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1 Joint Graduate Group in Bioengineering, University of California--Berkeley & San Francisco, Berkeley, CA, USA
2 Joint Graduate Group in Bioengineering, University of California--Berkeley & San Francisco, Berkeley, CA, USA; Department of Surgery, University of California--San Francisco, San Francisco, CA, USA; Veterans Affairs Medical Center, San Francisco, CA, USA
3 Veterans Affairs Medical Center, San Francisco, CA, USA
4 Department of Anesthesia, University of California--San Francisco, San Francisco, CA, USA; Veterans Affairs Medical Center, San Francisco, CA, USA
5 Department of Biomedical Engineering, University of Southern California, Las Angeles, CA, USA
6 Department of Biomedical Engineering, Duke University, Durham, NC, USA; The Center for in Vivo Microscopy, Duke University Medical Center, Durham, NC, USA
7 Joint Graduate Group in Bioengineering, University of California--Berkeley & San Francisco, Berkeley, CA, USA; Department of Radiology, University of California--San Francisco, San Francisco, CA, USA; Veterans Affairs Medical Center, San Francisco, CA, USA
* To whom correspondence should be addressed. E-mail: joseph_walker{at}berkeley.edu.
Objectives: Tagged-MRI and finite element (FE) analysis have become valuable tools in analyzing cardiac mechanics. To determine systolic material parameters in three-dimensional stress-strain relationships, this study used tagged-MRI to rigorously validate FE models of left ventricular aneurysm. Methods: Five sheep underwent antero-apical myocardial infarction (25% of LV mass) and 22 weeks later underwent tagged-MRI. Asymmetric, FE models of the left ventricle (LV) were formed to in vivo geometry from MRI and included aneurysm material properties measured with biaxial stretching, LV pressure measurements, and myofiber helix angles measured in detail with diffusion tensor MRI. Systolic material parameters were determined that enabled FE models to reproduce mid-wall, systolic myocardial strains from tagged-MRI (630±187 strain comparisons were made per animal). Results: When contractile stress equal to 40% of the myofiber stress was added transverse to the muscle fiber, myocardial strain agreement improved by 27% between the FE model predictions and experimental measurements (rms error decreased from 0.074±0.016 to 0.054±0.011, p<0.05). In the infarct border-zone (BZ), end-systolic, mid-wall stress was elevated in both fiber (24.2±2.7 to 29.9±2.4 kPa, p<0.01) and cross-fiber (5.5±0.7 to 11.7±1.3 kPa, p=0.02) directions relative to non-infarct regions. Conclusions: Contrary to previous hypotheses but consistent with biaxial stretching experiments, active cross-fiber stress development is an integral part of LV systole; FE analysis with only uniaxial contracting stress is insufficient. Stress calculations from these validated models show a 24% increase in fiber stress and a 115% increase in cross-fiber stress at the BZ relative to remote regions which may contribute to LV remodeling.
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