Cardiomegaly refers to excessive heart size in proportion to the diameter of the rib cage. Heart size is usually estimated based on the cardiothoracic ratio, which compares the maximum width of the cardiac silhouette on a frontal chest radiograph ("CXR") with the maximum internal diameter of the rib cage. Cardiomegaly is usually present if the maximum width of the cardiac silhouette is more than 50% of the maximum internal diameter of the rib cage.

However, not every heart that has a cardiothoracic ratio > 50% is automatically abnormal. There are a number of extracardiac causes for apparent cardiomegaly on CXR. One common cause is poor inspiratory effort; the patient has not inhaled completely, and the resultant lower air pressure within the chest cavity gives the heart a larger appearance. This is commonly seen in obese and pregnant patients, as well as many patients with severe respiratory complaints. Structural abnormalities may also cause "pseudocardiomegaly:" both pectus excavatum, an inward bowing of the sternum, and straight back syndrome, straightening of the normal curvature of the thoracic spine may compress the heart, producing a deceptively wide cardiac silhouette. Film technique may also increase the apparent size of the heart. If a chest film is portable, and obtained in the anteroposterior projection, the distance of the heart from the film will cause some magnification.

DIFFERENTIAL DIAGNOSIS:

  • Acute Pulmonary Embolism: Cardiomegaly is the most common chest radiographic abnormality associated with acute pulmonary embolism
  • Congestive Heart Failure
  • Pulmonic Valve Atresia: Nonperforation of the pulmonic valve of the heart
  • Sickle Cell Anaemia
  • Tetralogy of Fallot
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