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Research Papers

A Mechanical Characterization of the Porcine Atria at the Healthy Stage and After Ventricular Tachypacing

[+] Author and Article Information
Chiara Bellini1

Graduate Program in Biomedical Engineering,  The University of Calgary, Calgary, Alberta T2N1N4, Canadacbellini@ucalgary.ca

Elena S. Di Martino

Department of Civil Engineering,Center for Bioengineering Research and Education,  The University of Calgary, Calgary, Alberta T2N1N4, Canadaedimarti@ucalgary.ca

1

Corresponding author.

J Biomech Eng 134(2), 021008 (Mar 19, 2012) (14 pages) doi:10.1115/1.4006026 History: Received January 20, 2012; Revised January 24, 2012; Posted February 13, 2012; Published March 14, 2012; Online March 19, 2012

Atrial fibrillation (AF) is a cardiac arrhythmia that highly increases the risk of stroke and is associated with significant but still unexplored changes in the mechanical behavior of the tissue. Planar biaxial tests were performed on tissue specimens from pigs at the healthy stage and after ventricular tachypacing (VTP), a procedure applied to reproduce the relevant features of AF. The local arrangement of the fiber bundles in the tissue was investigated on specimens from rabbit atria by means of circularly polarized light. Based on this, mechanical data were fitted to two anisotropic constitutive relationships, including a four-parameter Fung-type model and a microstructurally-motivated model. Accounting for the fiber-induced anisotropy brought average R2  = 0.807 for the microstructurally-motivated model and average R2  = 0.949 for the Fung model. Validation of the fitted constitutive relationships was performed by means of FEM simulations coupled to FORTRAN routines. The performances of the two material models in predicting the second Piola-Kirchhoff stress were comparable, with average errors <3.1%. However, the Fung model outperformed the other in the prediction of the Green-Lagrange strain, with 9.2% maximum average error. To increase model generality, a proper averaging procedure accounting for nonlinearities was used to obtain average material parameters. In general, a stiffer behavior after VTP was noted.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Anatomy and microstructure of the rabbit atria above the valvular plane, after removal of the ventricles. (a) Pictorial illustration of the main anatomical features of the atria and surrounding structures (AO, aorta; PA, pulmonary artery; PVs, pulmonary veins). Approximate location and orientation of the tissue specimens from the body of the left (LAant , LApost ) and right (RA) atria are identified on the drawing. The fiber bundles visible to the naked eye on the external surface of the tissue samples are also indicated. Combined retardance magnitude and azimuth PLM images for representative specimens from (b) LAant , (c) LApost , and (d) RA. (e) Pictorial representation of an atrial appendage where the internal and external surfaces are both visible. (f) Photograph of an appendage tissue specimen that focuses on the ridges of the pectinate muscles located on the internal surface. (g) PLM image for a tissue specimen from the left atrial appendage (LAA). Two families of orthogonal fibers were observed for each tissue specimen analyzed under polarized light. All PLM images were taken with a 10× objective.

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Figure 2

Representative SII versus EII plots from planar biaxial tests on healthy atrial tissue specimens. The two graphs on each row are referred to the same anatomical location. From top to bottom: LAant , LApost , LAA, RA, and RAA. Data from five distributed tension ratios T2 :T1 are color-coded in each plot, with orange, 1:0.5; cyan, 1:0.75; purple, 1:1; green, 0.75:1; red, 0.5:1. All data were obtained from pig H1 in Table 1. (For interpretation of the references to colors in this figure legend, the reader is referred to the web version of this article).

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Figure 3

Representative SII versus EII plots from planar biaxial tests on atrial tissue specimens after VTP treatment. The two graphs on each row are referred to the same anatomical location. From top to bottom: LAant , LApost , LAA, RA, and RAA. Data from five distributed tension ratios T2 :T1 are color-coded in each plot, with orange, 1:0.5; cyan, 1:0.75; purple, 1:1; green, 0.75:1; red, 0.5:1. All data were obtained from pig VTP5 in Table 2. (For interpretation of the references to colors in this figure legend, the reader is referred to the web version of this article).

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Figure 4

Average SII versus EII plots for the anatomical region LAant at the healthy stage. Predictions from average Fung (top panels) and microstructurally-motivated (bottom panels) models are superimposed to the average data sets as continuous, dark lines.

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Figure 5

Contour plots for a representative Fung potential against two strain components with the third component set to zero. From left to right: E11 /E22 plane with E12  = 0, E11 /E12 with E22  = 0, and E22 /E12 with E11  = 0. A logarithmic scale has been used in all representations. Material parameters correspond to the average Fung model for the healthy LAant .

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Figure 6

Box-plots comparing the median and the spread of the parameters thickness (h), Ωnorm and Ω at the healthy stage (blue, left) and after VTP procedure (black, right).

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Figure 7

Box-plots comparing the median (horizontal line) and the spread of the absolute percentage errors in the prediction of the in-plane Green-Lagrange strain tensor components EII and the in-plane second Piola-Kirchhoff stress tensor components SII as computed for the specimen-specific material models. Fung-type model: blue, left. microstructurally-motivated model: black, right. Different scales have been used to represent the strain/stress errors.

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