In-vitro development of a novel method to noninvasively establish BScc valve integrity

Abstract

Purpose: the Bjork-Shiley convexo concave (BScc) mechanical heart valve has an increased occurrence of unexpected mechanical failure of the outlet strut of the valve compared to equivalent valve types, with a high incidence of mortality, when it occurs. Until now no noninvasive method to determine BScc valve integrity with adequate performance has been developed. The purpose of this study is the development of a novel method to establish BScc valve integrity in-vitro. Method: the proposed method analyses the movement of both legs of the BScc valves outlet strut during the cardiac cycle. It is based on the assumption that a broken leg will have increased movement compared to either the intact leg or the flange. BScc heart valves were mounted in the mitral position in an in-vitro pulse duplicator system, which is a plexiglass model of the left ventricle that can mimic the hydrodynamics in the ventricle and concomitant forces of heart valves. A focused single element ultrasound transducer (10 MHz) is excited using a sinusoidal wave to direct ultrasound, on a particular leg of the outlet strut. Correlation based time delay estimation is then used to estimate the difference in time of flight of the echoes during the cardiac cycle. These differences are subsequently converted to produce the movement of the outlet strut during the cardiac cycle. Results: the movement of two valves has been studied, one intact valve and a valve with a single leg fracture with both ends grating against each other (SLF). No significant difference in movement could be detected between both legs of the intact BScc valve (amplitude of movement 9.2 µm ± 0.1 µm). However, the amplitude of movement of the broken leg of the SLF valves outlet strut was 12 µm ± 1.6 µm versus 8.6 µm ± 0.1 µm for the intact leg. Conclusions: the detection of a difference in movement between the broken and the intact leg (or flange) is the most difficult in the case of an SLF BScc valve, as broken leg movement is damped the most in this case. Therefore, a larger difference in movement between both legs of the outlet strut (or the broken leg and the flange) is expected for valves with a nongrating type of fracture. The proposed method has shown to be feasible in vitro and has potential for the in vivo detection of BScc valve outlet strut fracture.