Experimental Considerations that Determine the Utility of Accelerated Durability Testing for Prosthetic Heart Valves
James C. Conti, Ph.D., Elaine R. Strope, Ph.D., Donald Rohde
Presented By: James C. Conti, Ph.D.
Dynatek Laboratories, Inc., Galena, MO
The reliability and usefulness of experimentally generated data is dependent upon, among other things, the generation of an experimental protocol that recognizes the importance of pertinent variables. Any experiment that fails to control or at least recognize those variables is probably better off not being performed. This experimental sloppiness is rampant throughout many of the world’s prosthetic heart valve testing laboratories. As a result of the uselessness of the generated data an attitude has developed that categorizes prosthetic heart valve and durability testing as useless and irrelevant.
Experimental protocols can indeed be written that result in the generation of durability data that is both relevant and can be correlated with in vivo wear. This paper addresses the steps that a researcher can follow to generate a meaningful protocol.
The Testing Instrument
Regardless of the supplier or designer of the accelerated tester, several assumptions are made in this paper.
1) Pressure measurements are free of inertial interference and are
frequency independent over the range of frequencies encountered.
2) The whole system has a near zero capacitance.
3) Pressure measurements do not change with time due to instrument variation.
4) Pressure measurements reflect true load being applied to valve.
There are three criteria that must be met for every valve being tested. The first and second of these are that the valve must fully open and then fully close each cycle. This will of course require stroboscopic analysis. The third criterion depends on the type of valve being tested. A mechanical valve demands a lack of cavitation. A tissue or polymer leaflet valve requires a lack of leaflet flutter. If all the criteria cannot be attained it is usually due to excessive testing speeds.
The most common flutter damage is fraying and tearing of leaflets. Leaflets should smoothly close, appose, stop, reverse and smoothly open. As a leaflet approaches the fully opened position the chances for leaflet flutter are maximized.
Cavitation occurs around the periphery of the occluder during maximum motion and at the instant of closure. It will appear as a momentary flash at the location of maximum occluder motion. It can be reduced by lowering temperature, flow rate, speed and raising systemic pressure.
The problem arises if the pressures measured are actually higher than the load experienced by the valve at closure. Researchers should therefore beware of situations in which valve wear is less even though the same closing pressures are measured.
Dynatek Laboratories, Inc., Fourth and Main, PO Box 254, Galena, MO 65656