Myxomatous mitral valve disease is a common disease in older dogs of small- to medium-size breeds. The mitral valve, and less commonly the tricuspid valve may be affected which causes them to leak blood from the ventricle to the atrium of the heart during systole. With increasing severity of mitral valve leakage the forward stroke volume ejected into the aorta decreases, which may lead to an increase in blood pressure in the pulmonary vein resulting in pulmonary edema. A decrease in cardiac output may lead to a decrease in blood pressure, but the body may respond in order to normalize the blood pressure. Franzén showed in 2007 that the systolic blood pressure was increased in dogs with severe mitral insufficiency.