Control of Heart Assist Devices

In this paper, we discuss several control techniques for Ventricular Assist Devices (VADs) and describe a hierarchical control structure that can be used to regulate the operation of a turbo-hydrodynamic VAD. Early assist devices were pulsatile, and control usually focused on regulating flow, atrial and/or arterial pressures. In recent years, non-pulsatile turbo-hydronamic devices have been introduced because of their reliability, small size, and energy efficiency. Open-loop control algorithms built into these devices maintain pump speed at a reference value, and the primary control problem is to determine an optimum desired reference value. Because of the possibility of regurgitation and/or ventricular suction, the pump speed must be maintained between a minimum and maximum value.. This is often done manually. Heuristic feedback control algorithms that use characteristics of the blood flow and/or motor current as feedback variables can also be used for this purpose. Additional physiologic constraints, such as maintaining arterial blood pressure within specific bounds, are satisfied using a multi-objective optimization technique. The resulting control can be implemented in feedback form, if sufficient and reliable measurements or system models are available to estimate these variables. In this paper, we discuss the characteristics of a hierarchical feedback controller that combines both heuristic and optimal approaches. The hierarchy includes a supervisor that continuously evaluates the available estimates of hemodynamic variables, reliability of a patient model, past history of the patient, and validity of the information available. Depending on this evaluation, the supervisor uses heuristic criteria, multiobjective optimization, a combination of both, or a default algorithm to determine the pump reference speed. It also evaluates the operating status of the device.