Effects of pressure overload on the passive mechanics of the rat left ventricle

Both myocyte growth and changes in the extracellular matrix may affect the passive mechanics of the left ventricle (LV). Pressure-volume (PV) relationships and midwall two-dimensional strains versus passive loading were measured in isolated rat hearts 2 and 6 weeks after ascending aortic banding. Collagen area fractions and perimysial fibril orientations were determined with picrosirius-polarization microscopy, and the equatorial region of the LV was modeled with finite element analysis of a transversely isotropic cylinder with the same material properties in hypertrophy and control. Compared with weight-matched shams, heart weight increased at 2 (19%) and 6 (22%) weeks, as did LV wall thickness (6% and 31%, respectively). The PV curve became less compliant with hypertrophy; only circumferential strain decreased after hypertrophy. Collagen area fractions were not different at either subendocardium or subepicardium (3.37 +/- 1.06 versus 3.96 +/- 0.76 at 2 weeks and 3.61 +/- 1.30 versus 4.22 +/- 1.50 at 6 weeks for banded and sham, respectively; subendocardium). Collagen and muscle fiber orientations also did not change with hypertrophy. The finite element model predicted trends in the strains similar to those found experimentally. Thus, in this model of pressure-overload hypertrophy, the decreases in compliance and circumferential strain of the passive LV are not due to changes in the percentage of extracellular matrix, but rather to global geometric changes.