Tagged MRI and finite-element (FE) analysis are
valuable tools in analyzing cardiac mechanics. To
determine systolic material parameters in
three-dimensional stress-strain relationships, we used
tagged MRI to validate FE models of left ventricular
(LV) aneurysm. Five sheep underwent anteroapical
myocardial infarction (25% of LV mass) and 22 wk later
underwent tagged MRI. Asymmetric FE models of the 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 with diffusion tensor MRI.
Systolic material parameters were determined that
enabled FE models to reproduce midwall, systolic
myocardial strains from tagged MRI (630 +/- 187 strain
comparisons/animal). When contractile stress equal to
40% of the myofiber stress was added transverse to the
muscle fiber, myocardial strain agreement improved by
27% between FE model predictions and experimental
measurements (RMS error decreased from 0.074 +/- 0.016
to 0.054 +/- 0.011, P < 0.05). In infarct border zone
(BZ), end-systolic midwall 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 noninfarct regions.
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 24% increase in fiber stress and 115%
increase in cross-fiber stress at the BZ relative to
remote regions, which may contribute to LV remodeling.
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