Implantable ventricular assist devices are a type of mechanical circulatory support for assisting the pumping of the heart. Accurate estimation of the flow rate through such devices is critical to ensure effective performance. A novel method for estimating the flow rate using the passively stabilized position of a magnetically levitated impeller was developed by our group. However, the performance of the method is affected by the gravity vector, which depends on the patient's posture. In this study, the effects of gravity on the flow estimation method are analyzed, and a compensation method is proposed. The magnetically levitated impeller is axially suspended and radially restricted by passive stability in a centrifugal blood pump developed by our group. The gravity effects were evaluated by analyzing the relationships between the radial position of the magnetically levitated impeller and the flow rate with respect to the gravity direction. Accurate estimation of the flow rate could not be achieved when the direction of gravity with respect to impeller was unknown. A mean absolute error of up to 4.89 L/min was obtained for flow rate measurement range of 0-5 L/min. However, analysis of the equilibrium of forces on the passively stabilized impeller indicated that the effects of gravity on the flow estimation could be compensated by performing additional measurements of the gravity direction with respect to impeller. The method for compensating the effects of gravity on the flow estimation should improve the performance of therapy with the implantable ventricular assist devices.
Keywords: Flow rate estimation; centrifugal blood pump; gravity effects; magnetic suspension; ventricular assist device.