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Clinical Evaluation of Heart Disease
Radiographic Evaluation
1. Can radiography diagnose heart failure and how?
Radiography cannot detect a reduction in cardiac output for the needs of the tissue (heart failure) but can provide evidence of pulmonary congestion to suggest congestive heart failure (pulmonary venous engorgement, pulmonary interstitial edema, and obscuring and enlargement of the cardiac silhouette). 

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. 

2. What are the radiographic features of pulmonary venous distention and pulmonary edema?
*See also the Thoracic Radiographic Tutorial section of these notes.

Radiographic criteria of pulmonary venous distention:

  • On the lateral view the pulmonary veins to the cranial lung lobes are greater than 75% the width of the proximal 1/3 of the fourth rib
  • The pulmonary vein to the cranial lung lobe is obviously larger than its accompanying pulmonary artery (normally they are of equal width)

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.

Radiographic Appearance:

  • Venous engorgement - distention of the veins.
  • Interstitial edema - shows a clouding (or silhouetting) of the pulmonary vasculature (perivascular pattern). The walls of the pulmonary vessels are obscured by edema fluid. A peribronchial pulmonary pattern (the most common sign of interstitial edema noted in the dog) also may occur as noted. This pattern is characterized by thickened airway walls. Normally the airway walls are not discernible radiographically after the third generation bronchi due to the lack of cartilaginous support. We typically infer the presence of a bronchus due its position between the artery and vein that lie on each side. The prominent appearance of airway walls throughout the lungs indicates a peribronchial lung pattern, which implies a cellular and/or fluid accumulation in the walls rendering them visible. On cross-section the airway walls are "donut-like", on long axis view they are "railway tracks-like."
  • Alveolar edema - (flooding of air spaces) shows as coalescing fluffy densities and/or air-bronchograms.

Location of edema:

  • Dog: Appears first in the central perihilar area progressing outward and caudodorsally. There is also a high incidence of edema in the anterior ventral area on the lateral view.
  • Cat: Variable distribution often occurring in a patchy, irregular pattern primarily in the caudal lobes.

Schematic Of An Air Bronchogram vs A Peribronchial Pattern

Note how the lumenal wall is readily identified in both examples, but the outside wall is obscured with air-bronchograms.

3. What other cardiac abnormalities can be diagnosed by radiography?
Radiography can also assist in the diagnosis of: 

  • chamber enlargement
  • great vessel enlargement
  • heartworm disease
  • pericardial effusion or pleural effusion
4. What are the radiographic features of cardiac chamber enlargement?
Radiography is a simple and effective means of diagnosing cardiac chamber enlargement. In most forms of heart failure cardiac enlargement is present.

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.

Always be consistent with your lateral (R or L) and D/V or V/D views. Always obtain films at end inspiration.

Some general guidelines on cardiac size in the dog (criteria from end inspiratory films):

On the lateral view normal cardiac dimensions:

  • horizontal plane:
    • < 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
  • vertical plane:
    • 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
    • as cardiac enlargement occurs in this plane the trachea tends to parallel the vertebral column and the vertical distance from the cardiac apex to the carina is increased

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.

On the D/V or V/D view normal cardiac dimensions:

  • 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:

  • In the Horizontal Plane:
    • Suggests right ventricular enlargement, however left ventricular enlargement tends to produce at least mild enlargement in this plane.
  • In the Vertical Plane:
    • Suggests left ventricular enlargement, however right ventricular enlargement tends to produce at least mild enlargement in this plane.

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:

  • The heart is more elongated and elliptical in shape than in the dog on the lateral view.
  • The ventricular area occupies about 2 to 2 1/2 intercostal spaces in the lateral view.
  • In the lateral view the cat heart tends to be more horizontal; as cats age, the heart tends to horizontalize even more (called a "lazy" heart).
  • In the D/V or V/D projection, the cat heart is more oval and thinner than the dog. The cardiac apex usually lies on the midline.
5. How can vertebral heart size (VHS) be used to evaluate heart size?
Vertebral Heart Size (VHS) is a technique that measures the width and breadth of the heart and compares it to the length of the vertebral bodies. It can be useful because the measurements are independent of respiration and the position of the heart within the chest. However, because of the test's low sensitivity, a normal VHS does not rule out an enlarged heart. It also does not rule out the possibility of cardiac disease that does not have an effect on heart size.

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:

  • Normal dogs: 8.5-10.7
  • Boxers: 10.3-12.6
  • Labrador Retrievers: 9.7-11.7
  • Cavalier King Charles Spaniels: 9.9-11.7
  • Cats: 6.7-8.1
6. What abnormalities of the great vessels might be identified by routine radiography?
Radiography is a simple means to determine changes in the great vessels.

  • Distention of the aorta in the region of the aortic arch with aortic stenosis:
    • may not be apparent unless aortic stenosis (or subaortic) is severe
  • Dilation of the main pulmonary artery (MPA):
    • due to pulmonic stenosis, heartworm disease, or pulmonary artery hypertension
  • Pulmonary over-circulation:
    • left to right shunt
  • Pulmonary under-circulation:
    • right to left shunt
    • under-perfusion/hypovolemia/dehydration
  • Caudal vena caval enlargement:
    • Caudal vena cava Present if the caudal vena cava is persistently of greater diameter than the descending thoracic aorta on the lateral view (normally they are roughly equal).
    • Suggests right heart failure with elevated volumes in the caudal vena cava.
7. What radiographic features might suggest the presence of heartworm disease?
  • Right ventricular enlargement
  • Dilation of main pulmonary artery (MPA)
  • Tortuosity of pulmonary arteries (PAs)
  • Truncated PAs
  • Enlargement of PAs
8. Can radiography diagnose pericardial effusion?
Radiography is useful to suggest a diagnosis of pericardial effusion.

  • A very rounded (globose) enlarged heart is present; individual chambers cannot be visualized
  • Pleural effusion is common (therefore, obscures the cardiac silhouette), ascites is common
  • Pulmonary edema is usually absent, however pleural effusion obscures our ability to visualize the features of pulmonary edema.
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