The three heart diseases I researched were chronic valvular disease, dilated cardiomyopathy, and heartworm. All three are life threatening and can be extremely fatal in canines. Chronic valvular disease is the most common heart disease in canines. It has an unknown cause and affects 20 to 40 percent of dogs. Smaller breeds of dogs are most affected. The disease is characterized by deteriorating changes in the heart valves. In most cases the mitral valve is affected. The tricuspid valve is affected in about one-third of cases. As one doctor writes, “The valve leaflets become thickened and distorted so that the free edges of the valves no longer make contact” (Dog Normal Heart Rate and Irregular Heartbeat). This can cause the cords that attach the valve leaflets to the lining of the heart to rupture, allowing the valve to flap in the bloodstream. These changes result in loss of valve function and a fall in cardiac output. The so called “trademark” of chronic valvular disease is a loud heart murmur, which can be heard over the left side of the chest. In most cases dogs with uncomplicated heart murmurs related to chronic valvular disease can remain symptom free for years. Treatment should be started at the first signs of heart failure. This includes coughing and becoming easily tired. Dilated cardiomyopathy is a disease of the heart muscle that reduces the pumping function of the heart. Symptoms usually include tiring, fainting, heart murmur, arrhythmias, and enlargement of the heart. As the heart becomes enlarged, its ability to pump blood out into the lungs and body deteriorates. This enlargement according to petmd.com “becomes overloaded, and this often leads to congestive heart failure.” Normally larger bred dogs are affected by dilated cardiomyopathy. The symptoms may be harder to notice in some dogs, especially older ones since they exercise less. For treatment “drugs may be administered to enhance heart contraction and to slow down rapid beating, and diuretics