(Dr. Ajit Yoganathan, advisor)
"Combined Heat and Mass Transfer in Gas-Liquid Two-Phase Systems"
Since the first successful implantation of a prosthetic heart valve four decades ago, over 50 different designs have been developed. Today, the most widely implanted is the mechanical bileaflet. Nearly all bileaflet valves have good bulk forward flow hemodynamics. However, blood trauma and thromboembolic events remain the major obstacles in the realization of the ideal bileaflet mechanical valve. In addition, recent clinical evidence suggests that the current standard regulatory assessments of forward flow hemodynamics are not sensitive enough to account for the reported differences in complication rates of these valves. Therefore, there is a need for a better understanding of how more subtle differences among bileaflet valves, such as variations in hinge shape and leakage gap width, influence both valve performance and the risk of blood damage.
The goal of this study was to investigate the leakage, hinge, and near-hinge flow fields of bileaflet mechanical heart valves. Multi-component laser Doppler velocimetry was used to quantify the velocity and turbulent stress fields in the upstream leakage and hinge flow fields of several bileaflet mechanical heart valves. The physiologic significance of the flow studies was demonstrated in a novel flow system primed with whole blood in which fluorescence activated flow cytometry was used to quantify the expression of various molecular markers of blood damage.
Even when tested under similar conditions, the properties of the leakage flow fields of the different bileaflet valves were design dependent. Within the hinge mechanisms, the flow fields were very complex and unsteady. However, the active motion of the St. Jude leaflet through its streamlined hinge design contributed to a more complete washout than did the comparatively passive motion of the Medtronic Parallel leaflet through the angled recesses of its hinge. In the presence of these leakage and hinge flow fields, the destruction of formed blood elements was manifested as decreases in the red cell, white cell, and platelet counts accompanied by elevated release levels of annexin V, b-thromboglobulin, and platelet factor 4.
The results of these studies provide new insight into the roles that subtle design features such as hinge shape and leakage gap dimensions have on the potential for blood damage. The methods presented in this research also represent a very useful set of in vitro screening tools that can be employed by manufacturers during the design and development phases, before prototype valves are tested and before expensive clinical trials are initiated.