The driving force of early filling is the atrioventricular pressure gradient. The gradient is enhanced by the ventricular elastic recoil or suction. During contraction, potential energy is stored at end systole in the form of a longitudinal gradient of circumferential rotation (twist) and released during early diastole as elastic recoil.94 Additional, so called restoring forces include myocardial compression and stretching of the mitral valve apparatus during contraction. The left ventricular twist is a special form of systolic deformation (torsion) and manifests itself as a counterclockwise rotation of the apex relative to the base of the left ventricle. At end systole the fast untwisting starts with a half of the twist already dissipated during the isovolumic relaxation period, followed by a slower untwisting during the early filling.95 Consequently, 60-80% of the stroke volume enters the ventricle during the first third of diastole.89 Restoring forces become more important when the end systolic volume is small, such as during exercise, tachycardia, and hypovolaemia.
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