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Heart murmurs are abnormal, extra sounds that are of a relatively long duration. A heart murmur occurs when there is turbulence to the normal flow of blood through the chambers of the heart, and is usually due to a problem in a heart valve. When a valve is thickened or damaged and unable to close fully, some of the blood escapes around the valve. A vet can hear a squishing sound which confirms that blood is leaking out of the heart valves during contraction.
Murmurs are graded from I to VI based on the loudness. Before you get alarmed at a murmur diagnosis, note that loudness does not always mean heart damage or even that it is severe. Conversely many cats with severe HCM have barely detectable murmurs. The best determinant of what's going on with your cat's heart is color Doppler echocardiography.
Murmurs are usually seen in older cats but it is not uncommon to see them in young kitties. In fact, many kittens are born with murmurs and outgrow them later in life. A lot of cats with murmurs live long healthy lives.
A murmur present since birth that does not cause problems e.g. kitten plays normally, isn't lethargic, or tires easily, is not as significant as one developed later in life especially if accompanied by symptoms.
When older cats have a valve-related murmur, the incomplete valve closure is often due to bacterial deposits on the valves. A very common source of this bacteria is dental disease, so it's important to keep your kitty's teeth and gums in good condition. A heart murmur can also be caused by severe anemia.
Congestive Heart Failure
When the volume of blood to the right side of the heart is greater than the heart's capacity to move it along, fluid builds up and congestion occurs behind it. This causes an abnormal accumulation of fluid in the abdominal cavity and/or the lungs and chest cavity, which results in congestive heart failure (CHF).
Sometimes you can hear the fluid by means of a stethoscope but this takes some practice (ask your vet or vet tech for help and/or check the references below). Often the cat displays some obvious signs like poor appetite, panting, coughing after even the slightest exertion and blue lips/tongue/gums. Kitty may appear to be out of breath, and not so often your cat may even throw up a reddish colored fluid. This may or may not be accompanied by fainting spells. CHF is a very serious disease and as such, any troubling signs warrant an immediate trip to the vet (emergency or regular. whichever can happen earlier).
Also, in advanced stages, CHF can lead to (or be caused by) renal failure, spleen and liver enlargement, and fluid in the abdomen. This fluid can be aspirated by a vet but sedation is required, so this is not a risk-free procedure :(
Aortic thrombosis or saddle thrombus
Another complication of heart disease in cats is the development of a blood clot. A clot can form in the heart’s left atrium and make its way through the blood stream. One place it can lodge is at the branch of the aorta that feeds the back legs. This effectively cuts off blood flow and can lead to either partial or full paralysis of the hind legs.
In some cases, the front leg or legs are affected, so be vigilant, and don't assume it's just the hind legs that can be involved.
Your cat will exhibit lameness or paralysis of the hind legs. The paw(s) might not look as pink as usual, and might also feel cold to the touch. Your cat might cry out in pain, which says a lot given how stoic cats can be. This is a very serious life-threatening condition, and extremely painful. It requires immediate care, so do not wait even if it means an ER visit.