Issue Description

The heart has four chambers. The upper chambers are called atria (singular: atrium), and the lower chambers are called ventricles. The heart is also divided into right and left sides. 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 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 pressures created when the left ventricle contracts and the eventual process of "wearing out," the mitral valve becomes leaky in many dogs. Because this is a progressive disease, we assign each dog to one of four stages based on clinical signs, historical findings, and x-ray findings. Stage 1 is the earliest stage of mitral valvular disease. Stage 4, the final stage, is the presence of life-threatening heart failure.

Symptoms

The earliest sign of a leaking mitral valve is a heart murmur. The murmur is created by turbulent blood regurgitating backward from the left ventricle into the left atrium. The murmur is audible through a stethoscope and may progress in intensity as the disease progresses. Another early sign of mitral valve disease is a chronic dry, hacking cough. This occurs because the enlarging left atrium puts pressure on the bronchus (a branch of the airway); this compression leads to a cough.

Heart Failure
The presence of a murmur does not mean that heart failure is imminent. But, as time goes on, the leak becomes more severe and more and more blood goes backwards. 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.

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. In other words, the body's cells become desperate and trigger a series of responses. 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. This fluid collection in the lungs produces very obvious signs and may be one of the first things an owner might notice. Noticeable signs include weakness, coughing or gagging, fainting or collapse, and obvious exercise intolerance.