1. Passive Biaxial Mechanical Properties of Different Anatomical Regions
of Normal Ovine Heart
Shahnaz Javani, Mostafa Abbasi, Matthew Gordon, Ali N. Azadani
Cardiac Biomechanics Laboratory, Department of Mechanical and Materials Engineering, University of Denver, Denver, CO
INTRODUCTION
METHODS
RESULTS
CONCLUSIONS
• A valuable key to treatment of many heart diseases is clear
understanding of mechanical properties of cardiac muscle.
• Left ventricular remodeling caused by myocardial infarction, and
right ventricular failure resulting from pulmonary hypertension are
among the most common causes of heart failure. MI and PH alter
myocardial architecture through re-orientation of myocytes and
collagen fibers presenting increased mechanical stiffness of
ventricular free wall.
• On the other hand, ventricular restoration techniques such as
injection of biomaterial or viable cells into the infarcted myocardium
deal with the mechanical properties of the ventricles.
• Similarly, knowledge of mechanical properties of atria and atrial
appendages are essential for design and development of medical
devices such as left atrial appendage closure devices.
• Therefore, as a baseline to better understand physiology and
pathophysiology of the heart, and to develop new treatments and
therapeutic methods, we need to determine mechanical properties
of normal heart.
• Ovine heart closely resembles human heart from the point of view of
physiology and anatomy.
• The goal of this study, therefore, was to determine passive
mechanical properties of different anatomical regions of normal
ovine heart.
• Fresh sheep hearts (n=19) were obtained from a local abattoir on the
morning of harvest. A total of 189 specimens were tested and
analyzed.
• Square specimens were excised from anterior and posterior portion
of the left and right ventricular free wall, anterior and posterior portion
of the left and right atria, and right and left atrial appendages, in
such a way that the edges were parallel and perpendicular to the
predominate fiber direction.
• A planar biaxial stretching system (CellScale, Waterloo, Canada) was
used to determine mechanical properties of the specimens (Fig 1).
• Samples were subjected to 10 cycles of preconditioning strain of
10% followed by an equibiaxial strain of up to 50%, with a 4s stretch
and a 4s recovery duration.
• The material constitutive coefficients were obtained by fitting the
Cauchy stress-Green Strain curves to a four parameter Fung-type
exponential strain-energy function (1).
𝑊 =
𝐶
2
𝑒 𝑄
− 1 , 𝑄 = 𝑐11 𝐸22
2
+ 2𝑐12 𝐸11 𝐸22 + 𝑐22 𝐸22
2
(1)
• Strain energy storage values of different regions at strain of 15%
were used for regional comparison through paired-sample t-test and
one-way ANOVA.
Figure 1: (Right) fresh ovine heart obtained from local slaughterhouse, (Middle) CellScale planar biaxial
stretching system, and (Left) CellScale LabJoy image tracking software was used to obtain strain maps.
• Cauchy stress-Green strain raw data from equibiaxial stretch
testing of different anatomical regions were obtained (Fig 2).
• Fung-type exponential equation was fitted to Cauchy stress-Green
strain data of each individual sample and the average curves, and
material coefficients were obtained (Table 1 and Fig 4).
• The average values of strain energy (kPa) at strain of 15% were
found to be 1.74±0.67 for the left ventricle, 1.21±0.47 for the right
ventricle , 0.54±0.16 for the left atrium, 0.36±0.15 for the right
atrium, 0.74±0.20 for the left atrial appendage, and 0.50±0.22 for
the right atrial appendage (Fig 3, left).
• Statistical analysis revealed no statistically significant difference in
strain energy between anterior and posterior portions of each
region (P>0.273), except for the right ventricle where strain energy
storage in posterior specimens were higher than that of anterior
specimens (P<0.019).
• Strain energy storage in left ventricle was significantly higher than
the right ventricle (p<0.001) (Fig 3, right). Likewise, the left atrium
was found to be significantly stiffer than the right atrium (P<0.001)
(Fig 3, right). Furthermore, strain energy values of left atrial
appendage were significantly greater than those of right atrial
appendage (P=0.002) (Fig 3, right).
• The results of one-way ANOVA between the three regions of each
side of the heart revealed that strain energy differed significantly
between each pair of the groups (P < 0.001), with ventricle having
the highest and atrium having the lowest values (Fig 3, right).
Overall, statistical analysis revealed that samples from each
anatomical region of the left heart tend to be stiffer than those from
the same region of the right heart. Furthermore, comparisons
between the strain energy stored in anterior and posterior samples
did not yield statistical significance, with the exception of right
ventricle in which the posterior samples stored higher energy than
the anterior ones. Finally, specimens from ventricle, atrial
appendage, and atrium were found to have the highest to lowest
tissue stiffness, respectively. This trend was consistent in both
sides of the heart.
Figure 3: Mean values±SD of strain energy storage at 15% strain for (left) anterior and posterior
portions of each region, and (right) different anatomical regions.
Figure 2: Cauchy stress-Green strain data for different anatomical regions
(anterior regions: circle, and posterior regions: asterisk).
Figure 4: Equibiaxial stress-strain curves and average curve for different anatomical regions. dashed lines represent extrapolated data.
Table 1 𝑐11 𝑐12 𝑐22 𝐶
𝐿𝑉 7.829 2.023 0.598 10.588
𝐿𝐴 1.112 0.058 1.016 20.542
𝑅𝑉 1.319 0.853 0.060 33.087
𝑅𝐴 0.129 2.500 0.029 5.331
𝐿𝐴𝐴 1.199 0.132 1.380 21.622
𝑅𝐴𝐴 0.720 0.612 0.560 17.052