Mitral valve disease is the most common cause of a heart murmur in dogs. Many cases are detected by a veterinarian after a routine examination (such as before vaccination) before dogs show any signs of illness. If your veterinarian suspects mitral disease they will offer further investigations so as to be able to advise as to whether treatment is necessary.

The dog's heart, like that of humans, is a muscular pump with four separate chambers. The right side of the heart sends blood to the lungs where it picks up oxygen. The left side of the heart pumps the blood around the body. The four chambers of the heart are separated by valves that ensure the blood always flows in the correct direction through the heart. The mitral valve lies between the 2 chambers on the left side of the heart. It prevents blood from flowing backwards (towards the lungs) rather than around the body when the heart contracts.

Damage to the mitral valves can occur in a number of ways. The most common disease is degeneration of the valve as a result of ageing (this is called endocardiosis). In other cases, the valves may fail to develop properly, eg mitral dysplasia, or can be affected by infections carried in the blood (endocarditis). Damaged valves do not work properly - abnormal valves allow leakage of blood between the heart chambers. When valves are leaking the abnormal blood flow can often be detected when listening to the heart with a stethoscope (a murmur). If the valve is leaking, not all the blood leaves the heart with each contraction and so the heart works less efficiently and has to beat faster and harder to pump the same amount of blood out into the circulation. The increased work eventually leads to further changes in the closure of the valves and progresses to changes in the shape and functionality of the heart and eventually can result in heart failure.

Many dogs with mitral valve disease have no outward signs of illness. The murmur is often detected by a veterinarian after a routine examination (often at a health check at the time of vaccination). Degeneration of the valve is more common in certain breeds of dog, particularly small breeds. In some breeds - notably the Cavalier King Charles Spaniel (CKCS) - dogs often inherit the condition from their parents. If your dog is from a breed with a high risk of developing mitral valve disease you might ask your veterinarian to check them for the presence of a murmur. Most animals with mitral valve disease are middle-aged or older but young animals can be affected. The initial sign of mitral valve disease may be reluctance to exercise or coughing due to heart failure or direct pressure on the large airways by an enlarged heart.

If your veterinarian hears a murmur when listening to your dog’s heart, they may want to perform further tests. Murmurs are common findings and if your pet is well there may be nothing to worry about, but your vet may suggest some additional tests so as to understand the cause. In some scenarios, it can be beneficial to start heart medications even when the pet is not showing signs of heart disease. Blood tests, scans and X-rays (radiographs) can be useful in the diagnosis and monitoring of heart disease. On the x-ray of your dog’s chest your veterinarian will be able to see the size of the heart and also any effects on the lungs and blood vessels. Enlargement of the blood vessels in the lungs or evidence of fluid in the chest may be the first indication of heart failure and, if these changes are present, treatment should be started immediately. Although examination and other tests may make your veterinarian suspect that your dog has heart disease, ultrasound scanning is the method of choice for finding the actual cause. With an ultrasound scan your veterinarian will be able to see the heart valves and it may be clear that they are not meeting correctly. With some special ultrasound machines, patterns of abnormal blood flow from the valve can be seen. Ultrasound examination of the heart requires considerable knowledge and experience, and your vet may wish to refer you to a veterinary cardiologist for an ultrasonographic examination known as an echocardiogram. Sometimes an ECG is required to measure the electrical activity of the heart. This may be performed in the veterinary practice, or a wearable device may be issued, to record your dog’s ECG trace over a longer period. Your veterinarian may also wish to perform blood tests, both cardiospecific and to check your dog’s general health before starting them on certain cardiac medications.

If your dog has damaged heart valves these will not repair themselves. The only permanent solution is to replace the valves. In man, replacement of damaged heart valves is possible and whilst this procedure has been carried out successfully in dogs (usually those born with a malformed valve) it is not considered standard practice in pets. Many dogs can lead good quality lives with mitral valve disease but may benefit from medication to slow progression and manage symptoms. If heart failure develops, medication can help to manage the symptoms and extend your dog’s life.

Many animals with mitral valve disease show no signs of heart disease and many of these patients require no specific treatment. Your vet and/or veterinary cardiologist will stage your pet’s heart disease and will advise treatment options depending on the stage your pet is at. In the early stages then it may be that no specific treatment or dietary changes are required, but as your pet progresses through the stages then medication and/or dietary changes may be recommended, even though your pet may seem clinically well. Early treatment can delay the onset of clinical signs. There are many drugs that can be used to help animals with heart failure these include:

• Diuretics (usually furosemide and spironolactone) to stop fluid retention in the circulation and the lungs
• ACE inhibitors to help to prevent fluid retention and make it easier for the heart to pump blood
• Pimobendan to improve efficiency of heart function
• Digoxin to slow the heart rate and may help it to function more effectively.