Research On Passive Shear Mechanical Properties Of Atrial Myocardium

As chambers of the heart,atria play an important role in the normal cardiac cycle and remain in a non-contratracted state for 85% of the atrial cycle.Therefore,the study of the passive properties of the atria is significant.There is a few literatures relevant to the atrial passive properties and they were tested through tensile experiment.Tensile tests cannot supply sufficient information to describe material characterization accurately.Therefore,the multi-axial shear test of atrial myocardium is very urgent.In this study,porcine hearts were used for the shear tests.Appropriate atrial myocardium were chosen through histological observation to investigate its passive shear mechanical properties,and the mechanical data of the atrial and ventricular myocardium was compared to determine whether there is a statistical difference in the passive shear mechanical properties among different regions of heart.The obtained experimental data not only provide effective material parameters for the finite element modeling of myocardium,but also can be used for the selection of materials in the heart patches of tissue engineering.It has a certain significance for the development of implantable medical devices,the choice of implant sites,and the improvement of related diagnostic techniques.The main contents of this paper and the corresponding results are as follows:(1)The design of a device suitable for the study of the passive mechanical properties of the biological soft tissue is described in detail.The stability of the device was examined by shear tests without samples.The accuracy of the equipment was verified by testing the gel with known parameters.(2)Two regions,left atrial appendage(LAA)and a region which is named PI by us(It is near the atrioventricular groove)were suitable for the study of passive shear mechanical properties of myocardium by the observation of histological techniques(paraffin sections).In order to know whether the atrial and ventricular muscle shear mechanical properties are different,a region in the left ventricle(LV)was selected for comparison study.(3)The results of shear tests on the myocardium of the three regions suggested that the myocardium can be described as a nonlinear,anisotropic(orthotropic),viscoelastic,strain-dependent soft biological material.The mean peak stress of the left atrial appendage was similar to that of the left ventricle(P>0.05).The stiffness of myocardium in PI region was significantly smaller than that in the other two regions(P <0.05).Further comparisons show that the average peak stress of the PI region at all strain levels and in all shear models is less than that of the other two regions,indicating that the myocardium is non-uniform in material properties.Based on the above results,one set of parameters of myocardial mechanics is not enough to describe the material characteristics of the entire atrium.The non-linear,anisotropic material properties of myocardial tissue are dependent on the local myocardial tissue components and the myocardial microstructure axis(fiber-,sheetand sheet-normal-directions).