Physiologically motivated control of rotary blood pumps

This thesis presents arguments and simulation results in favor of a novel strategy for control of rotary blood pumps. We suggest that physiological perfusion is achieved when the blood pump is controlled to maintain an average reference differential pressure. In the case of rotary left ventricular assist devices, our simulations show that maintaining a constant average pressure difference between the left ventricle and aorta results in physiological perfusion over a wide range of physical activities and clinical cardiac conditions. An integrated model of the human circulatory system with continuous flow ventricular assist devices (VAD) and a feedback VAD controller designed to maintain physiologically motivated perfusion were developed. The developed integrated model combines a network type model of the circulatory system with a nonlinear dynamic model of a brushless DC axial flow pump and a centrifugal flow pump. A gain scheduled proportional integral (PI) controller was developed to maintain the reference differential pressure for each pump. The differential pressure is initially measured requiring the use of two external pressure sensors that are unreliable in the long-term. The pressure differential is then estimated using an extended Kalman filter (EKF), which requires the implantation of only one flow sensor. A control algorithm that uses no external sensors and uses only the intrinsic pump parameters of revolutions per minute (rpm), current and voltage is then developed, which can be used to estimate the pressure differential using an EKF and Savitzky-Golay method, eliminating the need for any failure prone implantable sensors and improving the overall reliability of the system. Maintaining a reference differential pressure between the left ventricle and aorta leads to an adequate perfusion for different pathological cases ranging from the normal heart to left heart asystole, and widely varying physical activities from rest to exercise for both the pumps. Comparison with the traditional control strategy of maintaining a reference rotational speed (rpm) of the pump indicates that though the traditional approach has some degree of adaptability, it is only adequate over a narrow range of cardiac demand and clinical conditions of the patient. The viability of maintaining a constant pressure difference between the left ventricle and aorta was verified with preliminary experimental results using a mock circulatory system. Our results indicate that the proposed approach is superior to the alternatives in providing an adequate and autonomous adaptation of the total cardiac output over a broad range of exercise conditions (expected when an assist device is used as a destination therapy) and clinical statuses of the native heart (such as further deterioration or recovery of cardiac function), while having the potential to improve the quality of life of patients by reducing the need for monitoring and frequent human intervention.