Left ventricular geometric remodeling and residual stress in the rat heart

Theoretical considerations and observations of residual stress suggest that geometric remodeling in the heart may also alter residual stress and strai n. We investigated whether changes in left ventricular geometry during phys iologic growth were associated with corresponding changes in myocardial res idual strain. In anesthetized rats from eight age groups ranging from 2-25 weeks, the heart was arrested and isolated, and equatorial slices were obt ained. The geometry of the intact, unloaded state was recorded, as well as the "opening angle" of the stress-free configuration after radial resection of the tissue slice. The tissue was fixed and embedded for histological ex amination of collagen area fraction. Heart weight increased IO-fold with ag e and unloaded internal radius increased almost 4-fold. However, wall thick ness increased only 66 percent, so that the ratio of wall thickness to inte rnal radius decreased significantly from 2.22 +/- 0.29 (mean +/- SD) at 2 w eeks to 0.81 +/- 0.47 at 25 weeks. Opening angle of the stress-free slice d ecreased significantly from 87 +/- 16 deg at 2 weeks to 51 +/- 16 deg, and correlated linearly with wall thickness/radius ratio. Collagen area fractio n increased with age. Hence physiologic ventricular remodeling in rats decr eases myocardial residual strain in proportion to the relative reduction in wall thickness-radius ratio.