The heart has four chambers. The upper chambers are called atria (singular: atrium), and the lower chambers are called ventricles. In addition to the upper and lower chambers, the heart is also considered to have a right and a left side.
Blood flows from the body into the right atrium. It is stored there for a few seconds, then pumped into the right ventricle.
The right ventricle pumps blood into the lungs, where it receives oxygen. It flows from the lungs into the left atrium; it is held here for a few seconds before going into the left ventricle. The left ventricle contains the largest muscle of the heart so the blood can be pumped out to all parts of the body.
Dilated cardiomyopathy (DCM) is a disease wherein the heart muscle (myocardium) becomes much thinner than normal. In particular, the thick muscle wall of the left ventricle is affected. The pressure of the blood inside the heart causes this thinned wall to begin to stretch, resulting in a much larger left ventricular chamber. The three characteristics of dilated cardiomyopathy are a heart wall that is much thinner than normal, a chamber that is much larger than normal and decreased contractility of the heart wall.
Primary dilated cardiomyopathy is the most common cause of heart failure in large breeds of dogs. Small breeds are only occasionally affected. The most commonly affected breeds are Boxers, Doberman Pinschers, and Great Danes. Occasionally, medium sized breeds, notably Cocker Spaniels and English Springer Spaniels, develop this condition.
In some cases, DCM develops secondary to a chemical toxicity, nutritional deficiency, or inflammatory condition in the heart. Doxorubicin, a drug commonly used in chemotherapy, can induce DCM after repeated administration. Nutritional deficiencies of carnitine and taurine have been lined with DCM, although this cause is uncommon. Some non-cardiac conditions, such as pancreatitis and electrical shock, have occasionally been found as a cause of DCM. Unfortunately however, for most dogs, the cause is unknown. This is called primary or idiopathic DCM.
When the heart begins to fail, it is unable to deliver adequate oxygen to all the tissues of the body. This sets into motion a series of compensatory events. Various hormones are released by several organs in an attempt to correct the problem. These hormones conserve fluid in an effort to increase blood volume and the output of blood and oxygen by the heart. For several months, these compensatory responses help the situation. However, the increased fluid retention eventually becomes harmful.
Perhaps the most detrimental event occurs when this excessive fluid leaks out of the pulmonary capillaries and into the air spaces (alveoli) of the lung; this is called pulmonary edema. Noticeable symptoms include weakness, coughing or gagging, fainting or collapse, and obvious exercise intolerance.
Fluid may also collect in the abdominal cavity and body tissues. Congestive heart failure is a common cause of these signs. Dilated cardiomyopathy may have a very sudden onset. Some dogs go into severe heart failure in what appears to be a matter of hours. Rapid, heavy breathing, a blue tongue, excessive drooling, or collapse may be the first signs.
There are several tests that are used to look at different aspects of the heart’s structure and function.
1. Auscultation: a stethoscope is used to identify murmurs, their location, intensity and any abnormal heart rhythm (arrhythmia). It also allows us to hear sounds within the lungs.
2. Blood and urine tests: These allow us to understand other disorders in the body that may impact heart function and treatment of heart disease.
3. Thoracic radiographs (Chest x-rays): These give us the best look at the lungs and a view of the size and shape of the heart. In most cases, dilated cardiomyopathy causes enlargement of the heart. These changes are usually very apparent on the radiographs.
4. Electrocardiogram (ECG): This is an assessment of the electrical activity of the heart. It allows us to accurately determine heart rate and to more accurately identify any arrhythmias that might be present.
5. Ultrasound examination (sonogram, echocardiogram): This examination uses sound waves that bounce off the structures of the heart. It gives the most accurate determination of the size of each heart chamber, and permits measurement of the thickness of the heart walls. Certain measurements can be taken which allow the actual strength of the heart’s contraction to be measured as a number and compared to the normal animal.
The combination of all of these tests give us our best evaluation of the dog and its heart function.
There is not very much data to prove efficacy of therapy for dilated cardiomyopathy in dogs until after the occurrence of congestive heart failure. Human studies have indicated that therapy with beta blockers such as carvedilol and metoprolol should provide benefit. Enalapril or other angiotensin converting enzyme inhibitors(ACEi) may also be useful. Proper nutrition including taurine is also beneficial.
If the dog has a sudden onset of heart failure, rapid administration of appropriate medication is essential to survival.
Therapy for congestive heart failure for a dog with DCM might include:
1. An ACEi: These block some of the hormonal changes that accompany congestive heart failure. Some of these changes include higher blood pressure, saving fluid and salt and constricting blood vessels. This is generally a life long therapy. Kidney function can rarely be affected, so regular bloodwork is important.
2. Diuretics: These act on the kidney to make the dog produce more urine. Diuretics can be both an emergency and a long term therapy.
3. Pimobendan: This is a new therapy that dilates blood vessels and increases the heart’s ability to contract. It has markedly better survival than other medications used to increase contractility, perhaps because of its dilatory effects.
4. Digoxin: This medication slightly increases contractility and also can help slow some abnormal heart rhythms that can be associated with DCM. Digoxin has a very narrow therapeutic range (the toxic and therapeutic doses are similar) and it does not increase contractility very much. It has somewhat fallen out of favor as a primary therapy for DCM.
Prognosis varies widely given test results and relative responsiveness to medical treatment. However, while the long-term prognosis remains fair to poor, most dogs that stabilize quickly will live for a period of 6 – 12 months.