Cardiac Restraint and Support Following Myocardial Infarction - Cardiovascular and Cardiac Therapeutic Devices - page 194

[ 2 , 62 ]. Simulating acute infarcts as noncontractile regions with material properties
identical to passive myocardium predicted normal diastolic behavior (reflected in
an unchanged end-diastolic pressure-volume relationship, EDPVR) but severely
depressed systolic function (altered end-systolic function pressure volume rela-
tionship, ESPVR), in agreement with experiments. Increasing infarct stiffness in
the model improved systolic function as expected, shifting the predicted ESPVR
back towards baseline. However, hearts with a stiffer infarct also displayed
impaired filling, reflected in a left-shifted EDPVR, and reduced filling exactly
offset improved ejection, producing no overall change in predicted CO at matched
pressures (Fig. 12 ). These early models made a number of simplifications, par-
ticularly assuming that scar is mechanically isotropic (having the same properties
when stretched in any direction), but recent modeling studies using much more
sophisticated and better-validated finite element models reached similar conclu-
sions. We found that isotropically stiffening a large anterior infarct in a model of
an infarcted dog heart reduced both systolic and diastolic volumes at matched
pressures, producing no net benefit in overall pump function [ 63 ]. Similarly, Dang
et al. studied the impact of the stiffness of an isotropic patch applied in a simulated
Surgical Anterior Ventricular Restoration (SAVER) operation, and found that
increasing patch stiffness reduced systolic and diastolic volumes at matched
pressures, but actually decreased SV—in this setting, stiffer patches impaired
filling more than they improved ejection [ 64 ]. Overall, these computational studies
are remarkably consistent with the majority of the functional evidence reviewed
above for both global and local infarct reinforcement. We conclude that stiffer
patches or restraints are likely to be more effective in reducing or limiting LV size,
but that isotropic patches are unlikely to directly improve LV pump function,
regardless of stiffness.
6.2 Anisotropic Infarct Reinforcement
We recently proposed one idea for finessing the trade-off between systolic and
diastolic function apparent in the studies discussed above. Inspired by the fact that
some of the infarct scars we have studied are highly anisotropic (much stiffer in
some directions than others [ 13 ]), we tested whether any choice of material
properties in the circumferential and longitudinal directions could significantly
enhance predicted pump function in an FEM of a dog heart with a large anterior
infarct [ 63 ]. Those simulations suggested that an infarct that is quite stiff in the
longitudinal direction (the apex-base direction) but as compliant as passive
myocardium in the circumferential direction would have the best pump function;
this suggested to us that selective longitudinal reinforcement of an acute anterior
infarct could significantly improve LV pump function.
We tested this hypothesis directly by ligating the LAD in open-chest anesthe-
tized dogs and reinforcing the resulting acute infarcts with a modified Dacron
patch that was inextensible in the longitudinal direction but free to deform in the