Chronic valve disease (CVD), also called mitral valve disease or endocardiosis, is the most common form of acquired cardiac disease diagnosed in small- and medium-sized dogs. The mitral valve alone is affected in 60 percent of cases of chronic valve disease, whereas only the tricuspid valve is affected in 10 percent of cases. Thirty percent will have both the tricuspid and mitral valves affected. Endocardiosis is an age-related thickening of the mitral valve due to fibroblast proliferation and an increase in collagen and elastic fibers. The thickening of the mitral valve allows a regurgitant volume of blood to be forced from the high-pressure left ventricle into the low-pressure left atrium during systole. Over time, regurgitation can lead to progressive atrial and ventricular enlargement due to volume overload. Severe mitral regurgitation can lead to left-sided congestive heart failure and pulmonary edema formation. Long-term severe mitral regurgitation can lead to generalized heart failure. Right heart..