Briefly, how does the heart work?
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 left side.
Blood flows from the body into the right atrium. It is stored there briefly before it is pumped into the right ventricle. The right ventricle pumps blood into the lungs, where it receives oxygen. It then flows from the lungs into the left atrium and is held there before going into the left ventricle. The left ventricle contains the largest muscle of the heart, which pumps blood out to all other parts of the body.
What is heart disease (cardiomyopathy)?
Congestive heart failure is the inability of the heart to provide adequate circulation to meet the body’s needs. Unfortunately, this condition is quite common in dogs. In fact, about 10 percent of all dogs seen in primary care veterinary practices have some form of heart disease. This percentage continues to grow as dogs get older. Up to 75 percent of senior dogs experience some form of heart disease.
Heart disease in dogs falls into two categories: congenital and acquired. Congenital heart disease accounts for only about five percent of all canine heart disease and is generally diagnosed when the dog is very young. The vast majority—95 percent—of heart disease cases are acquired. Acquired heart diseases include those that a dog naturally acquires during his/her lifetime, usually as a result of normal wear and tear, infection or injury.
What is acquired heart disease?
Acquired heart disease is further subdivided into disease with a valvular cause and disease with heart enlargement. About 75 percent of dogs with acquired heart disease are afflicted with mitral valve disease (also known as atrial ventricular valvular insufficiency or AVVI) and dilated cardiomyopathy. The remaining 10 percent of acquired heart disease is caused by a group of other conditions, such as heartworm infection and endocarditis (heart valve infection).
What is mitral valve disease?
Mitral valve disease, or MVD, is the largest category of heart disease veterinarians see. Within this group, about 10 percent of dogs between the ages of 5 and 8 are affected, as well as 20 to 25 percent of dogs between the ages of 9 and 12 and 30 to 35 percent of dogs over 13. Smaller breeds are more likely to be affected by MVD and certain pure breeds are especially susceptible. These include the Boston terrier, Cavalier King Charles spaniel, Chihuahua, minature pinscher, miniature and toy poodle, Pekingese and Pomeranian.
In a case of MVD, the valves of the heart do not seal properly, allowing blood to leak backward into the left atrium. This eventually leads to a heart murmur. The valve leakage increases the load on the heart, which is not able to adequately pump blood to the rest of the body and may become enlarged from the excess blood. MVD is a degenerative progressive disease.
What is the mitral valve?
Each side of the heart has a valve to keep blood from going backward from the ventricles to the atria. The valve between the left atrium and left ventricle is called the mitral valve. Because of the pressure created when the left ventricle contracts, the mitral valve wears out in many dogs. This wearing out process begins with a small leak that gradually gets more severe.
What are the consequences of a leaking mitral valve?
The earliest sign of a leaking mitral valve is a heart murmur. This is produced by the turbulence created when some of the blood goes backward through the leaking valve and into the left atrium. This problem is especially common in small breeds of dogs. A murmur does not mean that heart failure is imminent, but as time goes on, the leak may become more severe as more and more blood travels backward into the left atrium. This results in reduced pumping efficiency and, eventually, congestive heart failure. From the time a murmur develops, it may be a few months to several years until heart failure occurs.
How will I know if heart disease is present?
When the heart is not properly pumping blood, small amounts of fluid can leak out of the capillaries into the air passageways. This fluid collection produces the earliest signs of heart failure. There may be attempts to gag up fluid from the lungs (as if trying to clear the throat), a chronic, hacking cough and lack of stamina when exercised.
Does that mean that heart failure will occur soon?
Congestive heart failure begins when the body is not able to provide blood with enough oxygen for the tissues. Without adequate oxygen, the body's cells become desperate and trigger a series of responses.
Various hormones are released in an attempt to correct the problem. These hormones conserve fluid in an effort to increase blood volume and output of blood and oxygen by the heart. For several months, these compensatory responses help the situation, but eventually, the increased fluid retention becomes a detriment. More and more fluid leaks out of capillaries causing increased gagging and coughing, reduced stamina and increased fluid collection in the abdominal cavity and body tissues. When these symptoms are present, congestive heart failure is likely.
