Some cats just have big hearts. Unfortunately, while that may be true metaphorically, it can also be true physically, and is the result of heart disease.
There are two primary heart muscle diseases that we see in cats, hypertrophic cardiomyopathy and dilated cardiomyopathy. We can also less commonly see valve disorders in the heart, arrhythmias, and of course, tumors. Hypertrophic cardiomyopathy (HCM). By far, the most common heart disease that we see in cats is HCM. This is a condition that results in the ventricular walls of the heart to get thicker and thicker over time. As the ventricular chambers get smaller, the blood has a harder time getting into the ventricles from the atria, and the atria start to stretch and weaken. In certain breeds of cats, primarily Maine Coons, this disease is sadly so common, that they have even isolated the gene that can potentially cause it. In most cases though we have no idea as to the cause, and unfortunately, we don't even always know it is present if there is no heart murmur or arrhythmia present, which isn't always, and in early stages, it won't be noticeable on an x-ray. The only way to definitively diagnose it is with an echocardiogram which is an ultrasound of the heart, or in middle to later stages, it can be seen on an x-ray by an experienced veterinarian.
There is absolutely nothing that can be done to prevent the progression of HCM. However, diet changes to a high protein low sodium diet and some medications can potentially slow down the progression of HCM. In later stages of the disease, diuretics and heart medications can prevent congestive heart failure and improve your cat's quality of life. Most cats will have a lifespan of 1-8 years after initial diagnosis. HCM progresses at different rates in different cats. The one bright spot about this disease is that it is not painful. As long as congestive heart failure, which is a back up of fluid into the lungs, can be prevented, the worst your cat will feel is tired. And, if the heart just gives out, which can happen, it isn't a bad way to go, very quick and painless.
Dilated cardiomyopathy (DCM). This has become a much more rare disease in cats since the discovery about 30 years ago that taurine is necessary in cat diets, and that lack of taurine can lead to DCM. Once cat food companies started supplementing taurine in their foods, DCM became very rare very quickly. A good reason not to feed your cat dog food, since dog food does not contain taurine. With DCM, the ventricular walls get thinner and more stretched out over time. Because the ventricular chambers become so enlarged and walls so thin and weak, the ventricles start having problems pumping the blood out to the body, which also results in the atria becoming enlarged because the ventricles are full of blood, and don't have room for more.
While this disease, when not related to taurine deficiency, also cannot be prevented, there are medications that can increase the ability of the ventricular walls to contract, which can greatly increase quality of life, although not necessarily longevity of life.
Cardiac valve disorders. The four valves of the heart are the aortic valve, the pulmonary valve, the mitral valve, and the triscupid valve. These valves closing are what cause the lub dub sounds that you hear when you listen to a heart. When blood passes through these valves abnormally, that is what causes the sound of heart murmurs. While muscle heart disease (cardiomyopathy) is the most common cause of heart murmurs, valvular disorders can also cause heart murmurs, and can result in cardiomyopathy. Valve disorders include but are not limited to infection growing on a valve, known as vegetative endocarditis, cancer in a valve, or the valve developing abnormally during development. Valves can also be damaged by disease or trauma, and can tear to some extent. The most common valve affected is the mitral valve which is the valve between the left ventricle and left atria. Valve diseases in people are treated by replacing the valve from a donor heart, either human or pig. In animals, very rarely is surgery done to replace a damaged valve. Instead we use cardiac medications to try and slow down the progression of cardiomyopathy as a result of the valvular disease. Prognosis depends on the valve affected and the degree to which it is affected.
Arrythmias. This when your cat's heart isn't beating at the normal consistent regular heart rate of lub dub, lub dub, etc. This can occur secondary to cardiac disease, such as HCM, DCM, valvular disease, or tumors. Or, it can also arise from electrical abnormalities in the heart. The heart has its own pacemaker called the sinoatrial, or SA node. This node tells the heart when to beat for the entire life of the heart. The signal travels from the SA node through the atria, down into the ventricles through another node called the atrioventricular, or AV node. The signal then travels through the ventricles and fades out. Various problems in the electrical system of the heart can cause electrical abnormalities. Disease in the SA node can cause an irregular rhythm to be put out. Disease in the AV node can slow down or prevent the signals from getting through to the ventricles. At various other places along the pathway the signal can be disrupted, usually from disease.
The heart does have two back up pacemakers. If the SA node stops sending signals to the ventricles, or if the signal gets stopped in the AV node, the ventricles have their own pacemaker that runs at about two thirds speed. So rather than a heart rate of 150, your cat might have a heart rate of 100. However, the ventricular pacemaker is like a back up generator. Eventually, it will run out of gas after a few years. If the SA node and the ventricular pacemakers stop working for any reason, there is one last backup pacemaker, and that it what is called the bundle of Purkinje. This pacemaker operates even slower, at about a third of a normal heart rate, about 40-50 beats per minute. This is like using a battery back up and has an ever shorter life span, just a few months.
If your cat develops an electrical abnormality, and it is not due to heart muscle or valve disease, then you have two options. You can either have a pacemaker implanted which is fairly expensive, but can last for many years, or we can try a medication to stimulate the passage of the electrical signal through the heart, but this is short term, and not always very effective.
Tumors. Thankfully, cardiac tumors are very rare in cats. The two most common that we can see are hemangiosarcoma, which is a tumor of the cells that line red blood vessels, and lymphoma, which is a cancer that can pretty much go anywhere in the body. Both of these cancers tend to occur at the base of the heart, where all of the large vessels come out of the heart by the atria. Neither cancer has a good prognosis.