The most common heart disease in dogs is myxomatous mitral valve degeneration. It is the cause for 75% of dog heart disease.
The mitral valves sit between the left atrium and the left ventricle. They prevent blood from flowing backwards when the left ventricle contracts. The left atrium receives oxygenated blood from the lungs and pumps it into the left ventricle which then pumps the blood into the aorta and out to the rest of the body. The degeneration of the valves is caused by an as yet unidentified process that leads to thickening and shortening of the valves. This results in a hole between the mitral valves’ two leaflets. If the mitral valve is not completely closed, then some of this blood goes backward into the left atrium rather than going into the aorta. This regurgitation of some of the blood can be accommodated by the left atrium to some degree by enlarging or dilating. There can come a point in time though when the left atrium cannot hold any additional blood, then blood backs up into the vessels bringing blood from the lungs to the heart. This can then lead to fluid leaking into the air exchange areas within the lungs or around the lungs. When this happens it is termed congestive heart failure. In spite of the grave nature of that term, this is frequently reversible at least temporarily with medication.
Not all dogs with this disease develop heart failure. It varies from dogs without any symptoms to those developing left-sided congestive heart failure, leading to death. Cavalier King Charles Spaniels often develop myxomatous mitral valve degeneration at a younger age than others where the average age at diagnosis is around 10 years. Breeds that are also predisposed to this disease include Chihuahua, Fox Terrier, Cocker Spaniel, Miniature and Toy Poodle, Pomeranian, Pekingese, Yorkshire Terrier and Dachshund. It is also more common in males than females. Symptoms are not always present but when present, may include coughing, rapid and difficult breathing, intolerance to activity, collapse and fainting.
On physical examination a murmur is typically present. A heart murmur is the sound made by turbulent blood flow. Normally blood flows through the heart in a quiet, smooth manner called laminar flow. If there is any narrowing or obstacles or backward flow in the heart, this leads to turbulent flow and the audible murmur. Many abnormalities at the heart can result in a murmur but a murmur by itself does not indicate treatment is needed. Treatment is based on identifying the cause and severity of changes at the heart. This is done with x-rays of the chest and a heart ultrasound examination. Even if the heart ultrasound examination finds myxomatous mitral valve degeneration, treatment is not always warranted.
If the heart ultrasound shows that the left atrium has significantly enlarged, then the use of a drug, Pimobendan, has been found to prolong the time until heart failure occurs and even prevent its occurrence. Strict guidelines for its use have been established such that if the ratio of the diameter of the left atrium to the aorta is equal to or exceeds 1.6 then the medication should be started. Pimobendan functions to decrease the resistance to blood flow leaving the left ventricle, making it easier for the blood to flow into the aorta rather than backwards into the left atrium. It also increases the ability of the left ventricle to contract. Follow-up ultrasound examination following use of Pimobendan frequently reveals a decrease in enlargement of the left atrium. This is due to its overall effect to encourage blood to flow in the correct direction rather than backwards. For patients that progress to heart failure they usually improve and can be maintained with the addition of diuretics. These are medications that encourage fluid to leave the lungs and prevent its return.
In a dog with myxomatous mitral valve degeneration and no symptoms of disease and mild changes at the heart it may be appropriate to simply re-check the heart ultrasound every 6 to 12 months. Monitoring the dog’s respiratory rate while sleeping is the best method to check for progression of the disease at home. Most dogs with mild myxomatous mitral valve degeneration never have a problem with their heart disease. Unfortunately, those that develop congestive heart failure are likely going to die from it. Fifty percent die within 9 to 10 months. Only a few manage to survive more than 2 years. Early intervention with drugs like Pimobendan, when appropriate, can prevent or prolong the time until congestive heart failure develops.
