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The VentrAssist implantable rotary blood pump (IRBP) is a centrifugal pump that uses a hydrodynamic bearing to support its impeller. The pump, is to be used as a left ventricular assist device (LVAD). Varying pump speed can control the degree of left ventricular assistance. By increasing impeller speed, it is possible to transition from the normal physiological state of ventricular ejection (VE) to a state where the aortic valve remains closed (AC) throughout the cardiac cycle. Using the non-invasive parameter of instantaneous impeller speed in an ovine experimental model (N=3), we investigated state transitions. The cardiovascular system of the animal was perturbed by pharmacological intervention or by exsanguination. A total of six pump speed set point changes that caused physiological state transitions (VE to AC) were examined. A state transition index (STI) derived originally from data obtained in an in-vitro mock loop setup was found to be directly applicable in the in-vivo studies and showed statistically significant (p<0.0605) reliability in differentiating between no change in state and change in state. These data indicate that the STI may be a valuable mechanism to in optimal LVAD control.