Cardiology was not a favorite subject of mine in vet school. This was not due to the subject matter. I'm fascinated by the heart and find it be to more logical than some subjects (neurology, for example). Unfortunately, it was a 5+ inch stack of notes, only 2 exams, and a high level of stress. Now that I'm finished with school, I deal with the real aspects of cardiology and still find it fascinating (although challenging at times).
Animals, as people, develop heart conditions as they age. Unlike people, true myocardial infarction/heart attack is rare. It is much more common for dogs, as they age, to go into congestive heart failure due to pre-existing heart disease. MI is practically unheard of. I am going to talk about the heart disease that occurs in dogs and cats below. We'll start with conditions that develop as pets age rather than congenital conditions.
Understanding blood flow in the heart is relatively easy and necessary in the discussion of heart disease. The heart is an efficient pump. It takes blood from the body that has already been "used" (de-oxygenated) back into itself via the vena cavas. This blood flows into the right atrium, through the tricuspid valve, into the right ventricle. From there, it is pumped into the pulmonary artery (via the pulmonic valve) to the lungs. Here it receives a fresh load of oxygen. From the lungs, it goes into the left side of the heart via the pulmonary veins, into the left atrium, through the mitral valve into the left ventricle, and out the aortic valve/aorta to the rest of the body.
With that understanding, let us discuss diseases.
The most common that I see is probably mitral valve insufficiency (75-80% of heart disease in dogs). This typically occurs in small breed dogs as they age - Schnauzers, Shih-Tzus, Lhasas, Dachshunds, etc. Large breed dogs can be affected, but this is not as common.
The hallmark is an older dog abruptly developing a heart murmur. For some reason that we fail to understand, the body starts to produce thicker layers (fatty) within the leaflets of the valves that separate heart chambers. This is called myxomatous valvular disease (also endocardiosis and degenerative valvular disease). This is very rare in cats.
The mitral valve separates the left atrium from the left ventricle. As the fatty deposits build up, the leaflets of that valve pull apart, causing improper blood flow between the chambers (regurgitation of fluid from the left ventricle backwards into the left atrium). Blood should flow forward - from the lungs to the pulmonary veins to the left atrium to the left ventricle out through the aorta. When the gates between chambers become weak, blood becomes turbulent, flows improperly, and the patient will develop a heart murmur.
As this problem worsens, the left atrium will become enlarged and thin-walled. This happens due to back pressure of the improper blood flow and turbulence. As the atrium becomes thin, it becomes unable to do its job properly. Blood starts to back up, as it is not being pumped forward correctly. The blood backs up into the lungs causing changes in the pressure gradient there. Eventually, fluid starts to leak into places it shouldn't and pulmonary edema ("fluid in the lungs") develops. This is congestive heart failure.
So, what should you do if your older, small breed dog is diagnosed with a new heart murmur (this link will let you listen to a normal heart versus a heart murmur)?
First and foremost, your veterinarian should thoroughly and carefully listen to all areas of the heart - both the right and left sides of the chest. By just listening carefully, a veterinarian can often determine where the murmur originates and thus, what the most likely cause is. They can also determine if any degree of heart failure is present by listening for crackling and wheezing in the lungs.
Then xrays of the chest should be conducted. These will help evaluate heart size and for the presence of pulmonary edema and heart failure. A vertebral heart score should be determined. This is the only objective way to identify heart enlargement on xrays. Your veterinarian will measure the width and height of the heart, compare those numbers to a specific vertebrae, and come up with a number. Normal VHS in dogs is 10-10.5. Anything greater indicates an enlarged heart.
Next, your veterinarian should recommend an echocardiogram performed by either a cardiologist or a specialist with ultrasound training. This will help determine what is causing the murmur. Many owners balk at this point due to cost. It is understandable, but an echo is the only way to know for sure where the murmur originates. If owners cannot pursue an echo, the veterinarian should treat based on the most likely cause of the murmur. (The murmur itself is not a disease, it is a symptom, and as such, it is not specifically treated.)
If mitral valve disease is determined to be present, your veterinarian will likely recommend xrays to re-evaluate heart enlargement every 3-6 months. Your vet will also discuss monitoring for signs of impending heart failure such as exercise intolerance, weakness, fainting/collapse, and coughing or wheezing. Some veterinarians will start an ACE inhibitor at that time (enalapril/Enacard or benazepril/Lotensin). It is important to know that at this point, there is NO way to prevent progression of heart disease. In people, valvular replacement can be done, but this is not done (or only done extremely rarely) in animals. The cost is prohibitively expensive.
There is no way to tell how each individual dog will progress. Some will live years with a heart murmur and no heart failure, some will live months and then go into heart failure. It is individual to each dog. If your older, small dog has a heart murmur, your veterinarian should be monitoring closely at 3-6 month check ups for changes in lung sounds, severity of the heart murmur, and clinical signs.
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