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You've just brought your healthy kitten to visit the veterinary office for her annual exam and vaccine updates. Now the vet is telling you she hears a heart murmur. How concerned should you be?

What Does That Murmur Mean?

Kitten with Muddy Creek Animal Care VeterinarianA murmur is simply a sound that tells the listener that something is causing turbulence of the blood flow as it travels through the heart.

Normally, the blood should move from chamber to chamber, and out of the heart valves, as smoothly as water running through a pipe. If something causes a partial blockage, if there's a leak in the system, or even if the liquid isn't being propelled efficiently, turbulence will result in the sloshing sounds we refer to as "murmurs" when we speak of heart function.

In kittens and young cats, congenital irregularities of the heart can cause these sloshing sounds. The majority of these are "innocent" murmurs that will resolve as the patient grows and its heart matures.

Persistent or loud murmurs, however, can be secondary to more severe defects and should be investigated if accompanied by any weakness, pallor, shortness of breath, failure to grow or thrive, or if they persist until the time the pet is due to be spayed or neutered.

In young adult to middle-aged cats, a new murmur is somewhat more concerning because it may be indicative of a developing heart condition. If the murmur is soft or subtle, it may be a long-standing congenital issue that is benign and the murmur may simply have been missed in prior exams (heart murmurs can occur off-and-on.) A leaky heart valve may become progressively more leaky as time goes on, leading to circulatory insufficiencies.

Although primary hypertension is extremely rare in cats, elevated blood pressure may occur secondary to other conditions such as kidney disease or hyperthyroidism, and a heart murmur may be the first indication that an underlying disease exists.

Heartworms Can Cause Murmurs in Young Adult Cats

Young adult cats may also become exposed to heartworms, a contagious parasite spread by mosquitoes. These foot-long worms take up residence in the right heart chambers and in the out-flow tract to the lungs, therefore causing murmurs that are more prominent on the right side. Heartworm disease can also cause chronic inflammation of the lungs, so a coughing cat may potentially have this condition.

Hypertrophic Cardiomyopath

Middle-aged felines, especially exotic and pure-bred cats, are prone to a condition called hypertrophic cardiomyopathy, a condition in which the heart muscle gradually overdevelops, becomes thickened and loses elasticity. Affected cats may be quieter, lose weight or have an increased respiratory rate, however these subtle changes may be the only sign that something is amiss.

Heart Disease

Cats with heart disease seldom cough unless the condition has progressed far enough to cause severe heart failure and a large amount of fluid in the lungs. Since cats tend to sleep and rest for a significant portion of the day, a decrease in activity may go unnoticed. The heart walls will often become so dense that the heart chambers cannot fill with adequate amounts of blood and the walls cannot compress properly to expel the blood into the peripheral circulation. Not only heart murmurs, but also an irregular heart rhythm, may be the result. These kitties cannot support their body's circulatory needs and will eventually go into heart failure. Sadly for some cats, sudden death may be the first indication of a problem unless a heart murmur is found on a routine annual exam. Fortunately, medications can often slow the progression of this disease.

Hyperthyroid Disease

Older cats are prone to a condition called hyperthyroid (or over-active thyroid) disease. This glandular disorder results in an excess of circulating thyroid hormone that over-stimulates the metabolism. The increased metabolism spurs the heart rate, stresses the heart muscle and can result in hypertension and murmurs. It will also cause weight loss, a ravenous appetite and stress to other internal organs such as the kidneys. Fortunately, medication options exist for this condition also.

How Are Heart Murmurs Diagnosed?

Depending on your cat's age and general health status, your veterinarian may recommend blood testing or cardiac imaging to diagnose the cause of a murmur. For older kitties, wellness bloodwork including thyroid testing is an appropriate first step to evaluate thyroid function and overall health. If a diagnosis of hyperthyroidism is made, many cats' heart murmurs will resolve once the condition is controlled with medication.

For adult cats and older cats with normal thyroid testing, a new blood test for a substance called pro BNP (Brain Natriuretic Peptide) is available that can assess heart stress. BNP is stored in the heart muscle and released in a "pro" form into the circulation when the heart muscle is stretched. Excessive stress to the heart muscle will result in elevated levels in the circulation, making this a marker for heart disease. Although this is a relatively inexpensive and simple test, it is also very non-specific, telling you only that there is an issue that needs addressing, but not what the specific disease process is. Cats with an elevated pro BNP should have an echocardiogram to determine the nature of the stress to the heart and what the most appropriate medications are.

Echocardiograms (or "echos") are ultrasounds that show the individual structures of the heart as it beats in real time. It allows tracking of the blood flow through the heart chambers and valves and can demonstrate any leaky valves or areas of back-wash. It can also allow measurements of the individual heart walls and chambers and may demonstrate the presence of parasites such as heartworms. This test is generally performed by Veterinary Cardiologists, Veterinary Radiologists or Internal Medicine Specialists and requires high quality imaging equipment.

An echo will not assess the health of the lung tissue, so for some cats, a chest x-ray and/or blood pressure testing may be recommended as well. Standard x-rays will show the outline of the heart and give an indication of the overall size. They will also show whether or not fluid has accumulated in the lungs or if the airways and cardio-pulmonary blood vessels are showing signs of stress. However, x-rays are not likely to provide a specific diagnosis and echocardiograms remain the gold standard for cardiac testing.

Is Testing Required?

For young and otherwise healthy cats, a wait-and-see approach may be appropriate. However, an echocardiogram will be recommended before any anesthetic event (like spaying or neutering), if the murmur has persisted. Some breeders will be willing to help cover the cost of this testing.

For adult cats, the wait-and-see approach does have some risks because cats with inefficient blood circulation can develop blood clots within the chambers of the heart. If one of these clots is released into the circulation, it may block an artery such as the one supplying the hind legs, and these cats will be acutely paralyzed. Sadly, once this event occurs, there are no good treatments and most of these cats require euthanasia.

A new murmur may mean nothing to the overall health of your kitty, but some investigation is usually warranted. Together with your veterinarian, you can make a plan that is appropriate for your cat.

Dr. Sylvia Reiser is a veterinarian at Muddy Creek Animal Care Center. A member of the American Association of Feline Practitioners (AAFP), she has an interest in feline care and behavioral medicine.