Continuous-flow left ventricular assist devices/systems (LVADs/LVASs) reduce symptoms and mortality in severe heart failure. The impeller or centrifugal designs provide challenges in assessing and titrating pump speed (revolutions per minute [RPM]), flow, and native heart pulsatility, and contribution to cardiac output. The Thoratec HeartMate II (HM II) LVAS is the most commonly used LVAD worldwide. The user sets the RPM and the monitor provides online data on RPM, power consumption, flow, and pulsatility. These parameters are routinely used by clinicians to assess native heart function and to optimize pump settings. However, little is known about their reliability, reproducibility, and variability. Therefore, we assessed HM II controller parameters and concurrent echocardiography during titrations of RPM to low and high values. We found that data displayed on the monitor and logged in the controller are consistent for power consumption and for flow at settings above 8,000 RPM but inconsistent and unreliable for flow at or below 8,000 RPM and for pulsatility throughout a range of common settings and specifically at 9,000 RPM. These findings have implications for clinicians attempting to optimize settings and assess pump and native heart function.