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

2𝑒𝑄 − 1 , 𝑄 = 𝑐11𝐸22

2 + 2𝑐12𝐸11𝐸22 + 𝑐22𝐸222 (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