Evaluating a new radiographic tool for canine heart disease diagnosis

Radiographic measurements of cardiac size and shape have long been used to detect cardiomegaly associated with heart disease.^1,2 While experienced veterinarians may recognize cardiomegaly readily, many practitioners could benefit from a quantitative approach.

Why this study?

The traditional quantitative measurement in veterinary cardiology is the vertebral heart score (VHS).¹ The VHS has limited use, however, because while it incorporates the overall size of the cardiac silhouette, it fails to encompass each cardiac dimension and chamber. Furthermore, breed variations and other factors can cause disparities in thoracic conformation and vertebral size, leading to nonrepresentative VHSs that may blur the lines between radiographically normal and enlarged cardiac silhouettes.^2,3

The manubrium heart score (MHS), on the other hand, uses the length of a single, easily identifiable structure, the manubrium. A strong and static correlation exists between the length of the manubrium and that of the corresponding cardiac axes. The MHS scoring system has been used experimentally to determine cardiac short-, long- and overall-MHSs in large- and small-breed dogs with radiographically normal cardiac structures.⁴

In a study recently published in the Journal of the American Veterinary Medical Association,⁵ investigators at the University of Florida College of Veterinary Medicine performed various cardiac measurements using radiographic images generated during routine care of dogs at their small animal hospital.

Study details and findings

The study subjects were 64 client-owned dogs (23 large breeds, 41 small breeds) diagnosed with cardiomegaly by echocardiogram between 2012 and 2015. The dogs had a mix of congenital and acquired cardiac diseases; the latter were predominant in both large and small breeds, and included heartworm disease, valvular endocarditis and degeneration, hypertrophic cardiomyopathy and dilated cardiomyopathy. More than two-thirds of the large-breed dogs had left-sided heart disease; in the small-breed group, the right:left ratio was roughly 50:50.

The control group consisted of thoracic radiographic images of 120 corresponding client-owned dogs (60 large breeds, 60 small breeds) whose cardiovascular status was clinically and radiographically normal (VHS < 10.9).^4,6

All evaluations were performed on right lateral radiographs. The following measurements were obtained:

• Cardiac long-axis length (cLAL): length of the line extending from the ventral border of the carina to the most ventrolateral margin of the cardiac apex; spans the left atrium and left ventricle
• Cardiac short-axis length (cSAL): length of the line drawn perpendicular to the cardiac long axis at the widest point of the cardiac silhouette; spans the atria
• Manubrium length (ML)

Using these measurements, several scores were calculated:

• MHS: cLAL + cSAL/ML
• Long-MHS: cLAL:ML ratio
• Short-MHS: cSAL:ML ratio
• VHS: cLAL + cSAL length expressed in the corresponding number of vertebrae (beginning at the cranial end plate of T4) representing the same length
• CSI (cardiac sphericity index): cSAL:cLAL ratio

Compared with controls, dogs with left-sided cardiac disease had significantly greater mean overall-MHSs, long-MHSs and short-MHSs, as well as significantly higher VHSs. Dogs with right-sided cardiac disease had significantly greater mean short-MHSs and CSIs than did control dogs.

Furthermore, the mean CSI was significantly greater in all case dogs with right-sided versus left-sided cardiac disease. For large-breed dogs only, the mean long-MHS was significantly greater in dogs with left-sided than in dogs with right-sided cardiac disease.

The study’s findings bolstered the investigators’ hypothesis that MHSs and CSIs are higher in dogs with cardiac disease and vary with the side of the heart affected. The findings also supported their theory that different MHS reference values may be needed to evaluate cardiac dimensions radiographically across different-size breed groups.

The authors also found that neither MHS nor VHS is reliable in diagnosing cardiac diseases that develop without eccentric heart enlargement.

Study limitations and conclusion

The limitations of the study included lack of echocardiography to rule out cardiac disease in control dogs, as well as physiologic variations in cardiac cycle, respiratory phase and blood volume that alter the radiographic appearance of the cardiac silhouette.

Overall, the study found the MHS to be useful in diagnosing cardiac disease, correlating wide (high short-MHS) and long (high long-MHS) cardiac silhouettes, as well as high overall-MHS, with left-sided cardiomegaly, and linking wide cardiac silhouettes (high short-MHS) with right-sided cardiomegaly.

While the authors maintain that echocardiography is the gold standard for diagnosing cardiomegaly in dogs, they concluded that radiographic assessments should incorporate MHSs to estimate heart size objectively, and CSI scores to represent cardiac sphericity.

Dr. Capuzzi is a small animal veterinarian and journalist based in the Philadelphia area.

References

1. Buchanan JW, Bücheler J. Vertebral scale system to measure canine heart size in radiographs. J Am Vet Med Assoc 1995;206:194-199.

2. Guglielmini C, Diana A, Santarelli G, et al. Accuracy of radiographic vertebral heart score and sphericity index in the detection of pericardial effusion in dogs. J Am Vet Med Assoc 2012;241:1048-1055.

3. Hansson K, Haggstrom J, Kvart C, et al. Interobserver variability of vertebral heart size measurements in dogs with normal and enlarged hearts. Vet Radiol Ultrasound 2005;46:122-130.

4. Mostafa AA, Berry CR. Radiographic assessment of the cardiac silhouette in clinically normal large- and small-breed dogs. Am J Vet Res 2017;78:168-177.

5. Mostafa AA, Pepper KE, Berry CR. Use of cardiac sphericity index and manubrium heart scores to assess radiographic cardiac silhouettes in large- and small-breed dogs with and without cardiac disease. J Am Vet Med Assoc 2020;256(8):888-898.

6. Gulanber EG, Gonenci R, Kaya U, et al. Vertebral scale system to measure heart size in thoracic radiographs of Turkish Shepherd (Kangal) dogs. Turk J Vet Anim Sci 2005;29:723-726.