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Cardiothoracic ratio

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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:

  • cardiac failure
  • pericardial effusion
  • left or right ventricular hypertrophy

Reference:

  • Wilson AG (1993). The chest radiograph in heart disease. Medicine International. 21:305-14.

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