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Durability/wear testing of heart valve substitutes
1Helmholtz-Institute for Biomedical Engineering, Aachen, Germany.
Background And Aims Of The Study:
The current standards for accelerated heart valve testing have considerable differences in test conditions. Another problem arises from the fact that such test systems are not standardized at all. It was shown earlier that different test systems generate totally different valve loading, even if operating at standard conditions. The present study aimed to improve this unsatisfactory situation and to develop a new concept where actual loading of valves is measured either in vitro or in vivo under physiologic conditions and subsequently to reproduce these conditions during accelerated testing.
Methods:
Integral loading forces at valve closure were measured for several valve types using a piezoelectric force ring within a real-time circulatory mock loop under physiologic conditions. This facilitated definition of a physiologic loading range. Physiologic loading was subsequently reproduced in a single-chamber accelerated test system. Working conditions obtained in terms of stroke, bypass flow and compliance served as design criteria for a new test chamber and a complete 12-chamber accelerated testing system.
Results:
The integral loading obtained using the force ring showed a correlation with previous in vitro and in vivo results of strain-gauged valves. Loading forces for mechanical valves are about one order of magnitude higher than for bioprosthetic valves and are strongly related to cardiac output for both valve types. At physiologic loading, however, the differential pressures across the valves are considerably below those given in FDA guidelines.
Conclusions:
This pilot study demonstrates that physiologic valve loading is reproducible over a wide range under appropriate testing conditions. It also showed that, at the back-pressures of the current standards, the loading forces during accelerated testing exceed the real-time loading forces by far and, thus, may provide unrealistically high valve loads. These initial findings indicate that amendments of the currently valid standards may be need to be accorded.