How does the left ventricle work? Ventricular rotation as a new index of cardiac performance

Abstract

Although simple cylindrical or ellipsoidal left ventricular (LV) geometry with transverse or circumferential muscle contraction has been traditionally used to estimate LV performance, the estimated LV ejection fraction (EF) with muscle fiber shortening up to 20% is less than 50% of maximum, which is lower than the normal EF observed in routine clinical practice. Thus, oblique fiber orientation and LV rotation, in addition to radial thickening and longitudinal shortening, is predicted as an essential component of effective LV pumping. This was confirmed by animal experiments using surgically implanted markers or invasive sonomicrometry. Demonstration of the muscle band extending from the pulmonary artery to the aorta, which connects the ventricular myocardium, both right ventricle and LV as a continuous band (muscle band theory) provides an anatomical backbone of helical configuration of the cardiac muscle band with descending and ascending segments wrapping the LV apex. Moreover, sequential, non-simultaneous, activation and contraction of the helicoids muscle band contributes to LV rotation or twist motion. Recently, magnetic resonance imaging and speckle tracking echocardiography (STE) techniques have provided an excellent noninvasive way to measure LV rotation and twist, which is expected to contribute to a more thorough evaluation of both LV systolic and diastolic function. Initial animal experiments showed that quantification of apical rotation or LV twist using STE is more accurate for estimating LV systolic function than conventional EF under a variety of LV inotropic conditions, irrespective of coronary ligation. As de-rotation or the untwisting rate can also be measured by STE, the role of ventricular untwisting as a temporal link between LV relaxation and suction can be addressed. Further clinical investigations are needed to determine the real clinical impact of these new indices of LV mechanical function.

Keywords: Echocardiography; Rotation; Ventricular function.

Fig. 1

Diagrams showing various fiber geometries and mathematical models of the left ventricle for calculation of ejection fraction (EF). Upon stimulation, uniform shortening of the individual fibers to k times their initial lengths results in a radius change from B to kB with the indicated EF. If k=0.8 in circular fibers, which corresponds to a 20% shortening, this corresponds to an EF of only 0.36 for the cylinder (A) or ellipse of revolution (B) and 0.488 for the sphere (C). In a cylindrical model with longitudinal fibers (D), 15% fiber shortening results in only a 15% reduction in chamber volume, whereas with circumferential fibers (E), 15% fiber shortening results in only a 30% reduction in volume. Depending only upon the cylinder dimensions and the pitch angle of the spiral fibers (F), virtually any ejection fraction (up to and including 100%) could be generated from spiral fibers capable of shortening only 15% (modified from references and 2).

Fig. 2

Representative illustrations demonstrating the muscle band extending from the pulmonary artery (PA) to the aorta (Ao) which connects the ventricular myocardium, both right ventricle and LV, as a continuous band. This band provides an anatomical backbone of helical configuration of the cardiac muscle band with descending and ascending segments wrapping the LV apex. The helical configuration of the cardiac muscle band results in two loops at the cardiac base and apex (modified from reference 5). LV: left ventricular.

Fig. 3

Representative images of apical and basal rotation angles with corresponding rotational velocity curves measured by speckle tracking echocardiography.

Fig. 4

Association between dP/dt_max, an invasive gold standard of LV contractility and left ventricular twist (A), apical rotation (B) and ejection fraction (C)(modified from reference 15). LV: left ventricular.