Cardiothoracic ratio

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.

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

1. Kelly B. The chest radiograph. Ulster Med J. 2012 Sep;81(3):143–148.