Guidelines for normal heart size on radiographs
I personally find the following guidelines easier than the vertebral heart score (VHS) method. Just as a reminder the normal VHS in a dog is 10-10.5.
In the dog, in inspiratory films:
lateral view:
< 3.5 intercostal spaces for:brachiocephalic breeds,immature dogs,small breeds
< 3 intercostal spaces for "average dog"
< 2.5 intercostal spaces for deep chested breeds
the vertical distance from the cardiac apex to the carina is normally 2/3 to 3/4 the vertical distance from the cardiac apex to the vertebral column
Right ventricular enlargement will result in a wider heart- more intercostal spaces
Left ventricular enlargement will result in a taller heart-therefore the trachea will be displaced dorsally
VD/DV view:
The width of the heart should be less than 2/3 of the thoracic width.
The distance between the heart and the thoracic wall should be equal on both sides. When this distance is reduced on the left side it suggests a left side enlargement, and when it is reduced on the right side it suggests a right side enlargement.
Exceptions:
In barrel chested dogs the heart always seemed to be enlarged even when it is normal.
In these dogs the VHS method is very useful in determining whether the cardiomegaly is real.
Yorkshire terriers also have "cardiomegaly" on radiographs and on the lateral view, the left atrium many times seems to be enlarged even though it is normal.
Note that in these breeds it can be difficult to obtain inspiratory films.
In these dogs, in the absence of a heart murmur the cardiomegaly is probably not real.
In deep chested dogs the cardiomegaly can be very subtle.
In Dobermans, even in heart failure, the left atrium is not always significantly enlarged.
In the cat, inspiratory films:
The guidelines for cats are less defined.
Lateral view:
The heart is more elongated and elliptical than in the dog
< 2-2 1/2 intercostal spaces
The cat's heart tends to be more horizontal and with age, the heart tends to horizontalize even more ("lazy" heart).
DV / VD
The cat heart is more oval and thinner than the dog, and the cardiac apex usually lies on the midline, unlike the dog in which the apex is shifted to the left. When shifted in a cat consider cardiomegaly.
If a cat's heart looks like a dog's heart on a radiograph there is definitely something wrong :)