BIAXIAL MECHANICS OF THE PASSIVELY OVERSTRETCHED LEFT-VENTRICLE

Overstretching the intact ventricle increases global compliance as a f unction of maximum previously experienced load and may have an importa nt role in the diseased heart, but the corresponding changes in local myocardial mechanics and structure are unknown. Therefore, we measured two-dimensional strain on the left ventricular (LV) epicardium in iso lated arrested rat hearts sequentially inflated to increasing cavity p ressures of 10, 30, and 120 mmHg. Strains at matched LV pressures incr eased significantly (P < 0.002) as the maximum pressure previously exp erienced by the LV (P-max) increased. Compared with P-max = 10 mmHg, r elative increases in fiber strain for P-max = 30 and 120 mmHg (100 and 149%, respectively) were significantly greater (P < 0.001) than the c orresponding increases in cross-fiber (51 and 78%, respectively) and f iber shear (57 and 86%, respectively) strains. Using an optimized prol ate spheroidal finite-element model of the rat LV that reliably reprod uced experimental strains, we estimated progressive decreases in epica rdial biaxial wall stiffness up to 87% with increasing P-max that were not different in the fiber and cross-fiber directions. Thus, although passive ventricular overloading causes direction-dependent increases in epicardial strain, these changes are the consequence of local myoca rdial softening that is actually independent of direction.