Revision 9 for 'Cardiothoracic ratio'

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Cardiothoracic ratio

The cardiothoracic ratio aids in the detection of cardiomegaly, or more broadly, enlargement of the cardiac silhouette

It is measured on a PA chest x-ray, and is the ratio of:

  1. maximal horizontal cardiac diameter
  2. maximal horizontal thoracic diameter (inner edge of ribs / edge of pleura)

A normal measurement should be less than 0.5. 

It should be noted however that this is a coarse tool, and if clinical concern exists then echocardiography is required. It will be not only able to assess cardiac function (cardiac size being a surrogate marker of function) but also exclude other causes of enlargement of the cardiac silhouette (e.g. pericardial effusion).

In some situations, the an increased cardiothoracic ratio on a PA radiograph may just result from a prominent epicardial fat pad and due to expiration rather than from due cardiomegaly.

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