Mechanics of ventricular torsion

Age-related changes in left ventricular twist assessed by two-dimensional speckle-tracking imaging

The aim of this study was to determine the normal value of left ventricular (LV) twist, and to examine the effects of aging on LV twist by newly developed 2-dimensional ultrasound speckle-tracking imaging. We acquired basal and apical LV short-axis second harmonic images in 118 healthy volunteers. Using commercially available 2-dimensional strain software, time-domain speckle tracking was performed, and mean value of LV rotation obtained at each plane. LV twist was defined as apical rotation relative to the base. Adequate data were obtained in 113 volunteers. During systole, the LV performs a wringing motion with a counterclockwise rotation at the apex and a clockwise rotation at the base. The mean value of peak twist was 7.7 +/- 3.5 degrees. Immediately after end systole, rapid untwisting develops. Different LV twist profiles are noted according to age. Peak LV twist was significantly higher, and the rate of LV untwisting significantly reduced and delayed, with advancing age. LV twist can be measured noninvasively by 2-dimensional ultrasound speckle-tracking imaging. The observed reduced and delayed diastolic untwisting with aging may contribute toward the tendency of diastolic dysfunction. This novel method allows the detailed study of diastolic function in various cardiovascular diseases.

Impaired systolic torsion in dilated cardiomyopathy: reversal of apical rotation at mid-systole characterized with magnetic resonance tagging method

Left ventricular (LV) torsion plays an important role in squeezing the blood out of the heart. To characterize the systolic torsion in LV dysfunction, we studied using magnetic resonance imaging myocardial tagging method in 26 subjects: 17 patients with dilated cardiomyopathy (DCM, LV ejection fraction [EF], 27 +/- 8%) and 9 healthy control subjects. Grid-tagged LV short-axis cine images were acquired at base, mid and apex levels. Tag-intersections were tracked during the systole, thereby determining rotation angle (positive indicated clockwise from the apex). Peak torsion was defined as the maximum difference in rotation angle between the base and apex. Time to peak torsion was expressed as % systole by dividing the time by a total systolic time. Amplitude of the rotation at peak was less in DCM than in controls at both the base (0.1 +/- 2.9 vs. 2.6 +/- 1.6 degrees , P < 0.05) and apex (-5.9 +/- 5.3 vs. -11.2 +/- 2.5 degrees , P < 0.01). Amplitude of peak torsion was then less in DCM than in controls (6.1 +/- 3.4 vs. 13.6 +/- 2.5 degrees , P < 0.001), and the timing of peak was earlier (66 +/- 22 vs. 104 +/- 16% systole, P < 0.001). The amplitude of peak torsion was correlated with LVEF (r=0.74, P < 0.001). In conclusion, amplitude of systolic torsion was impaired in proportion to LV function. Systolic torsion in LV dysfunction was characterized by the discontinuing counter-rotation of the apex to the base before end-systole.

Maturational and Adaptive Modulation of Left Ventricular Torsional Biomechanics

Background— Left ventricular (LV) torsional deformation, based in part on the helical myocardial fiber architecture, is an important component of LV systolic and diastolic performance. However, there is no comprehensive study describing its normal development during childhood and adult life.

Methods and Results— Forty-five normal subjects (25 children and 20 adults; aged 9 days to 49 years; divided into 5 groups: infants, children, adolescents, and young and middle-age adults) underwent assessment of LV torsion and untwisting rate by Doppler tissue imaging. LV torsion increased with age, primarily owing to augmentation in basal clockwise rotation during childhood and apical counterclockwise rotation during adulthood. Although LV torsion and untwisting overall showed age-related increases, when normalized by LV length, they showed higher values in infancy and middle age. The proportion of untwisting during isovolumic relaxation was lowest in infancy, increased during childhood, and leveled off thereafter, whereas peak untwisting performance (peak untwisting velocity normalized by peak LV torsion) showed a decrease during adulthood.

Conclusions— We have shown the maturational process of LV torsion in normal subjects. Net LV torsion increases gradually from infancy to adulthood, but the determinants of this were different in the 2 age groups. The smaller LV isovolumic untwisting recoil during infancy and its decline in adulthood may suggest mechanisms for alterations in diastolic function.

