In an adult, the cardiothoracic ratio should be less than 50% on a posterior-anterior (P-A) film.

Fig 1 highlights a cardiothoracic ratio of a/b = <50% Where:

• a = maximum transverse diameter of the heart
• b = maximum internal diameter of the thorax

NOTE: A misleadingly high cardiothoracic ratio can result from: Taking an A-P film rather than a P-A The heart being pushed more horizontally by upward abdominal pressure, such as obesity or pregnancy