It is measured on a PA chest x-ray, and is the ratio of:
- maximal horizontal cardiac diameter
- 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 not only be 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 simply result from a prominent epicardial fat pad and due to expiration rather than from due cardiomegaly.