Sensorless measurement of pulsatile flow rate using a disturbance force observer in a magnetically levitated centrifugal blood pump during ventricular assistance

Abstract

Implantable rotary pumps have been developed and used to assist the impaired heart ventricle because of lack of heart donors for transplant. Pulsatile flow rate measurement is important for controlling the flow rate of these rotary pumps. Conventional flow meters are not particularly compact; while the reliability and durability of small flow meters made using micro-electro-mechanical system technology is still uncertain. Several groups have proposed estimating flow rate using the motor power of the centrifugal blood pump (CBP), but none have succeeded in accurately estimating pulsatile flow rate. We have developed an implantable CBP, employing a two degrees-of-freedom radial controlled magnetic bearing (MB) to support the impeller without contact, which decreases the damage to blood cells and increases the durability of the device. The radial thrust acting on the impeller depends on the rotational speed of the impeller and the contractility of the heart ventricle. In this study we present a method to measure pulsatile flow rate during ventricular assistance, using the radial thrust estimated by the radial disturbance force observer of the MB of the CBP. The variable parameters of the dynamic model of the maglev impeller were identified for different flow conditions, assuming the density and viscosity of the water to be constant. The design of the disturbance force observers was based on the linear models obtained for a given rotational speeds. Under non-pulsatile conditions, the relationship between the measured flow rate and the estimated radial thrust was experimentally obtained. Using this relationship, under pulsatile conditions, a high correlation ( r > 0.94 ) between the measured and estimated flow rate was obtained, with a maximum estimation error of 0.3 L/min (15%) for average pulsatile flow rate and 0.5 L/min (7%) for the amplitude of pulsatile flow rate. Also, by observing the amplitude of the pulsatile flow rate, the backflow could be ascertained.