Background Myocardium undergoes complex cellular and histochemical alt
erations after acute myocardial infarction. These structural changes d
irectly affect the mechanical stiffness of infarcted and remote myocar
dia. Previous investigations of infarct stiffness have been limited to
uniaxial testing, which does not provide a unique description of the
tissue's three-dimensional material properties. This study describes t
he first serial measurements of biaxial mechanical properties of sheep
myocardium after anterioapical infarction. Methods and Results Anteri
oapical infarctions of 23.7+/-2.5% of the left ventricular mass were p
roduced by coronary arterial ligation in sheep. Biaxial force-extensio
n measurements were made on freshly excised squares (6.45 cm(2)) of re
mote, noninfarcted, and infarcted myocardia before and 4 hours, 1 week
, 2 weeks, and 6 weeks after ligation. Adjacent myocardial samples wer
e assayed for hydroxyprorine content. Force-extension data and a deriv
ed constitutive equation were used to describe stresses and strains an
d material properties of each sample. In sheep, anterioapical infarcti
ons evolve into thin left ventricular aneurysms that consist of predom
inantly fibrous tissue with disrupted groups of muscle cells encased i
n scar. In the infarct, Cauchy stresses at 15% extensions (control str
esses: circumferential, sigma(c), 19.4+/-3.3 g/cm(2); longitudinal, si
gma(L), 54.8+/-34.8 g/cm(2)) increase within 4 hours, peak at 1 to 2 w
eeks (sigma(c), 338.5+/-143.6 g/cm(2); sigma(L), 310.7+/-45.9 g/cm(2))
, and then decrease 6 weeks after infarction (sigma(c), 115+/-47.2 g/c
m(2); sigma(L), 53.2+/-28.9 g/cm(2)). Stresses in the remote myocardiu
m follow a similar time course but to a lesser extent than the infarct
ed region. Hydroxyproline content, a measure of collagen content, does
not correlate with infarct stiffness but progressively increases to 6
9.7+/-7.6 mu g/mg after 6 weeks. Stress-extension curves demonstrate d
irectional anisotropy of both infarcted and remote myocardia. Conclusi
ons The findings indicate that infarcted myocardium becomes more stiff
during the first 1 to 2 weeks after anterioapical infarction and then
more compliant. The infarct also exhibits directional anisotropy. The
se observations underscore the importance of ventricular material prop
erties during the remodeling process after acute myocardial infarction
and may partially explain the progressive left ventricular dilatation
and functional deterioration that occur in some patients after anteri
oapical infarction.