Prevention of disturbed flow (e.g., flow stasis) and consequent thrombosis in heart pumps is based upon design characteristics determined during laboratory bench tests. These tests employ optimal filling and emptying characteristics, such as the full-fill to complete empty mode in the Novacor left ventricular assist system. Filling characteristics of the Novacor LVAS were examined during the first 48 hours after implantation in 14 patients. Fill volume of the pump was reduced in pathologic states, such as cardiac tamponade, and following the initiation of right ventricular mechanical circulatory support. In addition, multiple regression analysis revealed that right ventricular function measured by the amount of inotropic support required, the right ventricular ejection fraction, and the total pulmonary resistance, significantly predicted left ventricular assist pump fill volume during the first 48 hours of support. Flow visualization simulating these clinical conditions of incomplete filling suggest inadequate valve washing, particularly around the inlet valve and its conduit, which may predispose to thrombus formation.