Evaluating Heart Size On Radiographs For Dogs And Cats

We've all had patients that could not afford a complete cardiac workup for their coughing dog or dyspneic cat. They may pay for the physical examination, blood work, and radiographs, but they just can't pay the cost of a specialist consultation and an echocardiogram. This can put a lot of pressure on primary care veterinarians to determine the cause of the cough.

Thoracic radiographs are an important tool in differentiating Congestive Heart Failure (CHF) from other conditions such as respiratory disease. However, primary care veterinarians are expected to read radiographs from different dog breeds (for example, a Dachshund, a Chihuahua, and a Great Dane) and be able to tell if the patient has an enlarged heart. Cats are even more difficult because we are often talking changes in millimeters, and much of their hypertrophy is concentric. With some populations this is easy, but for others, it can be a real challenge.

To address this dilemma, Drs. James W. Buchanan and Jorg Bücheler at the University of Pennsylvania developed the Vertebral Heart Size - VHS (sometimes called a score) to assist veterinarians in assessing heart size. The VHS is a method that allows veterinarians to evaluate the heart size across dog and cat breeds and provide an accurate assessment of true cardiac enlargement. There is a good correlation when scaling heart size and vertebral body length.

When performing a VHS measurement, it is important to obtain a good thoracic radiograph, which can be tough and sometimes impossible. The radiograph should be centered over the heart. Films that are centered over the diaphragm or abdomen can foreshorten the thoracic vertebral bodies, thus falsely elevating the VHS. The patient needs to remain still, and the radiograph should be taken when the dog is taking a deep inspiratory breath. The patient needs to be in a good straight lateral position, but taking a right or a left lateral doesn't really matter. Taking both laterals can sometimes be helpful. The right lateral is most commonly used.

To Perform a VHS in Dogs

1. Take a lateral view of the dog's heart
2. Measure the long axis, the length from base (ventral margin of the corina) to apex (L), and the short axis, width of the heart perpendicular to the length measurement, typically
   at the ventral margin of the caudal vena cava(S)
3. Take these dimensions and scale them against the length of the vertebrae dorsal to the heart, beginning with the fourth vertebral body on the spine (T4)
4. Count how many vertebral bodies the length (L) of the heart is and how many bodies can be included in the width (S) measurement. A vertebral body consists of the vertebral body starting at the cranial end-plate and includes the disc space immediately caudal to the that vertebrae
5. If the sum of these two measurements is higher than 10.5, the dog probably has an enlarged heart. Normal dogs tend to fall within a range of 8.4 to 10.5.

To Perform a VHS in Cats

• Using the Right Lateral projection:

1. Identify the long axis of the heart beginning at the point where the pulmonary vein crosses the trachea and end at the apex of the heart
2. Place another line perpendicular to the long axis at the widest point of the cardiac silhouette, which would correlate with the short axis
3. Identify the 4th thoracic vertebra (T4), and place 2 lines equal in length to the long and short axis lines at the cranial edge of T4
4. Determine the length of each line to the nearest 0.1 thoracic vertebra and add these two measurments for a VHS
5. Normal right lateral VHS in a cat should be 6.9 to 8.1
6. In addition, a short axis wider than 4 vertebral bodies is highly supportive of atrial enlargement.

VHS in cats using the lateral view

• Using the Ventrodorsal projection:

1. Identify the longest portion of the heart from base to apex (long axis of the heart)
2. Identify the widest dimension perpendicular to the long axis (short axis of the heart)
3. Identify the 4th thoracic vertebra (T4), and place 2 lines equal in length to the long and short axis lines at the cranial edge of T4
4. Determine the length of each line to the nearest 0.1 thoracic vertebra and add these two measurments for a VHS
5. Normal ventrodorsal VHS in a cat should be 7.3 to 9.1
6. In addition, a short axis wider than 4 vertebral bodies is highly supportive of atrial enlargement.

Take Away

Too many cardiac patients are euthanized because owners can't afford an echocardiogram or they are given a negative prognosis. Many owners who decline referral and echocardiography can afford $50 a month for medication, plus chest radiographs, and screening blood work. With appropriate treatment, a patient with degenerative valve disease and secondary CHF can live for more than a year and sometimes two while remaining relatively symptom-free. Many of these patients, despite the need for heart failure medication, will feel great and the owners will be very happy with the quality of life. Primary care veterinarians can treat mitral valve disease by obtaining accurate thoracic radiographs and vertebral heart scale.

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