Bioprosthetic valve tissue viscoelasticity: implications on accelerated pulse duplicator testing.
Most of our knowledge of heart valve mechanics has been gained from low strain-rate studies much lower than physiologic levels. Using a high-speed materials testing system, we compared the low and high strain-rate viscoelastic behavior of porcine aortic valve cusps at extension rates of up to 40 mm/s. Circumferential and radial strips were stretched and then held in their stretched configuration to measure their "stress-relaxation" behavior. During low strain-rate stretching, only 6% of the initial stress dissipated or relaxed after 1 second, whereas 25% of the stress dissipated during high strain-rate stretching. This considerable difference in stress relaxation suggests a rate-dependent viscoelastic behavior that has not been accounted for in valve design and may have important implications for accelerated pulse testing. Even though the valve cusp is loaded for only 0.4 seconds during each heartbeat, at least 15% of the stress may relax over that period. During accelerated pulse testing, however, sufficient time may not be available to allow the tissue fibers to relax back to their natural state before the subsequent loading cycle, leading to a higher baseline preload. In addition, because valve tissue is not given sufficient time to relax before the next cycle, pulse testing subjects the valves to lower-magnitude cyclic stresses than does physiologic loading. Because both the baseline preload and the magnitude of cyclic stresses may lead to early fatigue failure, accelerated wear testing may either overestimate or underestimate valve durability. Clearly, the mechanism of stress-induced failure of biologic tissues must be elucidated before too much validity is placed on pulse duplicator studies.