Using Visible Heart® methodologies, swine hearts were reanimated and implanted with a left ventricular assist device (LVAD). System components were modified to better understand the relationships between the data collected and system conditions. The viscosity of the blood-replacement buffer was increased to a more physiologic value (~3 cP). Pressure-Volume (PV) loop recordings from each heart on the ex-vivo perfusion system more closely mimic loops recorded from the heart in-vivo. This indicates that any system aiming to characterize or examine LVAD performance must include a buffer which mimics blood viscosity or data must be adjusted for the relative viscosity of the buffer.