Cardiomegaly

An enlarged heart. The cardiac silhouette is larger than the thorax it sits in allows for. On its own it is a sign, not a diagnosis: it is the shadow of a heart enlarged by disease of its valves or muscle, or by fluid around it.

Cardiomegaly on a lateral thorax — Cardiomegaly

Two different patients, so size and positioning differ as well as the finding. Compare what the finding changes, not the dog.

What to look for

On the lateral view the heart is wider than about three and a half intercostal spaces and touches more of the sternum.
The trachea is pushed dorsally, running closer to the spine and parallel to it.
A vertebral heart score above the normal range: above about 10.5 in most dogs (breed references differ), above about 8.1 in cats.
On the dorsoventral view the heart fills more than about two thirds of the width of the thorax.

Common causes

Mitral valve disease in small breeds, dilated cardiomyopathy in large breeds, hypertrophic cardiomyopathy in cats, pericardial effusion, congenital defects.

Against the normal

The normal heart sits between the third and sixth ribs with clear lung above and behind it.

Radiographs: VetXRay, Banzato T., Burti S., Zotti A., Wodzinski M., University of Padua, CC BY 4.0. Converted to web images; findings as annotated by the dataset's radiologist. Dataset · Licence. For learning: not a diagnostic service.