(515bl) Experimental Characterization Of In Vitro Venous Valve And Venous Tissue

The human veins are not a simple system, rather they can be described as collapsible tubes with special flexible structures embedded within the tubes. These flexible structures act as one-way restrictions that enable fluid flow to move against gravity. Hemodynamics within the venous system play an important role in venous return of blood from the extremities to the upper body and heart [1,2]. The opening and closing of the valve are caused by pressure gradients proximal and distal to the valve. When venous valves fail to perform chronic venous insufficiency (CVI), edema, and deep vein thrombosis (DVT) may occur. In a study on deep vein repair, Wilson et al. state that very little is known of the flow and pressure properties of venous valves [3]. One of the challenges of developing accurate mathematical models of venous blood flow is to describe the irregular, dynamically changing tube geometry (collapsible/expandable nature of the venous vein). The aim of studying venous, hemodynamic forces is to elucidate the biomechanical basis of venous valve failure and its link to cardiovascular diseases. The main goals of this study are to obtain the material properties and the functional, three-dimensional geometry of the venous valve. This work deals with obtaining experimental data from in vitro venous valve and vein segments excised from the deep veins in the legs of cadavers. It is hypothesized that the performance of the venous valve is a function of the material properties, pressure gradients, and valve geometry. Data from this study can be used to validate a mathematical model of the venous valve interaction with hemodynamic forces under different loads and to provide insights as to how the presence of DVT, originating in the valve sinus, results in CVI.

This study investigates the effects of venous valve geometry on valve performance and hemo-dynamic flow and identifies parameters that regulate the venous valve. Selected sections of the venous specimens will be subjected at least three different data collection experiments. The first, will be to measure fluid flow performance parameters in vitro venous segments using a specially designed pulsed flow device and data acquisition system. The second, will be concerned with obtaining the physical dimensions of the lumen interior in its physiologically extended state using silicone cast [1]. The third experiment will measure the material properties of the venous tissue specimens using sensitive force and displacement sensors. The minimum study data set will include at least three valves from the femoral triangle area of each thigh of each cadaver. Data to be collected about the origin of the specimen include: location in leg from which extracted i.e. left or right leg, deep or superficial, distance from hip joint, age, sex, weight, height of subject.

Pulsed fluid flow will be applied in vitro to venous valves, excised from the upper thighs of cadavers, to simulate dynamic fluid/structure interaction. The pulsed flow is used to test the performance of in vitro venous specimens under different but physiologically feasible fluid flow and pressure conditions. Data will be collected at either constant fluid flow conditions or with different impulse stimuli in the form of venous compression via an inflatable air bag. All data are recorded using National Instruments' LabVIEW? and will be synchronized at a rate of 200 Hz with images collected using a high speed CCD (charged couple device) camera. Performance data to be collected include: up to six hydrostatic or differential pressures at different locations in the venous test segment, one instantaneous mass flow rate, and one image of the valve leaflets from an arthroscopic viewpoint. Performance parameters include: valve opening, angle of leaflet flexion, leaflet length with antegrade or retrograde flow, and competency as defined by [4].

A novel experimental apparatus to visualize flow through the valve and new techniques to analyze the flexible motion will be employed. In order to analyze changes in the geometry, changes in the size of the valve opening will be captured with the use of the CCD camera attached to an arthroscope fitted with a specially designed bi-prism to capture stereo images. The images will be analyzed using stereo image analysis techniques to provide the dimensions of the valve [5]. These data, combined with flow and pressure data, will be used to estimate the stresses on the structure and to provide insight into the flow patterns developed around the valve.

A silicone mold of the interior lumen of the venous valve will be made using molds of silastic material [1]. Using the (negative) cast, parameters such as inlet-, outlet-, and sinus diameters; leaflet height; and sinus- and leaflet free edge lengths will be recorded and analyzed. Stress/strain properties of the venous tissue will be characterized by subjecting the vein tissue to experimentally controlled stresses and measuring the resulting strains using sensitive force and displacement sensors [6]. Each specimen section will be systematically tested in two different directions. This data combined with the mapping data will give an overall picture of the distribution of the material properties throughout the venous structure. The same approach will be used to determine the stress/strain properties of the valve leaflet.

Literature Cited

[1] L.B. Notowitz. Normal venous anatomy and physiology of the lower extremity. Journal of Vascular Nursing, XI:39?42, 1993.

[2] R. Gottlob and R. May. Venous Valves. Morphology, function, radiology, surgery. Springer-Verlag, New York, NY, 1986.

[3] N. M. Wilson, D. L. Rutt, and N. L. Browse. In situ venous valve construction. British J. of Surg., 78:595?600, 1991.

[4] C. D. Buescher, B. Nachiappan, J. M. Brumbaugh, K. A. Hoo, and H. F. Janssen. Experimental studies of the effects of abnormal venous valves on fluid flow. Biotechnology Progress, 21(3):938-945, 2005.

[5] D. Lee and I. Kweon. A novel stereo camera system by a biprism. IEEE Transaction on Robotics and Automation, 16(5):528?41, 2000.

[6] O. J. Deters, Bargeron C. B., F. F. Mark, and M. H. Friedman. Measurement of wall motion and wall shear in a compliant arterial cast. Journal of Biomechanical Engineering, 108:355?58, 1986.