Form follows function: developmental and physiological view on ventricular myocardial architecture

The arrangement of myocytes within the ventricle is critical for its contractile performance, as evidenced by significant functional impairment seen in cardiomyopathies associated with myofiber disarray or post-infarction remodeling. A review on this topic by Anderson and associates provides anatomical insight gained from a multitude of approaches, and concludes that the best concept is that of syncytial continuum with supporting collagenous matrix. The overall arrangement is in the form of several intertwined helices, and the authors find no support for a recently suggested ventricular myocardial band hypothesis. This commentary aims at providing a developmental and physiological perspective on this purely anatomical concept. Unlike some other organ systems, the developing heart has to function since very early stages to support the oxygen and nutrition demands of the growing embryo, thus putting some constraints on heart development. The ventricular myocardial architecture transforms from a single-layered tube through trabeculated stages into a mature form that relies on a multi-layered compact zone. The first evidence of helical patterns is found in trabeculated hearts during ventricular contraction, and layers with different helix pitch develop during later fetal stages as the compact zone thickens. The second major point determining ventricular contraction is the sequence of its electrical activation. The ventricular activation sequence changes concomitantly with its morphology, from slow peristaltoid through base-to-apex pattern found in looped trabeculated hearts, to mature apex-to-base direction. Thus, adult ventricular myocardial architecture is best understood when one also considers the way it developed together with its electrical activation sequence and contraction pattern.

Regional ventricular wall thickening reflects changes in cardiac fiber and sheet structure during contraction: quantification with diffusion tensor MRI

Dynamic changes of myocardial fiber and sheet structure are key determinants of regional ventricular function. However, quantitative characterization of the contraction-related changes in fiber and sheet structure has not been reported. The objective of this study was to quantify cardiac fiber and sheet structure at selected phases of the cardiac cycle. Diffusion tensor MRI was performed on isolated, perfused Sprague-Dawley rat hearts arrested or fixed in three states as follows: 1) potassium arrested (PA), which represents end diastole; 2) barium-induced contracture with volume (BV+), which represents isovolumic contraction or early systole; and 3) barium-induced contracture without volume (BV-), which represents end systole. Myocardial fiber orientations at the base, midventricle, and apex were determined from the primary eigenvectors of the diffusion tensor. Sheet structure was determined from the secondary and tertiary eigenvectors at the same locations. We observed that the transmural distribution of the myofiber helix angle remained unchanged as contraction proceeded from PA to BV+, but endocardial and epicardial fibers became more longitudinally orientated in the BV- group. Although sheet structure exhibited significant regional variations, changes in sheet structure during myocardial contraction were relatively uniform across regions. The magnitude of the sheet angle, which is an index of local sheet slope, decreased by 23 and 44% in BV+ and BV- groups, respectively, which suggests more radial orientation of the sheet. In summary, we have shown for the first time that geometric changes in both sheet and fiber orientation provide a substantial mechanism for radial wall thickening independent of active components due to myofiber shortening. Our results provide direct evidence that sheet reorientation is a primary determinant of myocardial wall thickening.

First Experimental Evaluation of Cardiac Apex Rotation with an Epicardial Coriolis Force Sensor

Cardiac apex rotation, quantified by sophisticated techniques (radiopaque markers and tagged magnetic resonance), has been shown to provide a sensitive index of left ventricle (LV) dynamics. The authors describe the first experimental assessment of cardiac apex rotation using a gyroscopic sensor based on Coriolis force, epicardially glued on the apex. Dynamics of apex rotation were evaluated in a sheep at baseline, after a positive inotropic drug infusion, and after impairment of cardiac function induced by coronary ligation. To evaluate the efficacy of the sensor to monitor cardiac function, results were compared to contractility variations expressed by the maximum value of the first derivative of LV pressure (LVdP/dtMAX). After inotropic drug infusion, a parallel increasing trend resulted for LVdP/dtMAX, for the maximum value of angular velocity measured by the sensor, and for apex rotation angle derived from velocity signal (+146%, +155%, and +11% from baseline, respectively), whereas a decreasing trend of all three parameters resulted after coronary ligation (-35%, -31%, and -65%). The twist pattern also was altered from baseline. These initial results suggest that the use of an implantable rotation sensor based on Coriolis force can be an efficient and effective tool to assess LV torsional deformation both in normal and failing hearts.

