The heart pumps blood around the body. Its most important component is the left ventricle (LV) because it pumps freshly oxygenated blood out to the entire body. Heart failure is when a diseased heart cannot provide the body with enough blood flow. One treatment option for heart failure is the Left Ventricular Assist Device (LVAD). It is an implantable mechanical pump that augments the LV function and ensures adequate blood flow to the body. One example is the Thoratec HeartMate II, a new and popular LVAD model. The mitral valve prevents blood from flowing backward from the left ventricle to the left atria. Mitral regurgitation, a common problem in heart failure patients, is when the valve does not function normally and blood does flow backwards across it. In this investigation the SDSU Cardiac Simulator was used to examine mitral regurgitation in LVAD patients with a closed aortic valve. Mitral regurgitation was found in simulations with an incompetent mitral valve, but not in simulations with a normal mitral valve. Surgical mitral valve repair should be considered in these patients to prevent mitral regurgitation and its detrimental effects.