Application of passive flow control to mitigate the thromboembolic potential of bileaflet mechanical heart valves: an in-vitro study
Date
2014
Authors
Forléo, Márcio Henrique, author
Dasi, Lakshmi Prasad, advisor
James, Susan, committee member
Orton, Christopher, committee member
Dinenno, Frank, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Implantation of a bileaflet mechanical heart valve (BMHV) continues to be associated with risk of thromboembolic complications despite anti-coagulation therapy. Mechanical heart valves have been the gold standard in valve heart replacement since the 1950s with BMHVs currently still being the valve of choice for younger patients. Given that a large body of literature points to thromboembolic complications due to poor hemodynamics, improvements to the hemodynamic performance of BMHVs are needed. In this study, we explore the concept of passive flow controls that have been widely used in aerospace industry as a novel approach towards improving BMHV design. Passive flow control elements are small features on solid surfaces, such as vortex generators (VGs), that alter flow to achieve desired performance. The specific aims of this study are (1) develop a methodology to evaluate thromboembolic potential (TEP) of BMHVs using in-vitro particle image velocimetry technique, (2) quantify the efficacy of rectangular VGs distributed on BMHV leaflets to reduce TEP, and (3) quantify the hemodynamic performance impact of rectangular VGs. An in-vitro pulsatile flow loop along with Particle Image Velocimetry (PIV) flow visualization technique was developed, validated, and utilized to acquire time-resolved velocity fields and shear stress loading: Lagrangian particle tracking analysis of the upstream and downstream flow during diastole and systole enabled the calculation of predicted shear stress history and exposure times corresponding to platelets. This information was then used in numerical models of blood damage to predict the TEP of test heart valves using established platelet activation and platelet lysis parameters. BMHV leaflets were constructed using 3D printing technology with VGs based on micro-CT scans of a model BMHV leaflet. Two configurations were constructed: co-rotating VGs and counter-rotating VGs. Co-rotating VGs consist of single features 1mm tall and 2.8mm long spaced equally apart (5mm) at an angle of attack of 23 degrees. Counter-rotating VGs consist of mirrored feature pairs 1mm from each other with the same dimensions as the co-rotating VGs. The leaflets were tested using the methodology described above to elucidate their effect on the TEP of the BMHV compared to the control leaflets. For systolic flow downstream of the valve, we report a decrease in the average platelet activation and average platelet lysis TEP (both normalized by the average exposure time) largely in the central jet, with the vortex generator equipped leaflets compared to the control leaflets at a p-value of 0.05. However, for diastolic flow upstream of the valve, we report an increase in the average platelet lysis TEP and average platelet activation TEP (both normalized by the average exposure time) largely in the regurgitant jet zone with the vortex generator equipped leaflets compared to the control leaflets at a p-value of 0.05. Also, steady and pulsatile flow experiments were conducted to calculate the transvalvular pressure drop across the model BMHV with control leaflets (no VGs) and leaflets containing VGs to calculate effective orifice area (EOA), which is an index of valve performance and is related to the degree to which the valve obstructs blood flow. We report a significant increase in EOA values for valves with leaflets containing passive flow control elements in both steady and pulsatile flow experiments compared to the control leaflets. Under steady flow, the co-rotating VGs configuration had the best EOA value compared to the control leaflet and counter-rotating vortex generator configuration. However, under pulsatile conditions, the counter-rotating VGs configuration had the best EOA value compared to the control leaflet and co-rotating vortex generator configuration. PIV measurements highlight the delay in flow separation caused by the VGs and corroborate the increased pulsatile flow EOA values. This study shows that the TEP of BMHVs can be accurately evaluated using in-vitro PIV techniques and that there is room for improvement in BMHV design using passive flow control elements. With optimization of passive flow control configuration and design, it is possible to further decrease the TEP of BMHVs while increasing their hemodynamic performance; thus creating a safer, more efficient BMHV.
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Subject
Bileaflet mechanical heart valve
passive flow control
thromboembolic potential
vortex generator
blood damage
cardiovascular