What is dilated cardiomyopathy?
Dilated cardiomyopathy, or DCM, affects the heart muscle itself. The weakening of the heart’s muscle hinders its ability to contract, which means blood is not pumped efficiently throughout the vascular system. The heart becomes flabby and enlarged, which further impairs cardiac output over time. DCM has a quick onset, progresses rapidly and produces dramatic effects. All four chambers become enlarged as the heart muscle stretches and thins out. This stretching also distorts the shape of the heart so that the valve leaflets are too far apart and can no longer close properly. 
DCM primarily affects middle-aged dogs. Large and giant breeds are most susceptible to DCM, including the Doberman pinscher, boxer, Great Dane, dalmatian, St. Bernard, Afghan hound, Newfoundland and cocker spaniel. It is important to note that cocker spaniels are susceptible to both DCM and MVD.
How common is dilated cardiomyopathy?
DCM is not the most common cause of heart failure in dogs in general. However, it is the most common cause of heart failure in large breed 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, are also affected.
Are there any signs of heart failure that would be noticeable to me?
When the heart is not pumping properly, blood backs up into the vessels of the lungs. Increased pressure within the vessels results in small amounts of fluid leaking out of the capillaries and eventually into the air passageways. This fluid collection in the lungs produces coughing and/or gagging, the most obvious sign of heart failure. Dogs in heart failure also tire very easily from minimal exercise. 
My dog seemed to get very ill just in the last day or two. How can this happen?
DCM develops over many months or even years. Its effects on blood flow also develop slowly. As heart function declines, the body is able to compensate for several weeks or months. However, at some point in time, the body’s ability to compensate is no longer effective and the animal goes 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.
What kinds of tests are done to assess the situation?
There are several tests that are used. All provide valuable information while determining different aspects of heart function.
  • Auscultation (stethescope). This valuable tool allows us to identify murmurs, their location and intensity, an abnormal heart rhythm (arrhythmia) and abnormal sounds within the lungs.
  • Blood and urine tests. These tests do not give direct information about heart function, but they allow us to understand other disorders in the body that may impact heart function and treatment of heart disease.
  • Chest radiographs. X-rays give us the best look at the lungs and the size and shape of the heart. In most cases, DCM causes tremendous enlargement of the heart. These changes are usually very apparent on X-rays.
  • Electrocardiogram (ECG or EKG). This is an assessment of the electrical activity of the heart. It allows us to determine heart rate and to more accurately identify any arrhythmias that may be present.
  • Ultrasound examination (sonogram or echocardiogram). This examination uses sound waves that bounce off the structures of the heart and create images on a TV-like monitor. Ultrasound exams give the most accurate determination of the size of each heart chamber and permit measurement of the thickness of the heart walls. This is seen on the monitor in real time so the contractions of the heart can be evaluated. In addition, certain measurements can be taken that allow the actual strength of the heart's contraction to be measured. Ultrasound may not be available in all private veterinary practices because of the additional training needed to learn how to perform the examination and because of the cost of the equipment.
The combination of all of these tests gives us our best evaluation of the dog and his/her heart function. However, if cost considerations prohibit every test, even two or three will provide valuable information.
Is there a treatment for heart failure caused by valvular disease?
Yes. Treatment centers on eliminating signs of congestive heart failure. We commonly use drugs, such as diuretics, ACE inhibitors and pimobendan (which helps the heart pump more efficiently), to correct these signs.
Is there a treatment for heart failure caused by dilated cardiomyopathy?
If a dog has a sudden onset of heart failure, rapid administration of the proper drugs is essential to survival. Treatment is based on a clinical presentation of each individual patient. Commonly used drugs include diuretics, ACE inhibitors and digoxin. In nutritional DCM, specific supplements are prescribed.
How much longer will my dog live?
There are many factors that must be considered before that question can be answered. The results of the tests are important and the response that occurs within the first few days is another indicator. If a response does not occur within a few hours to days, the prognosis is typically not good. It can be difficult to generate an accurate estimate for life-expectancy when a dog has heart disease because so many variables impact survival.
See Heart Disease in Cats