A degenerate valve becomes a leaky valve. Valves in the heart exist to ensure that flow occurs in only one way from the atria to the ventricles and from the ventricles to the lungs (right side) and the rest of the body (left side). A valve that leaks will lead to a certain amount of blood flowing back into the previous chamber – regurgitation – and the heart will have to cope with more blood in successive beats.
As a result of the surplus of blood in the left atrium caused by regurgitation, the left atrium will be forced to enlarge and the left ventricle will also have to become larger and pump more blood to compensate for the blood “loss” by regurgitation.
The heart may be able to cope well with the extra work for a long time up to the point where the pressure in the left atrium is so high that blood accumulates in the blood vessels of the lung – congestion – and fluid leaks into the lungs – congestive heart failure with pulmonary edema.
This is a chronic and progressive condition but not all affected dogs go on to develop heart failure. Some may live all their lives without ever showing any signs of disease. A classification exists to illustrate disease severity and patients may progress from one stage to the following throughout their lives:
Stage A – Predisposed breeds (see below) currently without any sign of disease
Stage B1 – Disease is present without signs of heart enlargement nor heart failure
Stage B2 – Disease is present with signs of heart enlargement but no heart failure
Stage C1 – Congestive heart failure in need of hospitalization
Stage C2 – Current or past signs of congestive heart failure on treatment at home
Stage D1 – Refractory heart failure in need of hospitalization
Stage D2 – Refractory heart failure treated at home