Contact
This project is sponsored by:
Sponsoring time: 2 years
Project beginning: July 07
Student Theses
For student theses click here.
ThromboSIM
- Fig. 1: Streamlines of an unsteady Flow simulation through a trileaflet heart valve prosthesis in the THIA2
Experimental and numerical Correlation of thrombogenic flow patterns in heart valve prostheses
Thrombosis is still the main problem related to implanted mechanical heart valve prostheses. Thrombus formation may not only cause more or less severe stenoses, but also total valve malfunction if thrombi are located in the pivot regions. This problem may not even be finally solved by a live long anticoagulation therapy of the patient /1,2/. In this project the results of in vitro thrombogenicity studies shall be correlated to numerical simulations of the flow patterns in mechanical heart valve prostheses. Thus, an efficient tool for design optimization of mechanical heart valve prostheses shall be developed.
State of Research
According to literature thrombo-embolic complications are mainly related to unphysiologic flow patterns and the respectively high (or low) shear load of the blood cells. The exact correlation and the limits of acceptable shear stresses are not fully clear yet. Some newer results regarding the critical amount of shear stresses and the exposure time to theses shear stresses, after which damage of the red blood cells appears, could be investigated at the Helmholtz-Institute /3/. Klaus extended these studies on platelet damage and compared porcine with human blood /4/. Until today the thromogenic potential of mechanical heart valve prostheses is exclusively tested in animal studies, whereas the selection of the appropriate animal model is still controversial.
According to today's knowledge the amount of shear stresses and the exposure time of blood cells to these shear stresses are crucial for blood damage. High shear stresses activate platelets and damage red blood cells (RBCs). Shear stresses may be used as a link between computable flow patterns and the physiologic process of thrombus formation. Thrombi are preferentially formed in areas of low shear from platelets that were before exposed to overcritical shear for an overcritical period of time.
Methods
The project ThomboSIM is subdivided in two parts. On one hand, the thrombogenicity of mechanical heart valve prostheses is tested in vitro in a self developed "thrombosis tester for heart valves". On the other hand, the flow patterns in these prostheses are analysed via transient flow field simulation.
In vitro Tests
- Fig. 2: Thrombosis tester (THIA3)
Complex pulse duplicators, as they are used for standardized heart valve testing, are not suitable for thrombogenicity testing because of their large volume. In the Cardiovascular Engineering group a thrombosis tester (THIA) was developed, that was among others successfully used for comparable studies of different anticoagulants /5/.
In this project a new test apparatus (THIA II) will be developed, which simulates the anatomic situation of the aortic valve region. Physiologic flow and pressure functions may be reproducibly generated in order to study activation of platelets and the coagulation system for different operational points of the heart.
In order to detect initial thrombus formation a sound detcetion and analysis will be implemented in the tester.
In result, reproducible and analytically describable conditions may be adjusted in the experimantal set up, which may then be reproduced in the numerical simulation.
Numerical Flow Simulations
- Fig. 3: Flow in a trileaflet valve prothesis
For evaluation of the flow induced blood damage the pattern of the blood flow through the heart valve is simulated. The approaching flow profile and the pressure ratios of the tester are reproduced in the simulations for the respective heart valve prosthesis. In particular, the shear stresses and their exposure times on blood cells within the valve region are evaluated as a characteristic parameter. Leaflet kinematics are implemented in the transient simulations in order to consider valve dynamics.
The simulations are experimentally validated with PIV (Particle Image Velocimetry).
Project Goals
The primary goal of the project is to clarify the relationship between geometry, flow patterns and risk of local thrombus formation in mechanical heart valve prostheses by correlation of experimental and numerical data. Furthermore, an efficient tool shall be developed for design optimization of mechanical heart valve prostheses, which helps to reduced the number of animal trials.
