The most common type of heart disease in small and medium breeds of dogs is endocardiosis. It is not as often noted in large dogs; however, it can occur and be problematic in these breeds as well. There are some breeds that are more prone to endocardiosis and these include Cavalier King Charles Spaniel, Chihuahua, Miniature Poodles, and the Boston Terrier. As with many hereditary conditions, males are more prone to the condition than females and it tends to be more problematic in older dogs than younger dogs.

Endocardiosis is also known as Chronic Mitral Valve Disease (CMVD) and affects the natural flow of the blood through the heart by affecting the actions of the valves in the heart. A dog's heart, just like a human's, is made up of four different chambers with valves in between that allow the blood to flow only one direction through the heart. The normal flow of blood is from the veins to the right atrium of the heart, through the right ventricle, then through the blood stream to the lungs. In the lungs the blood is oxygenated, then travels back to the left atrium of the heart. The mitral valve is the control system for moving the oxygenated blood from the left atrium to the left ventricle, where it is then pushed out of the heart through the body to the cells. Since the left side of the heart has to pump harder to force the blood through the body, the mitral valve is under a considerable amount of pressure as the left ventricle contracts.

Typically endocardiosis is not noticed until dogs are about six years of age, but it can occur in younger dogs as well. As the years of pumping blood start to weaken the seal of the valve, blood is forced by the contraction of the lower left ventricle against the mitral valve, then backwards up to the left atrium. This condition is usually mild at first and is only noticed at the yearly vet check as a mild heart murmur. There seems to be no evidence that this condition can be minimized or managed by special exercise or diet; however, dogs that are in good physical condition will have less complications with heart disease than overweight or obese dogs.

As the leak or back flow of blood through the valve becomes more severe additional symptoms and signs will be noted. Since the heart is no longer circulating the blood properly, dogs become lethargic and lack energy, although many owners mistake this as typical old age behavior. As the circulation further decreases, fluid begins to build up in the lungs, leading to the hacking and chronic cough that signals pulmonary edema or fluid on the lungs. In addition, the heart begins to swell and the swelling further reduces the amount of blood flow through the dog's system. Fluid begins to build in the stomach area and this in turn causes further pressure on the circulatory and digestive system. At this time the condition is usually fatal and death will usually occur in as little as a week to as long as a few months.

Dogs with heart murmurs should have further testing through electrocardiograms or EKGs. Ultrasound machines can also be very helpful in allowing the vet to see the action of the heart in real time. Various medications including diuretics to decrease the fluid build up, ACE inhibitors to dilate the blood vessels and allow the blood to circulate more freely, as well as medications such as digitalis that increase the strength of the heart muscles are usually used in combination in treatment. Early detection, drug therapies, and careful monitoring of the heart murmur are the most effective ways to manage the condition.