Heart Disease in Dogs: Mitral Valve Disease

MMVD (acquired myxomatous mitral valve degeneration) accounts for about 3 in every 4 cases ofHeart Disease in Dogs: Mitral Valve Disease heart disease in dogs.  It generally affects older, small-breed dogs most commonly, though any dog may be affected.  Predisposed breeds include miniature and toy poodles, Cavalier King Charles Spaniels, Cocker Spaniels, miniature Schnauzers, dachshunds, Shi Tzus, Lhasa Apsos, and various terrier breeds, as well as Doberman Pinchers.  Cavalier King Charles Spaniels are at especially high risk for this disease, which develops at an earlier age than other small breed dogs.  They should therefore be screened for the first three years of life.  About 30% of dogs with MMVD also have tricuspid valve degeneration, which may or may not be accompanied by signs of right heart failure.  This includes difficulty breathing due to fluid accumulation in the chest around the lungs and/or abdominal distension due to fluid accumulation around the organs in the abdomen.

Clinical signs of mitral valve disease in the dog

MMVD is often discovered during routine physical examination, though it may evade detection ifMitral Valve and Heart Disease in Dogs annual exams are not performed.  Though mitral disease may be present and worsening for some time, clinical signs only become apparent after heart function deteriorates beyond the body’s ability to compensate.  This is the point at which heart disease becomes heart failure, and pulmonary edema results.  Lethargy, which you might erroneously chalk up to simple aging, may be your pet’s first sign of mitral valve disease.  Heart failure can also manifest as decreased exercise tolerance, even after normal daily activities around the house.   Further deterioration of heart function results in coughing, rapid breathing, and severe respiratory distress.

Description of mitral valve disease in the dog

The one-way mitral valve should allow blood to flow from the left atrium to the left ventricle, but not the reverse.  It is made of two tough, thin fibrous leaflets anchored to the ventricular heart muscle as shown in the illustration.  They resemble two parachutes that fill, and therefore close, when blood flows backwards.  Retrograde leakage is called regurgitation or insufficiency, and results most commonly in dogs from degeneration of the leaflets which become too stiff and thickened to close completely.  This is called “acquired myxomatous mitral valve degeneration” (MMVD) and has an unknown cause.  MMVD is different than “Mitral Valve Prolapse”, which occurs in human women and does not involve valve  thickening

Click here  to see a video demonstrating normal mitral valve function and mitral regurgitation.

Increased pressure within the left atrium increases work on the heart, increases the size of the left atrium, and causes blood to back up in the lungs (pulmonary hypertension).  This can lead to left heart failure (congestive heart failure), where fluid collects around or within the airways (pulmonary edema), and is characterized by coughing, increased respiratory rate and effort, exercise intolerance, fatigue, and even collapse (syncope).

Prognosis of mitral valve disease in the dog

Fifty percent of dogs with MMVD that have mild or no signs of heart failure at the time of diagnosis live for 1 year, and 25% live to 2 years.  Fifty percent of dogs with MMVD with severe signs of heart failure at the time of diagnosis live to 6 months, and 25% live to 1 year. Despite these published statistics, many dogs that are properly managed live comfortable, albeit quiet, lives for much longer that the textbooks predict.

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Dr Roger Johnson and Dr Derek Calhoon

Roger K. Johnson, DVM, Diplomate ACVIM (internal medicine) is a board-certified veterinary internal medicine specialist. His professional interests include cardiology as well as using advanced diagnostics to help his patients. His particular favorites include echocardiography, abdominal ultrasonography, and endoscopy. Derek Calhoon, DVM is a veterinary general practitioner.

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