Twist Mechanics of the Left Ventricle.

Left ventricular (LV) function is determined by the complex interactions between myocardial tissue architecture, myocardial contractility, and loading conditions. Myocardial fibers within the LV wall present a double-helical arrangement that surrounds the circumferential fibers in the midwall.1 In the LV myocardial wall, the myofibers orientation changes gradually from a righthanded helix in the subendocardium to a lefthanded helix in the subepicardium, being almost horizontal in the midwall. This counter-directional helical arrangement of fiber layers also results in sliding or shear deformation. The contraction of subepicardial fibers will rotate the apex of the LV in counterclockwise and its base in clockwise direction. Conversely, the contraction of subendocardial fibers will rotate the LV apex and base in exactly the opposite directions. The larger radius of rotation for the outer epicardial layer produces higher torque and allows the subepicardial fibers to prevail in dominating the overall direction of rotation when both layers contract simultaneously1 and resulting in global counterclockwise LV rotation near the apex and clockwise rotation near the LV base during ejection. Functionally, this twist contributes to maintain a uniform distribution of LV fiber stress and fiber shortening across the wall,1 and to produce a relatively high ejection fraction (≈60%) despite limited (≈20%) myofiber shortening.1–5 Moreover, LV twisting and shearing of the subendocardial fibers during ejection deform the myocardial matrix and result in storage of potential energy, which is subsequently used for diastolic uncoiling of circumferential fibers and untwisting of helices, together producing diastolic suction.6 Thus, LV twist provides a key mechanistic link between systole and diastole. Both loading conditions and contractility alter the extent of LV twist.7–9 LV twist increases with higher preload (ie, higher enddiastolic LV volumes, with constant end-systolic volume, produce higher LV twist), and increasing contractility (eg,positive inotropic interventions such as dobutamine infusion and paired pacing). Conversely, an increase in afterload reduces LV twist. Several imaging modalities and techniques can be used to quantify LV twist mechanics: echocardiography (tissue Doppler, 2and 3-dimensional speckle tracking, vector velocity imaging) cardiac magnetic resonance (tagging and phase contrast velocity mapping), and sonomicrometry. Currently, there is no gold standard for the assessment of LV twist mechanics, but the imaging modalities listed above have been compared against one another and show good agreement.10–12 Because of its safety, wide availability, cost/effectiveness and feasibility, echocardiography (namely, 2-dimensional speckle tracking echocardiography) has been the most frequently used imaging modality to assess LV twist mechanics in both normal subjects and patients with various cardiovascular diseases.13 © 2019 American Heart Association, Inc. Circulation: Cardiovascular Imaging