Heart Disease: Mitral Valve Disease

What is the mitral valve?

The heart has four chambers. The upper chambers are the atria (one chamber is called atrium) and the lower the ventricles. The heart is also divided into right and left sides. 

The atrium and ventricle on each side of the heart is separated by a one-way valve which prevents the blood from going back into the atrium when the heart contracts. The valve between the left atrium and the left ventricle is the mitral valve.  

Because of the pressure that this valve has to withstand during the dog’s life it often starts to wear out. This can be detected with a stethoscope and is called a mitral murmur.

How common is mitral valve disease?

Mitral valve disease is the most common cause of heart failure in small dogs. In dogs, it represents 75% of all cardiovascular disease. Although it is more commonly diagnosed in small breeds, it can occur in large breeds. It is more commonly seen in older animals and is particularly common in some breeds such as the Cavalier King Charles Spaniel.

"Mitral valve disease is the most common cause of heart failure in small dogs."

How serious is a leaking mitral valve?

Many dogs of the smaller breeds have an audible mitral murmur from 6 years of age onwards. Often this is asymptomatic, in other words it causes no obvious signs. However as time goes on the leak becomes more severe and as more blood leaks back into the atrium this results in reduced pumping efficiency. Eventually congestive heart failure occurs. However this can sometimes be months or years from the time when the murmur was first detected.

When I took my little dog for his annual booster the vet told me he had a mitral murmur but said he was not going to treat it at this stage. Is this correct?

Many dogs have audible left side murmurs and provided there is no exercise intolerance, cough or other signs of a problem. It is not unusual to do nothing initially except perhaps reduce exercise if the dog is normally very energetic and perhaps also ensure that any excess weight is reduced.

How will I know if heart failure is present?

When the heart is not pumping the blood properly and some is leaking back into the left auricle the blood moves more slowly through the lungs. This results in a collection of fluid in the lungs. The usual sign is that a cough develops as the dog attempts to get rid of this fluid. It is as though he is trying to clear the throat. Exercise tolerance will also be reduced.

Does this mean that he will have a heart attack and die?

No, not necessarily, because initially the body tries to compensate and apart from a cough you may not notice much difference except the dog will appear to be slowing down. Ultimately, however, without treatment congestive heart failure will develop. We have a separate education handout on this condition.

How is the leaky valve assessed?

With any heart problem a similar collection of tests is carried out. 

These include cardiac auscultation, listening to the heart with a stethoscope, which is the usual way the valve defect is first picked up.  

Chest radiography is then used to check the lungs and also the size and shape of the heart. 

Blood and urine tests are performed to give an indication of any other disorders in the body, particularly those affecting the liver and kidneys which may have significance on heart function.  

An electrocardiogram (ECG) may also be carried out. This assesses the electrical activity of the heart and allows accurate determination of both heart rate and rhythm. Any abnormal rhythms, (arrhythmias or dysrhythmias) can be detected and evaluated. 

Ultrasound examination (echocardiography) utilises sound waves which are bounced off the various structures of the heart and are processed by computer and can be read as a trace on a monitor.  Using ultrasound the contractions of the heart can be evaluated in the conscious patient without risk either to the dog or to personnel.

A combination of these tests will give good evaluation of heart function. However a full work up of this nature can be quite expensive and if your dog is not insured it is worthwhile discussing cost prior to any tests since these only lead to a diagnosis. Additionally there is the cost of treatment which depends on the medication required.

What does treatment usually involve?

Today we have a wide collection of drugs which are effective in control of the condition although cure is not possible. These include diuretics and also sophisticated drugs acting directly on the heart muscle, improving contraction and regulating rhythm. In addition, special drugs are used which dilate the arteries, sometimes also the veins, thus reducing load on the heart and so easing the pressure on the diseased valve.

Is this treatment costly and is it long term?

Unfortunately most dogs have to remain on treatment for the rest of their lives. However treatment is tailored according to each patient’s needs and sometimes relatively inexpensive drugs are effective.

As a result of all the treatment, how much longer will my dog live?

Unfortunately it is difficult to answer this question with certainty. It depends on the severity of the condition and the initial response to treatment. The more rapid the response once treatment is instituted, the better the prognosis is. Many dogs, once stabilised, will live for months, or even years with no reduction in quality of life.