Radiography provides the most readily available means to identify pulmonary edema and pulmonary venous congestion. Because the vast majority of cases of pulmonary edema are due to congestive heart failure - then the finding of pulmonary edema is strong evidence of congestive heart failure.
Since pulmonary venous congestion (distention) will/must occur prior to the development of cardiogenic pulmonary edema, to identify the presence of pulmonary venous congestion is also strong evidence of congestive heart failure.
Pulmonary edema refers to an abnormal accumulation of fluid in the interstitium and/or the alveoli of the lungs. As fluid weeps out of the capillaries, at first it accumulates in the perivascular and peribronchial interstitial spaces (producing silhouetting of the vessels, and/or peribronchial pattern on radiographs). Continued fluid accumulation results in edema of alveolar walls and ultimately, alveolar edema (producing air-bronchograms or coalescent [so called cotton-like] pulmonary densities).
Although alveolar edema is usually preceded by interstitial edema, many clinical cases represent a mixture of interstitial and alveolar edema.
Location of edema:
Note how the lumenal wall is readily identified in both examples, but the outside wall is obscured with air-bronchograms.
It is important to determine the cardiac structures that contribute to the silhouette of the heart on the lateral and D/V or V/D view.
On the lateral view normal cardiac dimensions:
A schematic diagram of a lateral radiographic view of the chest. Normally A is approximately 1/3 to 1/4 of A + B, and B is approximately 2/3 to 3/4 of A + B.
- the greatest horizontal cardiac dimension should be < 2/3 of the chest wall to chest wall thoracic dimension at that location.
A schematic diagram of a V/D or D/V radiographic view of the chest. In normal hearts R is approximately equal to L; B is < 2/3 of A.
Significance of cardiac enlargement in the lateral view:
Significance of cardiac enlargement in the V/D or D/V view:
Single chamber enlargement is unusual and therefore enlargement in one chamber tends to cause enlargement in other areas of the heart.
Some general guidelines on cardiac size in the cat:
To calculate VHS, use a lateral view that clearly shows the T4-T13 vertebrae, with minimal rotation of the thorax (rib arches and costochondral junctions should be aligned).
Long axis: Measure the distance (using a ruler or piece of paper) from the carina to the apex of the heart.
Short axis: Measure the widest part of the heart on an axis perpendicular to the long axis.
Compare to vertebrae: Starting from the cranial edge of the T4 vertebral body, measure the length of the axes by the number of vertebrae. Measure to 0.1 of a vertebral body. The vertebral heart size is the sum of the length of the short and the long axes in vertebrae (VHS = long + short).
Reference intervals for VHS: