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Myxomatous Mitral Valve Degeneration

Your dog had been diagnosed with myxomatous mitral valve degeneration. This condition is also known by other names including mitral regurgitation, degenerative mitral valve disease and endocardiosis.

In myxomatous mitral valve degeneration, a degenerative process affects the mitral valve on the left side of the heart. It is not uncommon for dogs to have the tricuspid valve on the right side of the heart also affected. These valves are one-way valves. The mitral valve opens to allow blood to flow from the left atrium to the left ventricle. At no time should blood flow backwards from the left ventricle back to the left atrium. When this occurs, it is called mitral regurgitation and your vet will report a heart murmur. In myxomatous mitral valve degeneration, thickening of the valve occurs which causes it to leak. When this leak is small, it is of no consequence. However, over time, the leak can progressively worsen so that the left ventricle is pumping a large amount of blood backward into the left atrium rather than forward to the body. As there is now extra blood on the left side of the heart, the left atrium and left ventricle slowly enlarge to accomodate this

If there is a severe leak, over time, the pressure in the left atrium increases. This high pressure is transmitted back along the vessels that are draining blood from the lungs.  If this pressure gets high enough, fluid can leak out into the air sacs. This is called pulmonary oedema and means your dog has developed congestive heart failure. This is a life threatening condition that requires prompt treatment. Signs of congestive heart failure include an increased breathing rate and effort, lethargy, poor tolerance to exercise, reduced appetite and, occasionally, collapse. Dogs with myxomatous mitral valve degeneration often take years to reach congestive heart failure, however, some can progress faster and some never reach heart failure.


How do I test for my dog for myxomatous valvular degeneration?

There are a number of tests that can be done to diagnose myxomatous mitral valve degeneration and determine the stage of the disease in the individual dog


The Physical Examination: Dogs with mitral valve degeneration have a murmur on the left side of the chest. Heart murmurs are graded from 1-6 depending on the level of intensity of the murmur. A middle aged to older small breed dog that develops a murmur in this location is most likely to have mitral valve degeneration as the cause.


Echocardiogram (Heart Ultrasound):  An echocardiogram is the quickest, most accurate and least invasive way to the confirm the cause of the murmur, determine if any heart chambers have become enlarged from the mitral leak and look for any additional factors that may affect your dog and his or her treatment plan. Echocardiography is usually performed in a quiet, dim room on a padded table with no, or occasionally, minimal sedation.


Thoracic Radiographs (Chest Xrays): Chest xrays may be recommended to get an initial assessment of the disease severity, monitor the progression of heart disease, look for pulmonary oedema or look for other causes of respiratory disease in an individual patient. Chest xrays only take minutes to perform but 3 different views of the chest are recommended. Occasionally, mild sedation may be required to achieve this.


Blood work: Once your dog starts medication for this disease, it is important to monitor kidney function as some medications can have adverse affects on the kidneys. This is usually done when a new medication is started, when dosage adjustments are made or if your dog stops eating and drinking.


Electrocardiogram (ECG): Some dogs can develop abnormal heart rhythms (arrhythmias) due to the cardiac muscle damage caused by an enlarged heart.  If an arrhythmia is heard during the physical exam, an ECG can be performed to determine which part of the heart it is coming from and if treatment is required.


Blood pressure: High blood pressure can hasten the progression of mitral regurgitation. Some dogs can develop high blood pressure without any other signs. It is often recommended to measure the blood pressure upon diagnosis of the disease and then periodically in case additional treatment is required.


Monitoring: Once your dog has been diagnosed with myxomatous mitral valve degeneration, yearly check-ups with the cardiologist are recommended if there is no cardiac enlargement. Once cardiac enlargement occurs, a medication called Pimobendan will be recommended to try and delay further progression of the disease. Once there is cardiac enlargement, at home monitoring of sleeping respiratory rate and observation for lethargy and collapse are essential. If your dogs sleeping respiratory rate is > 40 breaths/minute, he or she has developed intolerance to exercise or collapses, an immediate veterinary visit is recommended.

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