Expansion
Maximum cardiac width divided by maximum thoracic width, normally less than 0.5
Expansion
- Cardiothoracic ratio = maximum transverse cardiac diameter ÷ maximum internal thoracic diameter
- Normal is less than 0.5 (50 per cent) in an adult
The technical caveats matter as much as the number. On an anteroposterior film the heart lies further from the detector and is magnified, so a ratio above 0.5 cannot be called cardiomegaly. A poor inspiration raises the diaphragm, splays the heart transversely and does the same.
A truly enlarged cardiac silhouette has two broad explanations that anatomy helps separate: chamber enlargement (dilated cardiomyopathy, valvular disease) or a pericardial effusion, which gives the characteristic globular, flask-shaped outline with sharp borders and clear lung fields.