The mitral or left atrioventricular valve is one of four one-way valves in the heart. It controls blood flow from the left atrium, where oxygenated blood coming from the lungs collects, to the left ventricle, where arterial blood is pumped out into the body. When the left ventricle contracts, the mitral valve closes, thus preventing blood from going back into the atrium. With a slight delay, the aortic valve, the outlet valve from the left ventricle, opens and blood is pumped out into the arterial system.
Two kinds of mitral valve disease occur. Stenosis or narrowing of the valve results in interference with the blood’s ability to flow into the left ventricle. Insufficiency is the inability for the valve to close properly, and allows blood to be pumped back through the valve into the left atrium. Either condition can result in the valve not closing properly resulting in blood leaking, regurgitating, or flowing back into the left atrium. When this happens a murmur can usually be detected.
Mitral valve disease can be the result of a birth defect, or acquired as the result of bacterial or viral infections, some types of cancer that affect the heart muscle or just as a result of the aging process, particularly in smaller breeds of dogs. When the mitral valve does not function properly, the ability of the left atrium to empty is compromised and the larger-than-normal volume of blood in the left atrium causes the pressure in that chamber to increase. As a result, blood flow out of the lungs is compromised. Depending upon the severity of the lesion, the outcome can be congestive heart failure characterized by pulmonary edema, the collection of fluid in the lungs.
Congenital mitral valve stenosis is more commonly found in Newfoundland and bull terrier breeds but can occur in any breed including mixed-breeds. Acquired mitral valve disease, particularly age-associated degenerative valve disease, can occur in any breed of dog but appears to happen more frequently in the smaller breeds and is endemic in King Charles spaniels. Mitral valve disease in the King Charles spaniel has been shown to be a polygenetic disease that can afflict over 50 percent of all individuals of this breed by the time they are 5 years old. By age 10, any of these dogs that survive almost always demonstrate signs of the condition.
Depending upon the severity and progression of the valve disease, many dogs will have no clinical signs in the early stages. We usually notice that as the dog gets older, it seems to lose energy. Your veterinarian will usually detect a murmur, the result of the blood regurgitating through the diseased valve. This results in turbulent flow and can be detected before any clinical signs are noticed. The loudness of the murmur is not always associated with the severity of disease. A small area of leaking can result in a very turbulent and noisy jet while a large area might not create enough turbulence to create a loud murmur. If the disease progresses the dog may exhibit exercise intolerance, coughing, trouble breathing, increased rate of respiration, weakness and collapse with exercise.
The diagnosis is usually made by auscultation, use of the stethoscope. If the dog is showing clinical signs of congestive heart failure, your veterinarian, or the veterinary cardiologist to whom you have been referred, may need to take X-rays, an electrocardiogram, an ultra-sound exam or even catheterize the animal to determine the severity of the disease, the prognosis and the level of treatment required.
Treatment for this condition is palliative, designed to control the symptoms and delay the progression of the disease. Medical treatment cannot cure the problem. Because the valve usually degenerates slowly the treatment can change over time. A variety of drugs are used depending on the stage and progression of disease. These include diuretics, vasodilators, positive inotropic drugs (drugs that increase the force of contraction of the heart muscle) like digitalis, and other agents that may prove beneficial in certain individuals. In humans, if the patient is showing signs of heart failure as a result of mitral valve disease, the treatment of choice is open-heart surgery and heart valve replacement with a prosthetic valve. This is possible to do in dogs, and available in some very specialized institutions, but it is expensive and usually not an option to be considered.
This disease can also occur in cats and almost any other species of animals but is most commonly identified in dogs. The problem is reasonably easy for your veterinarian to detect and another good reason for regular physical exams.
— By Dr. David Gross
Dr. David Gross of Edmonds graduated from Colorado State University’s veterinary school in 1960 and was in private practice for 10 years. He retired in 2006 as Professor and Head of Veterinary Biosciences, at the University of Illinois, Urbana-Champaign. Dr. Gross is the author of “Animals Don’t Blush,” which describes the unique patients and even more unique clients of a veterinary practice in Sidney, Montana in the early 1960s.