(C-92) Viscoelastic Properties of Healthy and Hypertrophied Human Septum

Introduction:: Hypertrophic cardiomyopathy (HCM) is often caused by genetic disorders, resulting in abnormal sarcomere, thickening of the heart muscle, and overgrowth of the left ventricular septum. This overgrowth can block the outflow of blood, disrupt the mitral valve coaptation, and increase the mechanical stress experience by the heart. Besides the hypertrophied and disarrayed cardiomyocytes, overexpressed fibrotic collagen alters the septum constitutions and microstructures [1]. As a result, cellular hypertrophy and the fibrotic environment change the mechanical properties of the hypertrophied septum, contributing to the abnormal biomechanical behavior of the septal tissues [2,3]. However, the mechanical properties of human hypertrophied septum are not well investigated. In this study, we performed viscoelastic mechanical tests such as stress relaxation testing and creep testing, as well as shear mechanical testing on human hypertrophied septum obtained from myectomy surgeries and compared with the healthy septum.


Materials and Methods:: Hypertrophic septal samples were obtained from the surgical waste of clinical myectomy of the HCM patients (UTSW: IRB STU 082017-072). Healthy human septal tissues were obtained from a tissue bank under the same IRB protocol. Samples prepared for viscoelastic and shear tests were aligned with the longitudinal direction of the septum. For viscoelastic tests, tissue samples were mounted onto the TestResources Universal Testing Machine (Shakopee, MN) and tested in 1X Phosphate Buffered Saline (PBS). After preconditioning, the following testing protocols were completed: (1) Stress Relaxation: hypertrophied and healthy samples were loaded to 100g and the achieved strain was kept constant while recording the stress decay up to 15 minutes. (2) Creep: hypertrophied and healthy samples were loaded to 100g and kept at this constant load while deformation was continuously monitored up to 15 minutes. (3) Failure test: hypertrophied and healthy samples were loaded until failure. Each sample was pulled at a rate of 0.5 mm/s. Simple shear testing was performed to assess shearing property of the septal tissues. 3 mm x 3 mm x 3 mm cubed septum samples were mounted onto custom-built steel shear plates with minimal cyanoacrylate. The shear direction is along the longitudinal direction of the septum tissue. The shear plates were then mounted on the TestResources Universal Testing Machine. The samples were subjected to positive and negative shear up to 50% strain with a loading rate of 0.02 mm/s.

Results, Conclusions, and Discussions:: We found that the healthy septum exhibited a stress decay similar to most soft tissues, 57 ± 6.3%. On the other hand, hypertrophied septum exhibited a much larger stress decay, 68 ± 11%. Both healthy septum and hypertrophied septum showed a small amount of creep over time. The final creep amount at 15 minutes reached 9.1 ± 1.0% for healthy septal tissues and 9.8 ± 4.2% for hypertrophied septum. We also observed that the healthy septum showed a very typical nonlinear concave upward stress-strain relation. However, the hypertrophied septum showed a stress-strain trend that was much less nonlinear. The hypertrophied septum had an initial modulus that was higher than healthy septal tissues, most likely due to the abundant fibrotic collagen associated with HCM. An important observation was that the healthy septum had much higher mechanical strength than the hypertrophied septum (failure stress - healthy: 2,561 ± 919 kPa vs HCM: 494 ± 206 kPa). In the high strain region, the healthy septum was also stiffer than the hypertrophied septum. Both healthy and hypertrophied septum demonstrating a nonlinear shear behavior. The hypertrophied septum appears much stiffer with greater energy dissipation than the healthy septum. Overall, human hypertrophied septum had larger stress relaxation, larger creep, and was stiffer at the initial phase of tissue loading, less nonlinear, less stiff in the linear region, and much weaker in mechanical strength when compared to healthy human septum. We also found the human hypertrophied septum a stiffer shear behavior and a greater energy dissipation during simple shear test. The stress relation and creep behavior, the failure behavior, and the shear property all reflected the fact that hypertrophied septum had more disorganized fibrotic collagen fibers when compared with the healthy septum. The poorly organized, pathological compositions in hypertrophied septum seemed to lose the benefit of native tissue design and hence exhibited an inferior viscoelastic and shear mechanical behavior as described above. Our viscoelastic and shear mechanical data build a strong foundation to understand tissue behavior of hypertrophied septum and the degree of its deviation from the healthy septum.

Acknowledgements (Optional): : NIH R15HL159599, R01HL157050

References (Optional): : [1] Varnava et al, Heart 84(5) (2000) 476-482.

[2] Urbano-Moral et al, Circulation: Cardiovascular Imaging 7(1) (2014) 11-19.

[3] Hinojar et al, International journal of cardiology 249 (2017) 467-472.