Heart Murmur in Cats: Causes, Symptoms and Treatment

What is a heart murmur?

A heart murmur is an abnormal sound of the heart caused by the vibration of turbulent blood flow. Heart murmurs may be a sign of a serious underlying condition such as hypertrophic cardiomyopathy and birth defects, or functional, innocent or physiologic (no disease).

The heart muscle or heart valves may be affected.


  • Anemia – Decreased number of red blood cells, with a number of causes. 
  • Bacterial endocarditis – Bacterial infection of the heart valve.
  • Congenital (present at birth) heart defects such as pulmonic stenosis.
  • Heart valve disorders.
  • Heartworm – A potentially fatal type of roundworm which lives in the heart, lungs and pulmonary arteries.
  • High blood pressure (hypertension) – A condition in which the blood is pumped at abnormally high pressure through the arteries.
  • Hypertrophic cardiomyopathy – Thickening of the left ventricular wall, papillary muscles, and septum.
  • Hyperthyroidism – Benign tumour of the thyroid gland.

Heart murmurs which are present at birth are usually due to congenital defects or functional murmurs. Some young kittens will have a detectable heart murmur which vanishes by the time they are a few months old.

The veterinarian will grade heart murmurs from one to four by the intensity with the use of a stethoscope.

  • I – Barely audible
  • II – Soft, but can be heard easily
  • III – Moderately loud
  • IV – Loud
  • V – Can be heard with stethoscope off the chest
  • VI – Can be heard without a stethoscope


A heart murmur is a symptom in itself and not a disease. Symptoms will depend on the cause of the heart murmur, and many cats will be asymptomatic.


A thorough physical examination, medical history and diagnostic workup will be needed to determine the underlying cause and evaluate the overall health of the cat.


  • Baseline tests: Biochemical profile, complete blood count, and urinalysis to evaluate organ function and check for anemia.
  • Heartworm test: Antigen and antibody tests are quick blood tests that detect exposure to heartworm larvae.
  • Chest x-ray: X-ray can show an image of the heart, lungs and blood vessels and reveal if the heart is enlarged.
  • Echocardiogram: A test using ultrasound waves that allows the veterinarian to evaluate the size and movement of the heart as well as the location of the heart murmur. A Doppler is a specialised echocardiogram that evaluates the direction and speed of blood flow across the heart chambers and valves.


Treatment depends on the cause of the murmur. An innocent heart murmur generally won’t require treatment, however, if an underlying cause is found, it will be necessary to treat.

  • Anemia: Treating the cat for internal and external parasites, blood transfusions for severe anemia, antibiotics to treat feline infectious anemia.
  • Bacterial endocarditis: Antibiotics to control the infection.
  • Surgery to correct birth defects, if possible.
  • Pulmonic stenosis: Balloon valvuloplasty is the insertion of a balloon into the pulmonic valve to break down the obstruction.
  • Heartworm: At the time of writing, there are no approved medications to treat heartworms. Treatment is on a case by case basis. If there are no other symptoms, the vet may choose to sit and wait; it takes approximately 2-3 years for the heartworm to die. Cats who display symptoms of heartworm disease can receive prednisone to help reduce inflammation. Bronchodilators if the cat is experiencing breathing difficulty. Cage rest will also be necessary.
  • High blood pressure: Find and treat the cause, if possible. Medications to bring down the blood pressure such as calcium channel blockers, beta-blockers, ACE inhibitors and diuretics.
  • Hypertrophic cardiomyopathy – There is no treatment other than supportive care for HCM, which may include diuretics, blood-thinning drugs, beta-blockers to help the muscle relax.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio

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