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MATTERS OF THE HEART

By John MacGregor, DVM, DACVIM | Sep 01, 2019

Myxomatous mitral valve degeneration (MMVD) is the most common acquired heart disease and cause of newly discovered murmurs in dogs. A heart murmur is a sound that can be heard with every beat which is caused by turbulent blood flow in the heart. MMVD is a manifestation of a process that can affect all heart valves, but most commonly involves the mitral valve. MMVD most commonly affects small breeds of dogs, though it can occur in larger breeds. Some breeds such as the Cavalier King Charles Spaniel can develop the disease earlier in life than most other breeds.

The mitral valve is the one that is positioned between the left atrium and left ventricle. The left atrium is the chamber that receives oxygenated blood from the lungs. Blood then flows through the mitral valve into the left ventricle and is pumped out to the body. The mitral valve closes when the left ventricle contracts, preventing blood from flowing backward into the top left chamber. A normal mitral valve is thin, supple, and anchored in place by strands of fibrous tissue called chordae tendonae (CT). Myxomatous degeneration is essentially valvular “wear and tear,” resulting in a thickened, stiffer valve with small nodules on its edges. These changes prevent complete valve closure and allow blood to flow backwards into the left atrium. This is referred to as “mitral regurgitation.”

Over time, the atrium and ventricles compensate by enlarging. Though this process continues to worsen, the exact time course is highly variable among canine patients. The increasing volume of the leakage eventually leads to increased pressure within the left atrium. Less commonly, the pressure can increase suddenly if a chordae tendonae ruptures. The mitral valve, being partially untethered, is now an open flood gate. Referred to as mitral prolapse, the high pressures within the atrium are now transmitted upstream to the lungs with fluid exuding from the lung capillaries. This is what the term “pulmonary edema” refers to in both dogs and humans. The patient would now be considered in “congestive heart failure” (CHF), requiring immediate treatment. Spoiler alert, this is not necessarily where the story ends!

Signs & Symptoms

The onset of MMVD disease is usually discovered when your veterinarian detects a “new” heart murmur during your adult dog’s physical examination (though not every murmur means MMVD). From this point, symptoms of heart disease do not usually occur for three to four years. The first symptom is often a worsening cough. Dogs with MMVD cough because the enlarged left atrium directly pushes on the airways and/or there may be lung congestion as described above. More studies show that rising breathing rates when the dog is sleeping are indicative for progressive heart disease.

Diagnosis

The first stage in the diagnosis of MMVD is the detection of a heart murmur during examination. Screening blood tests called BNP or NT-proBNP are available to detect levels of a hormone that is known to elevate when the heart becomes physically enlarged. They can be useful in determining progression and severity of the disease. Thoracic radiographs (chest xrays) are very helpful in determining heart size, the presence or absence of lung diseases and are the definitive means for diagnosing congestive heart failure. Pulmonary edema is the hallmark of CHF and can usually be readily detected. An echocardiogram (ultrasound of the heart) can noninvasively distinguish between MMVD and other causes of heart murmurs. Actual heart chamber measurements can be obtained and used to track the patient’s disease and response to therapy. Veterinary cardiologists and ultrasonographers commonly use “color flow” Doppler which is an ultrasound technique that detects direction of blood flow within the heart.

Treatment

Currently, there are no medications that are proven to slow or halt the progression of MMVD in our beloved canines. This is especially true in early onset disease. A simple cough suppressant may be all that is initially prescribed. However, the treatment for CHF IS beneficial and can improve quality of life as well as extend patient survival time.

Treatment for CHF must be timely and includes a combination of diuretics (medications that help the kidneys remove water from the body), and blood pressure-lowering drugs, which reduce the workload of the heart. The addition of pimobendan, a specific cardiac drug, helps the heart work more efficiently and is well tolerated by patients. It has literally been a lifesaver for thousands of dogs!

Veterinary cardiologists recommend patient monitoring every 6-12 months in the early stages of MMVD, and every 3-4 months if CHF has developed. Tests may include chest xrays, blood tests to determine organ function, and additional echocardiograms. The prognosis, as mentioned, is variable, but most owners find that their pet’s response to medical therapy can be very rewarding.

 

John MacGregor, DVM, DACVIM

Board -certified veterinary cardiologist

Portland Veterinary Emergency and Specialty Care