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Figure 10 Integrated overview of the left ventricular mechanical sequence during a cardiac cycle. Electric and mechanical activation are initiated in the apical subendocardial region. The subendocardial myofibers (right-handed helix) shorten, producing a brief twist along the right-handed helix direction during the isovolumic contraction period (phase 1) (33). During ejection (phase 2), the subendocardial and subepicardial layers shorten sequentially from the apex toward the base. The direction of apical twist is along the left-handed helix direction owing to dominating intrinsic twist imparting features of subepicardial myocytes (21), while the basal region torques briefly along the right-handed helical direction. Shortening of the subepicardium near the apex and the subendocardium near the base continues beyond aortic valve closure during the isovolumic relaxation (phase 3). This is accompanied with untwisting of the relaxed subendocardial layer with lengthening and enlargement of the left ventricular (LV) cavity at the apex. Shortening of the subepicardium near the apex continues briefly beyond the isovolumic relaxation period and is accompanied with further opening of the left ventricular cavity during early diastole (phase 4). The subsequent period of diastole is characterized by relaxation in both layers. The relaxation is further facilitated in the later phase of diastole by atrial contraction (phase 5). Phases 1 to 5 are described in Figure 6.
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