The effects of cross-fiber deformation on axial fiber stress in myocardium

We incorporated a three-dimensional generalization of the Huxley cross-brid ge theory in a finite element model of ventricular mechanics to examine the effect of nonaxial deformations on active stress in myocardium. According to this new theory which assumes that macroscopic tissue deformations are t ransmitted to the myofilament lattice, lateral myofilament spacing affects the axial fiber stress. We calculated stresses and deformations at end-syst ole under the assumption of strictly isometric conditions. Our results sugg est that at the end of ejection, nonaxial deformations may significantly re duce active axial fiber stress in the inner half of the wall of the normal left ventricle (18-35 percent at endocardium, depending on location with re spect to apex and base). Moreover, this effect is greater in the case of a compliant ischemic region produced by occlusion of the left anterior descen ding or circumflex coronary artery (26-54 percent at endocardium). On the o ther hand stiffening of the remote and ischemic regions (in the case of a t wo-week-old infarct) lessens the effect of nonaxial deformation on active s tress at all locations (9-32 percent endocardial reductions). These calcula ted effects are sufficiently large to suggest that the influence of nonaxia l deformation on active fiber stress may be important, and should be consid ered in future studies of cardiac mechanics.