A healthy canine heart has chamber walls that are flexible – they contract easily in order to pump oxygenated blood out to the body. Hypertrophic Cardiomyopathy (HCM) is a type of heart disease with characteristics of thickening and concentric enlargement of the walls of the left ventricle. This causes the ventricular walls to become increasingly rigid and less flexible, which in turn leads to a reduction in the outflow of blood and increased resistance to filling. Instead of flowing forward into circulation, the blood goes backwards to the left atrium and pulmonary vein. In some cases, the blood becomes stagnant and increases the dog’s risk of thromboembolism. This disease is fairly rare in dogs, but it does occur. It is common in cats.

Much like Dilated Cardiomyopathy, there is no real known cause of HCM. There is a suspected hereditary component that causes a defect in the heart muscle cells within the left ventricle. Once the thickening occurs, and the blood flow is affected, the body’s organs and systems fail to receive a sufficient amount of oxygenated blood. This disease almost always results in congestive heart failure.

Because there is no known cause for HCM, there is no way to prevent it. Dogs that show signs of this condition should be removed from breeding programs. Breeding pairs that have dogs with this condition should not be bred to each other again. Siblings of affected dogs are sometimes removed from breeding programs as well.

Many times, a dog with HCM will show no clinical signs – instead, the disease is detected during a routine check-up or when a chest x-ray for another problem shows signs of the enlarged heart.

Symptoms of this form of heart disease include vomiting, panting, increase respiratory rate, coughing, respiratory distress, lack of appetite, exercise intolerance, weakness, depression, lethargy or listlessness, collapse, acute onset of hind-limb pain or lameness, weight loss, and sudden death. If you notice any signs of heart disease, get your dog to a veterinarian as soon as possible.

Dogs at Risk
This is a very rare disease in dogs. Breeds with a higher-than normal incidence of HCM are German Shepherd Dogs, Dalmations, Rottweilers, Boston Terriers, and some Pointers. Male dogs seem to be affected more often than females, although female dogs have been diagnosed with this type of heart disease. Most show signs before three years of age, although Boston Terriers do develop this disease as mature dogs.

Testing and Diagnosis
When a vet notices that a dog’s left ventricle walls are thickened and stiff, he or she must first rule out all other possible causes before coming to a diagnosis of Hypertrophic Cardiomyopathy (called diagnosis on the basis of exclusion). Other, less rare conditions that may cause these symptoms include subaortic stenosis, hyperthyroidism, systemic hypertension, and acromegaly. Usually, the best way to identify heart problems is through a specialized ultrasound of the heart, called echocardiogram. It is non-invasive, although the dog generally needs to be sedated to make sure it lies still for accurate results. The echocardiogram can show blood pooling, mitral valve regurgitation, outflow obstruction, and abnormal blood turbulence. These abnormalities are all signs of HCM.

Other methods of diagnosis include auscultation of the heart to check for abnormal heart sounds; electrocardiogram to measure the rate, rhythm, and electrical activity of the heart; blood work and urinalysis; thoracic x-rays to check for thickening of the walls; and measurement of the dog’s systemic blood pressure.

Treatment and Prognosis
Treatment goals are to relieve discomfort and restore the dog’s quality of life. To do so, the vet will reduce any fluid that has built up around its lungs and heart, and stabilize its heart rhythms. Several drugs may be used to help stabilize or reduce the dog’s heart rhythm and rate, or to prolong ventricular filling time. These drugs are called vasodilators, calcium channel blockers, ACE inhibitors, and beta blockers. If required, the vet will draw fluid off the chest wall – called thoracocentesis – with a needle. Oxygen therapy may provide relief. Finally, if the dog’s electrolyte or fluids are imbalanced, IV fluids should be administered.

Dogs with HCM should eat a diet low in salt and should be kept from being placed under undue stress. Exercise should be done to the dog’s tolerance level only. Some vets prescribe an anticoagulant drug to prevent arterial thromboembolism.

Prognosis depends upon the progression of the disease when it is diagnosed. In mild cases where the dog shows no clinical signs, prognosis is from good to fair. Once the heart’s walls become severely thickened, however, and the dog experiences pulmonary edema and/or pleural effusion in addition to left atrial enlargement, prognosis may be guarded to poor. In cases where the dog develops thromboembolisms, prognosis is grave.