The heart has four chambers. The upper chambers are the atria (auricles, singular atrium). The lower chambers are the ventricles. The heart is also divided into right and left sides.
Blood flows from the body into the right atrium. It is stored there briefly and 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 where it is held a few seconds before going into the left ventricle. The left ventricle is surrounded by the largest and strongest of the heart muscles. This large muscle is necessary to pump blood to all parts of the body.
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 very large 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.
This is the most common cause of heart failure in small dogs. Large breeds have a lower incidence.
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. Many dogs develop a murmur from the mitral valve as early as 6 years of age. This problem is especially common in small breeds of dogs; most of them will have a murmur by 10 years of age.
However, a murmur does not mean that heart failure is imminent. As time goes on, the leak becomes more severe and more and more blood goes backwards into the 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 before heart failure occurs.
When the heart is not properly pumping blood, the blood moves more slowly through the lungs. This results in small amounts of fluid leaking out of the capillaries into the air passageways. This fluid collection produces the earliest signs of heart failure. The dog 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.
Congestive heart failure begins when the body is not able to provide blood with adequate 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 the output of blood and oxygen by the heart. For several months, these compensatory responses help the situation. However, eventually, the increased fluid retention becomes detrimental. More fluid leaks out of capillaries causing increased gagging and coughing, reduced stamina, and increased fluid collection in the abdominal cavity and body tissues. Fluid in the lungs is called pulmonary oedema, fluid below the skin is called peripheral or limb oedema, and fluid in the abdomen is called ascites. When these are present, congestive heart failure is present.
There are several tests that are used. All provide valuable information while looking at different aspects of heart function.
The combination of all of these tests give the best evaluation of the dog and its heart function. However, if cost considerations prohibit performing all of them, two or three will provide much valuable information.
A leaky heart valve can be replaced surgically in people. However, this is usually not feasible in dogs. There are several drugs that will improve heart function, even in the presence of a leaky valve.
Not all of these drugs are used in each dog in heart failure. The results of the various tests will determine which ones are appropriate.
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 ways is another indicator.
If response does not occur within a few hours to days, the prognosis is not good. However, most dogs that stabilise quickly will live for many months or a few years.