Fatigue and wear testing of prosthetic heart valves
Background and principle function
An important aspect for the certification of a new prosthetic heart valve is the proof of durability. To obtain sufficient information about durability and wear within an acceptable period of time, the valves are tested with accelerated rates up to 1000 cycles/min, nearly equivalent to a time compression of factor 14 in comparison to physiological conditions.
- Figure 1: Fatigue test system HIA-FT1
- Figure 2: Fatigue test system HIA FT2
With the HIA-FT1 (Helmholtz Institute Fatigue Tester 1) fatigue test system the fatigue strength of biological and mechanical heart valve prostheses can be examined according FDA and ISO standard. The static pressure difference across the closed valve at physiologic conditions is reproduced at high test rates. According the above mentioned standards mechanical heart valves have to be tested 600 million cycles (FDA-Guidelines) respectively 400 million cycles (ISO 5840). Bioprostheses are tested for 200 million cycles. The pressure difference across closed valve has to be 90 - 120 mmHg + 20 mmHg depending on valve size and fully opening and closing of the valves has to be ensured.
An additional important parameter, however, is the dynamic loading force at valve closure, which is higher than static pressure load and the main factor for impact wear at valve bearings and housing. Therefore, the current standards only take into account partial aspects of total valve loading. In view of theses findings a new concept of durability testing was developed, where physiologic impact loading of valves is reproduced during accelerated testing.
Within the newly developed 12 cylinder fatigue tester HIA-FT2, 12 valves of different sizes can be tested simultaneously. By means of 12 separate test compartments (fig. 2) loading conditions for each valve can be individually adjusted after previous calibration. For calibration purposes a specific calibration compartment is used, where a piezoelectric force ring is integrated directly upstream of the valve. By means of this transducer the loading forces, previously measured with the same transducer under physiologic real-time conditions within a pulse duplicator.
Operating mode HIA-FT1
Fluid is pumped from the bottom reservoir to the lower test housing (2). Fluid then passes through a rotating disc (5) with two oblong holes. Above this disc the valves are located within the mounting disc (4) above which the upper test compartment is located. Because of the rotating plate, the fluid is pumped through two opposite valves simultaneously. Pressure within the upper test compartment is adjusted by means of a throttle within the backflow line (10). When the valves are closed, the space below the valves is connected to atmospheric pressure via a ring channel (12).
Pressure difference is measure directly upstream and downstream the valve. Cycle number is measured by means of a light barrier, which also triggers a stroboscope to observe the valves. Test frequency is controlled by the rotational speed of the ring slit plate.
- Figure 3: schematic cross section HIA-FT1
1 Base plate
2 Housing
3 PMMA cover
4 Heart valve mounting disc
5 Rotating slit disc
6 Drive shaft housing with bearing
7 Seal
8 Drive shaft
9 Fluid inlet
10 Fluid outlet
11 Ring channel, connected with 12
12 Dome connected with 13
13 Outlet to atmospheric pressure
Operating mode HIA-FT2
A swash plate generates a sinusoidal flow through the test valve by compression and extension of a metallic bellow. the valve is closed, fluid from the upper chamber flows to the lower chamber via an adjustable bypass. The pressure difference is measured directly upstream and downstream of the valve. The pressure difference can be controlled by adjustment of bypass throttle. Stroke volume and test frequency are constant for all test compartments. Valve motion can be observed by means of a triggered stroboscope.
- Figure 4: schematic cross section single compartment HIA FT2
