Heart Disease in Dogs: Chronic Valvular Disease and Dilated Cardiomyopathy

The leading cause of heart failure in dogs is chronic valvular disease. Next is dilated cardiomyopathy, followed by congenital heart disease and heartworms. More infrequent causes include bacterial endocarditis and myocarditis. Coronary artery disease is rare in dogs. It occurs only in dogs with severe hypothyroidism accompanied by extremely high serum cholesterol levels.

Chronic Valvular Disease

This common heart disease of unknown cause affects 20 to 40 percent of dogs. It occurs most often in toy and small breed dogs, particularly Cavalier King Charles Spaniels, Miniature and Toy Poodles, Chihuahuas, Lhasa Apsos, Yorkshire Terriers, Schnauzers, and Cocker Spaniels.

Chronic valvular disease is characterized by degenerative changes in the heart valves. The mitral valve is affected in nearly all cases; the tricuspid valve in about one-third of cases. The valve leaflets become thickened and distorted so that the free edges of the valves no longer make contact. The cords that attach the valve leaflets to the lining of the heart may rupture, allowing the valve to flap in the bloodstream.

These changes result in loss of valve function and a fall in cardiac output. When the ventricles contract, some blood is ejected backward into the corresponding atrium. This is called regurgitation. Regurgitation increases the blood pressure in the atrium and causes it to enlarge. Because the mitral valve is invariably involved, chronic valvular disease is also sometimes called mitral valve disease or mitral regurgitation.

The hallmark of chronic valvular disease is a loud heart murmur heard over the left side of the chest. A chest X-ray, ECG, and echocardiogram may show an enlarged left atrium, thickened valves, or a ruptured cord (muscle band). If the tricuspid valve is involved, there will be a loud heart murmur heard over the right side of the heart. It is important to exclude heartworms as a cause of a right-sided heart murmur.

Signs of congestive heart failure can be attributed to low cardiac output and lung congestion. They include a cough that occurs after exercise and/or is worse at night; lethargy and tiring easily; and fainting spells often related to cardiac arrhythmias.

Treatment: Many dogs with uncomplicated heart murmurs associated with chronic valvular disease remain asymptomatic for years. The disease, however, is chronic and progressive. Treatment should be started at the first signs of impending heart failure (coughing, easy tiring). The outlook depends on how far the disease has progressed and the general health and age of the dog.