Early relaxation in the cardiac cycle is characterized by rapid torsional recoil of the left ventricular (LV) wall. To elucidate the contribution of the transmural arrangement of the myofiber to relaxation, we determined the time course of three-dimensional fiber-sheet strains in the anterior wall of five adult mongrel dogs in vivo during early relaxation with biplane cineangiography (125 Hz) of implanted transmural markers. Fiber-sheet strains were found from transmural fiber and sheet orientations directly measured in the heart tissue. The strain time course was determined during early relaxation in the epicardial, midwall, and endocardial layers referenced to the end-diastolic configuration. During early relaxation, significant circumferential stretch, wall thinning, and in-plane and transverse shear were observed (P < 0.05). We also observed significant stretch along myofibers in the epicardial layers and sheet shortening and shear in the endocardial layers (P < 0.01). Importantly, predominant epicardial stretch along the fiber direction and endocardial sheet shortening occurred during isovolumic relaxation (P < 0.05). We conclude that the LV mechanics during early relaxation involves substantial deformation of fiber and sheet structures with significant transmural heterogeneity. Predominant epicardial stretch along myofibers during isovolumic relaxation appears to drive global torsional recoil to aid early diastolic filling.

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Laminar, or sheet, architecture of the left ventricle (LV) is a structural basis for normal systolic and diastolic LV dynamics, but transmural sheet orientations remain incompletely characterized. We directly measured the transmural distribution of sheet angles in the ovine anterolateral LV wall. Ten Dorsett-hybrid sheep hearts were perfusion fixed in situ with 5% buffered glutaraldehyde at end diastole and stored in 10% formalin. Transmural blocks of myocardial tissue were excised, with the edges cut parallel to local circumferential, longitudinal, and radial axes, and sliced into 1-mm-thick sections parallel to the epicardial tangent plane from epicardium to endocardium. Mean fiber directions were determined in each section from five measurements of fiber angles. Each section was then cut transverse to the fiber direction, and five sheet angles (β) were measured and averaged. Mean fiber angles progressed nearly linearly from −41° (SD 11) at the epicardium to +42° (SD 16) at the endocardium. Two families of sheets were identified at approximately +45° (β + ) and −45° (β − ). In the lateral region (n = 5), near the epicardium, sheets belonged to the β + family; in the midwall, to the β − family; and near the endocardium, to the β + family. This pattern was reversed in the basal anterior region (n = 4). Sheets were uniformly β − over the anterior papillary muscle (n = 2). These direct measurements of sheet angles reveal, for the first time, alternating transmural families of predominant sheet angles. This may have important implications in understanding wall mechanics in the normal and the failing heart. Keywords cardiac microstructure; sheets LEFT VENTRICULAR (LV) myofibers are connected by an extensive extracellular collagen matrix (3) to form myolaminar "sheets," two-to-four cells thick, which are also interconnected Copyright © 2005 by a collagen network (16). This sheet architecture has been proposed as an anatomic basis for myofiber rearrangement throughout the cardiac cycle (17,22), and sheet deformation is thought to underlie LV mechanics during contraction (6) and relaxation (2).The three-dimensional geometry of fibers and sheets is critical. Maximum contraction of fibers that lie in planes tangent to the epicardial surface is only 15% along their long axis (21); yet LV ejection fractions of 60% and systolic radial wall thickening of 40% are typically observed. The helical arrangement of the fibers may account for some of this disparity (20), but an additional important mechanism appears to be sheet deformation (19,22). Laminar shear, extension, and thinning or thickening are thought to contribute to wall thickness changes (6,7,24).Fiber orientation, which can be measured directly, has been found in several species to vary from approximately −60° at the epicardium to +60° at the endocardium (4,10,12,14,23,28). Until recently, however, sheet orientation could only be inferred indirectly from the directions of fiber and cleavage planes from three orthogonal views (6). Recently, our group and Ashi...

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Background-Mitral valvular-ventricular continuity is important for left ventricular (LV) systolic function, but the specific contributions of the anterior leaflet second-order "strut" chordae are unknown. Methods and Results-Eight sheep had radiopaque markers implanted to silhouette the LV, annulus, and papillary muscles (PMs); 3 transmural bead columns were inserted into the mid-lateral wall between the PMs. The strut chordae were encircled with exteriorized wire snares. Three-dimensional marker images and hemodynamic data were acquired before and after chordal cutting. Preload recruitable stroke work (PRSW) and end-systolic elastance (E es ) were calculated to assess global LV systolic function (nϭ7). Transmural strains were measured from bead displacements (nϭ4