This is the transverse cardiac diameter (the horizontal distance between the most rightward and leftward borders of the heart seen on a postero-anterior (PA) chest radiograph) divided by the transverse chest diameter (measured from the inside rib margin at the widest point above the costophrenic angles on a PA chest film).
A cardiothoracic ratio of more than 50% is considered abnormal in an adult; more than 66% in a neonate (1).
The cardiac diameter itself can also be measured and, in normal individuals, is less than 15.5 cm in males, and less than 14.5 cm in females. A change in diameter of greater than 1.5 cm between two X-rays is significant.
Possible causes of a ratio greater than 50% include:
Reference:
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