There are several factors, usually working in combination, that lead to cardiac disease:


They can buildup on the heart valves inhibiting their ability to flow blood in the proper direction through the heart.


They can directly affect the heart muscle (myocardium), causing cardiomyopathy. In the early years of parvo virus there was a form of the disease that attacked the myocardium and caused rapid death.

Amino Acids

Taurine, an essential amino acid in cats, can lead to dilated cardiomyopathy. Humans and dogs can produce taurine naturally (non-essential) and do not need it in their diet. A deficiency of taurine in the cat will also cause degeneration of the retina. Taurine was not included in adequate amounts in cat food many years ago. This problem has been corrected in almost all commercial cat foods, so we rarely see this problem anymore.


An inadequate amount of the thyroid hormone thyroxin can predispose a dog to heart disease. This problem is diagnosed with a blood sample and corrected with thyroxin hormone replacement. This problem is almost exclusively seen in dogs.


This cat problem occurs when the thyroid gland has a tumor and secretes excess thyroid hormone (thyroxin). You can learn about it in detail in our hyperthyroidism page. In this disease the heart is racing as it tries to keep up with the increased metabolic rate of the organs as they respond to the increased thyroid hormone.

Valve Disease

Congenital abnormalities of the heart valves, usually a problem in young dogs, can lead to significant heart disease. These can include any of the valves. We get an indication of this early in a pets life by the presence of a heart murmur. Young animals can have what are called “innocent murmurs”. These are heart murmurs that don’t cause any problems and eventually resolve. If these murmurs persist, are of high intensity, or the pet is ill, they should be pursued further with diagnostic tests.

Older dogs get a problem with heart valves that also can be significant. We will go into this in more detail soon because it is the most common cause of heart disease in the dog.


Some drugs are toxic to the heart. It is ironic that one of the drugs used to treat heart disease, called digitalis, can be toxic to the heart also. This drug is commonly used to help slow down a racing heart. When used, we monitor digitalis levels with electrocardiograms and with a blood sample at least every 6 months to make sure it does not go into the toxic range. With the advent of new and better drugs we do not use this drug any more.


A heart that receives a severe blow can have problems with adequate pumping of blood or the normal electrical beating.


The heartworm parasite (dirofilaria immitis) can cause severe heart disease. This problem is not uncommon in dogs, and has been diagnosed with increasing frequency in cats lately.

Lung Disease (cor pulmonale)

Problems with hypertension in the lungs can cause the right heart to fail. These problems can include cancer, heartworms, infections, trauma, toxins, hypothyroidism, and Cushing’s disease.

Metabolic Disease

When vital internal organs like the kidney and liver are diseased there can be many changes that effect the heart. These include electrolyte abnormalities, calcium irregularities, and waste product buildup.


Some poisons selectively target the heart. Curare, the drug used in the tips of poison arrows and darts, is one of these drugs.


Typical symptoms of cardiac disease include:

  • lethargy
  • poor appetite (anorexia)
  • coughing- initially caused by an enlarged heart pushing upwards on the mainstem bronchi (windpipe) when the left atrium enlarges. As time goes on, this coughing is exacerbated by fluid buildup in the lungs (pulmonary edema).
  • passing out (syncope)
  • weakness
  • difficult breathing (dyspnea)
  • distended abdomen (especially if the right heart is diseased) called ascites
  • pale or bluish colored gums (cyanosis)
  • racing heart (tachycardia)

These symptoms are found in many other diseases also, especially of the respiratory tract. To determine which disease is causing these symptoms we need to strictly adhere to the diagnostic process.

In some heart diseases, notably cardiomyopathy in cats, there might be no symptoms prior to a sudden death. These cats seem fine until a stress causes their diseased yet compensating heart to reach its limit.

Continue on to:


How we Diagnose Heart Disease


Go back to:


Vascular Anatomy & Physiology


Heart Anatomy and Physiology


Causes and Symptoms of Heart Disease


Heart Disease Overview Page


 Diseases List Page


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