Transmural left ventricular mechanics underlying torsional recoil during relaxation

Abstract

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.

Fig. 1

Sites of marker implantation. A: schematic representation of the heart. X₁, circumferential axis; X₂, longitudinal axis; X₃, radial axis; LAA, left atrial appendage; RCA, right coronary artery; LAD, left anterior descending artery; D1, D2, first and second diagonal branch of LAD, respectively. B: schematic representation of excised tissue block containing the bead set. Fiber angle (α) was measured in the circumferential-longitudinal (X₁-X₂) plane at each transmural depth with reference to the positive circumferential axis (X₁), with a positive angle defined as rotation toward the longitudinal axis (X₂). Sheet angle (β) was measured in the plane perpendicular to the fiber angle at each transmural depth with reference to the radial axis (X₃), with a positive angle defined as rotation towards the positive cross-fiber direction (X_cf). X_f, fiber axis; X_s, sheet axis; X_n, axis oriented normal to the sheet plane. The X_f, X_s, and X_n axes present a Cartesian system (see text for details).

Fig. 2

Early relaxation. Early relaxation was defined as the period beginning at end systole (ES) and ending at minimum LV pressure (LVP, solid line). □, Circumferential strain (E11) at midwall.

Fig. 3

Measured fiber angle (α) and sheet (β) angles vs. % wall depth (n = 5; means ± SE). Mean α ranged approximately from −60° to +60° from epicardium (epi) to endocardium (endo), resulting in a transmural gradient of ~120°. Mean β was predominantly negative, with small variations across the wall (approximately −36° to −2°).

Fig. 4

Time course of finite strains during early relaxation in local cardiac coordinates. Note significant endocardial circumferential stretch (E₁₁), endocardial wall thinning (E₃₃), epicardial torsional recoil (E₁₂), and transverse shear (E₂₃, E₁₃) during early relaxation (P < 0.05). Values are means ± SE. mid, Midwall; ER, early relaxation; ES, end systole, time = 0%; end of ER, time = 100%. Note different scales for each strain.

Fig. 5

Time course of finite strains during early relaxation in fiber-sheet coordinates. Note significant epicardial stretch along the fiber direction (E_ff), endocardial sheet shortening (E_ss), and fiber shear (E_fs, E_fn) during early relaxation (P < 0.05). Values are means ± SE. Note different scales for each strain.