Bioactivity amines, principally, serotonin, liberated from a malignant tumor causes deformation of the tricuspid valve that leads to tricuspid regurgitation. The pulmonary valve becomes deformed by the plaque-like material resulting in a leaky, incompetent valve (pulmonary regurgitation) or a tight, stenotic valve (pulmonary stenosis). In a clinical study of carcinoid heart disease, 97% of patients had right-sided valvular involvement; severe tricuspid valve regurgitation occurred in all patients and severe pulmonary valve regurgitation in 72%.
A whitish colored plaque forms mainly on the right side of the heart and only in less than 3% of cases are the mitral and aortic valves of the left heart affected. The lesions of the valves and endocardium are caused by serotonin that reaches a high concentration in the right heart. Mini¬mal quantities reach the left side of the heart because 5-hydroxytryptamine is destroyed in the lungs by mono-amine oxidase. Some serotonin is destroyed in the liver and in the brain.
The anorectic drugs fenfluramine and dexfenfluramine exert their effects through interference in serotonin meta¬bolism. It is interesting that they were associated with lesions identical to that seen in carcinoid syndrome.