in the first year, decreasing to 50 percent by age 5.
On the chest x-ray, the heart and major vessels are visualized in the middle mediastinum and are readily recognized. The heart as well as the great vessels all appear to have the same density, relative to the radiolucent lungs. On the left side, there is usually air around the cardiac border, but, on the right side, the heart blends in with the density of the liver. In normal individuals, two-thirds of the heart lies to the left of the midline.
Was this article helpful?