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Structure and Function of the Cardiovascular System in Dogs

By: Virginia Wells

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What Are the Common Diseases of the Cardiovascular System?

  • Acquired valvular disease. Various diseases can develop during the life of a dog that affect the valves and alter the normal function of the valves. Acquired valvular disease of the mitral valve is the most common form of heart disease encountered in small and medium-sized dogs. It affects many different breeds, including cocker spaniels, Cavalier King Charles spaniels, poodles, miniature schnauzers, Chihuahuas, terrier breeds, etc. As dogs age, the mitral valve degenerates and begins to leak. The degeneration is usually progressive, and causes the left side of the heart to change as more and more blood leaks back into the left atrium. Eventually, left-sided heart failure may develop, as the workload of the heart continuously increases.

  • Heart failure. Heart failure is the inability of the heart to maintain a circulation sufficient to meet the body's needs. Congestive heart failure (CHF) occurs as a result of impaired pumping ability and is associated with water and sodium retention. Heart failure may develop on either the right or left side of the heart, and sometimes involves both sides. It may arise with severe acquired valvular disease, certain forms of cardiomyopathy, from heartworm disease, from inflammation of the heart, secondary to pericardial diseases, and tumors.

  • Dilated cardiomyopathy (DCM). Dilated cardiomyopathy is disease where the heart muscles become weak and very flabby. As the muscles weaken, the chambers of the heart enlarge in size or become dilated. This disease primarily affects the left side of the heart. Weakening of the muscles decreases the ability of the heart to pump blood to the rest of the body. In dogs, this disease occurs most often in large breeds, such as the boxer, Doberman pinscher and Great Dane. It may also occur in some smaller dogs, such as the American cocker spaniel. In cockers, the disease has been associated with a deficiency in the amino acids, carnitine and taurine. The cause in most other dogs is unknown, but may involve genetic factors.

  • Arrhythmias. Cardiac arrhythmias are disturbances in heart rate or rhythm. The electrical activity of the heart is altered, which may greatly affect the ability of the heart to coordinate the contractions of its different chambers. Arrhythmias can be mild and insignificant, or be serious enough to cause heart failure and sudden death. Arrhythmias may arise in conjunction with almost any form of heart disease, and can also develop with other diseases in the body, such as high potassium levels, low oxygen levels, infections, hormone imbalances, drugs, trauma, and organ failure.

  • Heartworm disease. Heartworm infection is disease of both domestic and wild canines, and it occurs in many areas of the United States. It is caused by a parasite (Dirofilaria immitis) that enters the body through the bite of an infected mosquito. The adult worms prefer to live in the pulmonary vessels that lead from the right heart to the lungs. Heartworm disease in the dog most often causes obstruction of the pulmonary artery, with secondary right heart failure.

  • Congenital heart defects. Several congenital heart defects occur in the dog. Aortic stenosis (narrowing of the opening of the aorta as it leaves the left ventricle) is an inherited condition that affects primarily large-breed dogs, such as the Newfoundland, rottweiler, German shepherd dog, golden retriever, etc. Pulmonic stenosis (narrowing of the opening of the pulmonary artery as it leaves the right ventricle) is seen in the English bulldog, beagle, keeshond, bullmastiff, mastiff, terrier breeds, etc. Patent ductus arteriosus (PDA) is a deformity that arises when a small communication between the aorta and the pulmonary artery (that is present before birth) fails to close after birth. This disease affects female dogs more often than male dogs, and is seen in many different breeds. Small holes in the septum of the heart may also occur. They may be located between the two atria (atrial septal defects) or between the two ventricles (ventricular septal defects). Most of these congenital disorders affect the function of the heart and may give rise to clinical signs of heart disease and eventual heart failure.

  • Infectious endocarditis. Infectious endocarditis is an inflammation of the heart caused by some sort of infectious agent, such as bacteria, protozoa, and viruses. This condition is uncommon in the dog, but may arise with bacterial or viral infections.

  • Pericarditis and pericardial effusion. Pericarditis is inflammation of the pericardium, which is the fibrous sac that encloses the heart. Pericardial effusion is the accumulation of fluid within the pericardial sac. As fluid accumulates in this sac it applies pressure to the heart and decreases the ability of the heart to pump blood. Pericardial diseases in the dog arise with infections (e.g. bacterial and fungal), tumors, blood clotting disorders, trauma, foreign bodies, kidney failure, and for unknown reasons. Pericardial effusion occurs most often in large breed dogs.

  • Hypertrophic cardiomyopathy (HCM). Cardiomyopathy is a disease of heart muscle. In hypertrophic cardiomyopathy the muscles of the heart become tremendously thickened. This disease most often affects the muscles of the left ventricle and septum of the heart. As the muscular walls increase in size, the size of the heart chambers become smaller, which reduces the amount of blood that can flow through the heart. This form of cardiomyopathy is rare in the dog and occurs for unknown reasons.

  • Thromboembolism. A thrombus is a blood clot that develops within the heart or a blood vessel. An embolus is a blood clot that arises in one area of the circulatory system and is transported in the bloodstream to a distant site, where it becomes lodged in a blood vessel. The most common form of this disease in dogs occurs with heartworm disease. When adult heartworms die or are killed, they break apart and float to the lungs where they lodge in the small blood vessels. This causes a severe inflammatory reaction in the lungs and may significantly decrease the functional capacity of the lungs.

  • Arteritis or vasculitis. This is an inflammation of arteries or veins that may be caused by viruses, bacteria, parasites or immune-mediated diseases. This type of inflammation is uncommon in the dog.

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