Determination of optimal hemodynamic and pressure-volume loading conditions for patients undergoing veno-arterial extracorporeal membrane oxygenation (VA-ECMO) would benefit from understanding the impact of ECMO flowrates (QE) on the native cardiac output in the admixing zone, i.e. aortic root. This study characterizes the flow in the aortic root of a pig with severe myocardial ischemia noninvasively using contrast-enhanced ultrasound particle image/tracking velocimetry (echo-PIV/PTV). New methods for data pre-processing are introduced, including auto contouring to remove surrounding tissues, followed by blind deconvolution to identify the centers of elongated bubble traces in images with low signal to noise ratio. Calibrations based on synthetic images show that this procedure increases the number of detected bubbles and reduces the error in their locations by 50 percent. Then, an optimized echo-PIV/PTV procedure, which integrates image enhancement with velocity measurements, is used for characterizing the time-resolved 2D velocity distributions. Phase-averaged and instantaneous flow fields show that the ECMO flowrate influences the velocity and acceleration of the cardiac output during systole, and secondary flows during diastole. When QE is 3.0L/min or higher, the cardiac ejection velocity, phase interval with open aortic valve, velocity-time integral (VTI), and mean arterial pressure (MAP) increase with decreasing QE, all indicating sufficient support. For lower QE, the MAP and VTI decrease as QE is reduced, and the deceleration during transition to diastole becomes milder. Hence, for this specific case, the optimal ECMO flowrate is 3.0L/min.