Three-dimensional myofiber architecture of the embryonic left ventricle during normal development and altered mechanical loads

Mechanical load influences embryonic ventricular growth, morphogenesis, and function. To date, little is known regarding how the embryonic left ventricular (LV) myocardium acquires a three-dimensional (3D) fiber architecture distribution or how altered mechanical load influences local myofiber architecture. We tested the hypothesis that altered mechanical load changes the maturation process of local 3D fiber architecture of the developing embryonic LV compact myocardium. We measured transmural myofiber angle distribution in the LV compact myocardium in Hamburger-Hamilton stages 21, 27, 31, and 36 chick embryos during normal development or following either left atrial ligation (LAL; LV hypoplasia model) or conotruncal banding (CTB; LV hyperplasia model). The embryonic LV was stained with f-actin and then z-serial optical sectioning was performed using a laser confocal scanning microscope. We reconstructed local 3D myofiber images and computed local transmural myofiber angle distribution. Transmural myofiber angles in compact myocardium (in LV sagittal sections) were oriented in a circumferential direction until stage 27 (-10 to 10 degrees). Myofibers in the outer side of compact myocardium shifted to a more longitudinal direction by stage 36 (10 to 40 degrees), producing a transmural gradient in myofiber orientation. Developmental changes in transmural myofiber angle distribution were significantly delayed following LAL, while the changes in angle distribution were accelerated following CTB. Results suggest that mechanical load modulates the maturation process of myofiber architecture distribution in the developing LV compact myocardium.

Cardiac torsion and electromagnetic fields: The cardiac bioinformation hypothesis

Although in physiology the heart is often referred to as a simple piston pump, there are in fact two additional features that are integral to cardiac physiology and function. First, the heart as it contracts in systole, also rotates and produces torsion due to the structure of the myocardium. Second, the heart produces a significant electromagnetic field with each contraction due to the coordinated depolarization of myocytes producing a current flow. Unlike the electrocardiogram, the magnetic field is not limited to volume conduction and extends outside the body. The therapeutic potential for interaction of this cardioelectromagnetic field both within and outside the body is largely unexplored. It is our hypothesis that the heart functions as a generator of bioinformation that is central to normative functioning of body. The source of this bioinformation is based on: (1) vortex blood flow in the left ventricle; (2) a cardiac electromagnetic field and both; (3) heart sounds; and (4) pulse pressure which produce frequency and amplitude information. Thus, there is a multidimensional role for the heart in physiology and biopsychosocial dynamics. Recognition of these cardiac properties may result in significant implications for new therapies for cardiovascular disease based on increasing cardiac energy efficiency (coherence) and bioinformation from the cardioelectromagnetic field. Research studies to test this hypothesis are suggested.

Alterations in Left Ventricular Torsion and Diastolic Recoil After Myocardial Infarction With and Without Chronic Ischemic Mitral Regurgitation

Background— Chronic ischemic mitral regurgitation (CIMR) is associated with heart failure that continues unabated whether the valve is repaired, replaced, or ignored. Altered left ventricular (LV) torsion dynamics, with deleterious effects on transmural gradients of oxygen consumption and diastolic filling, may play a role in the cycle of the failing myocardium. We hypothesized that LV dilatation and perturbations in torsion would be greater in animals in which CIMR developed after inferior myocardial infarction (MI) than in those that it did not.

Methods— 8±2 days after marker placement in sheep, 3-dimensional fluoroscopic marker data (baseline) were obtained before creating inferior MI by snare occlusion. After 7±1 weeks, the animals were restudied (chronic). Inferior MI resulted in CIMR in 11 animals but not in 9 (non-CIMR). End-diastolic septal-lateral and anterior-posterior LV diameters, maximal torsional deformation (φ max , rotation of the LV apex with respect to the base), and torsional recoil in early diastole (φ 5% , first 5% of filling) for each LV free wall region (anterior, lateral, posterior) were measured.

Results— Both CIMR and non-CIMR animals demonstrated derangement of LV torsion after inferior MI. In contrast to non-CIMR, CIMR animals exhibited greater LV dilation and significant reductions in posterior maximal torsion (6.1±4.3° to 3.9±1.9°* versus 4.4±2.5° to 2.8±2.0°; mean±SD, baseline to chronic, * P <0.05) and anterior torsional recoil (−1.4±1.1° to −0.2±1.0° versus −1.2±1.0° to −1.3±1.6°).

