Educational Resources

Heart Murmurs

A cat’s heart has four chambers. Unoxygenated blood returning from the body enters the heart in the right atrium and passes through the tricuspid valve on its way to the right ventricle. From the right ventricle, the blood passes through the pulmonic valve and on to the lungs to obtain oxygen. Oxygenated blood then returns to the heart and passes through the left atrium, then the mitral valve, and the left ventricle. Finally, the blood passes through the aortic valve to be distributed to the body.

Heart murmurs are abnormal sounds created by a disturbance in the normal flow of blood through the heart. Heart murmurs may be classified as congenital, or present at birth, or acquired, meaning that it has developed during the cat’s lifetime. Heart murmurs can be ‘innocent’ or ‘physiological’, where no apparent disease is causing the murmur. However, a murmur may also be the first indication of heart disease.

Heart murmurs can be caused by mechanical changes within the heart like narrowing of the blood vessels or chambers of the heart, blood leaking between the heart valves, or through defects within the heart muscle. Systemic illnesses, like anemia, high blood pressure, or thyroid disease, may also cause murmurs. Occasionally, stress may cause a benign or physiologic murmur.

Heart murmurs are detected when your veterinarian auscults (listens) to your cat’s heart. The murmur is then classified, or graded, to characterize its loudness. Low grade murmurs (Grade 1 and 2) are quieter than Grade 3–6 murmurs. The loudness of the murmur does not always correlate with the degree of heart disease.

Once a heart murmur is detected, your veterinarian may recommend further diagnostic tests in order to determine the cause, if treatment is needed, and the prognosis. Tests may include:

  • Blood work and urinalysis
  • Blood pressure measurement
  • Chest radiographs (x-rays)
  • ECG
  • Echocardiogram (heart ultrasound)

In most cases, an echocardiogram is the best diagnostic test to determine if heart disease is present and to determine the severity of the disease. It allows the ultrasonographer to look within the heart in order to measure the thickness of the heart muscle and the size of the heart chambers, to look for heart defects or blood clots, and to determine if there is blood leaking between the heart valves.

After the diagnostic tests are performed, your veterinarian can then make a treatment plan that best suits the needs of your cat. Most diseases that cause heart murmurs, including primary heart disease, are treatable through medical management. Surgical intervention is very rarely recommended and is often contraindicated for many types of heart disease. Your cat’s prognosis will depend on the type and severity of disease that is present. However, early detection and treatment will give your cat the best chance of survival.

Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common type of acquired heart disease in cats. HCM occurs when the heart muscle of the ventricles becomes hypertrophied, or overly thickened. While cats may show no outward symptoms at first, the disease can be progressive and may cause clinical signs in time. The domestic shorthair cat is most commonly diagnosed with HCM, although there is a form of HCM that is familial and manifests at a young age (usually less than 3 years old) in Maine Coon, Sphynx, and Ragdoll cats. Evidence also shows that American shorthair, Persian, and oriental breeds may be predisposed to HCM. HCM may develop at any age, but most cases are diagnosed in middle age to older cats. Males are diagnosed more frequently than females.

Effects of HCM

When the heart muscle becomes hypertrophied, it cannot relax normally after contraction. Relaxation is necessary so that blood can refill the ventricle in order to be pumped to the body. The blood that should be shuttled to the body then builds up and the pressure forces fluid from the blood vessels into the lungs or chest cavity (this is commonly known as congestive heart failure (CHF). In some cats, HCM will cause abnormal heart rhythms or arrhythmias and can result in sudden death. The build-up of blood within the heart may also cause the formation of thrombi, or blood clots. These clots are then pumped through the blood stream and may cause blockage of blood flow. This occurs most commonly when blood reaches the end of the aorta and splits to deliver blood to the back legs. Clots may also occur in the front limbs, and happen more commonly on the right side. HCM cats that develop an aortic thromboembolism (ATE) will develop sudden paralysis.

Clinical Signs

The clinical signs of HCM are variable. In most cases, the degree of heart muscle thickening determines the severity of the disease. Cats with early or mild forms of the disease may show no evidence of disease while more advanced cases may have non-specific symptoms like decreased appetite or lethargy or evidence of CHF, like difficulty breathing. Cats with ATE will often have acute, sudden paralysis.


The earliest indication that HCM may be present is detection of a heart murmur or arrhythmia upon chest auscultation, although some cats with clinically significant HCM will not have detectable abnormal heart sounds. Many times, this is noted during routine physical examinations. Your veterinarian will make determinations as to the next appropriate diagnostic step, which may include blood work, chest x-rays or a cardiac ultrasound (echocardiogram). An echocardiogram can identify how thick the heart muscle is and whether or not blood clots are present in order to determine the severity of the disease and if treatment is needed.


Therapy for HCM is supportive, meaning there is no cure for the disease. The changes that occur in the heart muscle are irreversible, with the exception of those cats with hyperthyroid induced cardiomyopathy. Medications (atenolol) can be used to try to help the heart muscle relax. Cats with CHF may be placed on diuretics (furosemide or spironolactone) or ACE inhibitors (enalapril). In severe cases, it may be necessary to remove fluid from the chest cavity if the cat is having difficulty breathing. In some cases, the other diseases, like high blood pressure and hyperthyroidism may cause heart muscle hypertrophy. Medications may be prescribed to treat these conditions specifically. Lastly, medications (low-dose aspirin or clopidogrel) may be prescribed to prevent blood clot formation.


The prognosis for cats with HCM is variable. If only mild, non-progressive disease is present, a cat may do well for many years. In most cases, the disease will continue to progress over time and heart function will become compromised. Cats that develop heart failure have a guarded prognosis and survival times are usually estimated to be 12 to 18 months after diagnosis. Only 30% of cats that develop ATEs survive to discharge from the hospital and they often require intensive at home care. Recurrence of ATEs is very common. In those cats with both CHF and ATE, prognosis is poor.

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