Flow Rate Estimation of a Centrifugal Blood Pump Using the Balance of Control Currents Through the Electromagnets in the Magnetic Bearing

Abstract

Implantable ventricular assist devices (iVADs) are commonly used to treat patients with severe heart failure. To ensure safe and reliable operation of iVADs, it is crucial to develop a method for estimating the flow rate (Q) of the centrifugal blood pump used in iVAD without relying on a flowmeter. To address this need, we previously developed a flow rate estimation method (FEM-r) that focuses on the correlation between Q and the levitation position (r) of an impeller suspended by a magnetic bearing in a centrifugal blood pump. The FEM-r maintains a high level of estimation accuracy even when changes in blood viscosity are expected during treatment. This advantageous characteristic distinguishes FEM-r from the conventional method (FEM-I_MT) that employs motor current as an estimation index. However, a drawback of FEM-r is the requirement for an additional displacement sensor to measure r. In this study, we propose a solution to this issue by leveraging the correlation between r and the balance of currents (I_EMs) flowing through the electromagnets in a magnetic bearing. Thus, we propose an estimation method called FEM-I_EM, which utilizes the balance of I_EMs as a flow rate estimation index. The balance of I_EMs can be obtained using the minimum number of sensors required for iVAD control. First, we developed an estimation equation for FEM-I_EM and determined its estimation coefficients by multiple regression analysis based on the data obtained from experiments using a mock loop. The root mean square error (RMSE) and determination coefficient (R²) of the multiple regression analysis were 0.293 L/min and 0.965, respectively. The RMSE between the estimated and measured Q values were 0.284, 0.347, and 1.069 L/min for FEM-I_EM, FEM-r, and FEM-I_MT, respectively, indicating that the accuracy of FEM-I_EM was comparable to that of FEM-r and significantly higher than that of FEM-I_MT. Thus, the simple FEM-I_EM proposed in this study demonstrates excellent performance for clinical use, allowing accurate flow rate estimation of a centrifugal blood pump without requiring additional position sensor, viscosity measurement, or compensation method, even when the blood viscosity fluctuates.