K. Schichl

The use of image processing in the investigation of artificial heart valve flow.

A circulation model enlarged ten times with a likewise enlarged model of the artificial heartvalve leads to a very slow motion of the fluid. Many methods of flow visualization can therefore be applied. The particle method has been chosen as the most appropriate for further processing. The flow is videotaped, two consecutive frames are selected and with the help of image processing the vector field, the streamlines and the velocity profiles are computed and drawn.

A computer controlled versatile pulse duplicator for precision testing of artificial heart valves.

Numerous devices and mock circulations have been described for the measurement of pressure loss, closure time, closing and leakage volumes and energy loss in artificial heart valves. However, all the devices have been troubled with difficulties in generating and assessing the precise flow through the valve, and problems in defining the arterial load, i.e. the artificial aorta. The new test device follows a radically different approach: a computer controlled piston forces the fluid through the test valve only — with no afterload. During systole, outflow follows a physiological curve which is identical for all types of heart valves of a given size. During diastole a mathematically defined physiological pressure difference curve is followed. Consequently, the measurements are independent of the individual machine, the lab where testing takes place, the scientist who executes the test, the time when measurements are taken and all other external influences.

A Ten Times Enlarged Model of Artificial Heart Valve Flow

The flow through artificial heart valves has been studied at great length using a variety of methods. The most versatile and lately most popular method is Laser Doppler anemometry (LDA), introduced into the field of artificial heart valve research in 1979 [1,2]. Measurements performed with this method do not interfere with the flow, because only laserbeams and no physical probes are present within the flow. The velocity at one point within the flow field can be measured with a good timewise resolution. The spatial resolution is determined by the accuracy of the traversing unit. With several laserbeams it is possible to measure all three components of the velocity [3]. From this, turbulent information, such as the Reynolds shear stresses can be calculated. A great number of artificial heart valves have been systematically investigated in this way [4].

FLOW SEPARATION IN ARTIFICIAL HEART VALVES

Flow separations are most unwelcome in the flow through artificial heart valves as they are in most technical flows. The bulk flow properties as the resistance of the valve or its closing behaviour are not so much influenced by flow separations as another phenomenon which is unique to blood flow — the generation of a thrombus. This phenomenon which is normally useful by closing leaks in the vessel system becomes harmful, when a mechanical device such as an artificial heart valve is implanted. In areas of stagnant flow the blood solidifies and can impede or even completely close off the flow. At a rate of 3 to 5% the patients die each year of complications related to this undesired interaction of the blood with the implant. From detailed in vitro experiments one knows the flow conditions, which are relevant for the generation of a thrombus. The first condition is the flow of blood through a volume of high shear stress, either in a jet or at a wall, which exceeds τ = 100 N/m2 and lasts for more than 30 ms [1]. This high shear stress activates the cells, which are responsible for a thrombus formation, the platelets. The second condition for the generation of a thrombus is the presence of an area of recirculation close to a wall of a foreign material. The recirculation is found in a flow separation and allows a concentration of platelets. If they are activated by the mechanical shear stress and then accumulated densely enough and in addition placed in the vicinity of foreign material to which they can attach, then the formation of a thrombus is likely. Investigations of the flow through artificial heart valves in the past have been concentrated on flow properties as resistance or turbulence, however, investigations of the flow in regard of the thrombus generation have been scarce, and it was the aim of this project to illucidate the role of the flow in thrombus generation.