The heart is made up of four chambers. Upper two chambers are called atria (singular atrium) while lower two chambers are called ventricles. Between each pair of atrium and ventricle, there is a valve. Right sided atrium-ventricle valve is called tricuspid valve while left sided atrium-ventricle valve is known as mitral valve. These valve are essential for normal flow of blood in the right direction and they function to prevent backflow of blood when a specific part of the heart contracts.
The term endocardiosis refers to long-term (chronic) formation of excessive fibrous tissue of the atrioventricular valves.
Atrioventricular valve endocardiosis is a condition in which excessive fibrous tissue formation in the valves affects both the structure of the valves as well as their function. This acquired condition is characterized by degeneration and fibrosis of the cardiac valves. Degeneration and fibrosis of the cardiac valves results in the valvular insufficiency — inability of the mitral and/or tricuspid valve to work properly. With progression of this condition, the valves become thick, stiff and distorted, leading to inability of the heart to pump adequate blood to the body — a condition known as congestive heart failure. The affected valves become incapable of maintaining normal blood flow within the heart. These changes ultimately lead to heart failure and death in affected dogs.
The insufficiency of the affected valve(s) allows blood to jet back into the atrium, increasing the pressure in this chamber and decreasing venous (de-oxygenated blood from lungs) flow from lungs. This results in pulmonary venous congestion and ultimately pulmonary edema. Dilation of the atria (either left or right or both) may result in atrial arrhythmias, further decreasing cardiac output. In addition, constant jetting back of blood into atria physically damages endocardium and atrial rupture in chronic cases. Decreased cardiac output forces several compensatory mechanisms into action, further complicating the condition.
The condition is frequently seen in small to medium breed dogs including Chihuahuas, Maltese, Pomeranians, cocker spaniels, Pekingese, Cavalier King Charles spaniels, Boston terriers, fox terriers, miniature poodles, miniature schnauzers, miniature pinschers, whippets, and toy poodles.
Due to the chronic nature of this condition, eventual heart failure is seen in dogs older than ten years whereas initial symptoms are seen in younger dogs. The common symptoms associated with this condition include heart murmurs, coughing, weakness, fatigue, difficulty breathing, abdominal distention, orthonea (breathing become more difficult in lying position), bluish skin, loss of conciousness and ultimate death.