Conclusion— MI associated with CIMR resulted in greater perturbations in torsion and recoil than inferior MI without CIMR. These perturbations may be linked to more LV dilation in CIMR, which possibly reduced the effectiveness of fiber shortening on torsion generation. Altered torsion and recoil may contribute to the “ventricular disease” component of CIMR, with increased gradients of myocardial oxygen consumption and impaired diastolic filling. These abnormalities in regional torsion and recoil may, in part, underlie the “ventricular disease” of CIMR, which may persist despite restoration of mitral competence.

Undersized Mitral Annuloplasty Alters Left Ventricular Shape During Acute Ischemic Mitral Regurgitation

Background— Underlying left ventricular (LV) dysfunction contributes to poor survival after operation to correct ischemic mitral regurgitation (IMR). Many surgeons do not appreciate that a key component of the Bolling undersized mitral ring annuloplasty concept is to decrease LV wall stress by altering LV shape, but precise 3-dimensional (3-D) geometric data do not exist substantiating this effect. We tested the hypothesis that annular reduction decreases regional circumferential LV radius of curvature (ROC) in a model of acute IMR.

Methods— Eight adult sheep underwent insertion of an adjustable Paneth-type annuloplasty suture and radiopaque markers on the LV and mitral annulus. The animals were studied with biplane videofluoroscopy during baseline conditions, then before and after tightening the annuloplasty suture during proximal left circumflex occlusion. End-systolic circumferential regional LV ROC and mitral annular area were computed.

Results— Acute IMR was eliminated (MR grade 2.1±0.4 to 0.4±0.4, mean±SD, P <0.05) by tightening the Paneth annuloplasty suture. Paneth suture tightening during circumflex occlusion also decreased end-systolic regional circumferential radii of curvature at the basal (anterior, 3.40±0.16 to 3.34±0.14 cm; posterior, 3.31±0.23 to 3.24±0.26 cm; P <0.05) and equatorial levels (anterior, 2.99±0.21 to 2.89±0.29 cm; posterior, 2.86±0.38 to 2.81±0.41 cm; P <0.05).

Conclusions— Acute proximal circumflex occlusion caused IMR and increased end-systolic LV radii of curvature in this experimental preparation. Annular reduction sufficient to abolish IMR also decreased end-systolic anterior and posterior LV ROC, which would be expected to reduce LV wall stress and oxygen consumption in these regions, both potentially beneficial effects. The long-term effects of undersized annuloplasty on LV remodeling and function, however, will require further study in chronic animal preparations or patients with chronic IMR.

The regional elastic properties analysis of myocardium based on echocardiographic 3-D reconstruction of the left ventricle

The present study evaluates the myocardium regional elastic properties on the basis of relative thickness change (DeltaHWT) in the left ventricular (LV) wall during the diastolic filling phase. Two-dimensional (2-D) LV long-axis images were obtained with a Powervision-380 (Toshiba) transesophageal echocardiographic imager. Three-dimensional (3-D) reconstruction of the LV was carried out by rotation of the transducer in calibrated steps. Endocardial and epicardial surfaces were approximated to the shape of heart wall by means of spherical functions. At the beginning of the diastolic filling phase, LV endocardial surface was divided into equal angular segments sized about 4 x 4 mm in a spherical coordinate system. To define the displacement direction of the heart wall surface fragments at every moment (frame) of diastolic filling, a new algorithm was developed. The elastic properties of LV wall regions were represented as regional DeltaHWT maps. A qualitative test of the method was implemented according to data from clinical and instrumental inspections of the patients with ischemic heart disease (IHD). Possible error sources were considered to evaluate the method quantitatively. The method root-mean-square error was about 5.4%, including errors of initial data, approximation and rounding off.

Haemodynamics and mechanics following partial left ventriculectomy: a computer modeling analysis

Mechanics following partial left ventriculectomy is still poorly understood. A computational cylindrical model of the left ventricle was developed, based on the myocardial fibre behaviour for the evaluation of the mechanical and haemodynamical effects of the operation. A healthy left ventricle with physiological geometry and function and a dilated hypokinetic heart were investigated. Haemodynamic and mechanical data were obtained at baseline and compared with those obtained at different degrees of volume reduction. Data included: ejection fraction (EF); stroke volume (SV); end-systolic and end-diastolic pressure-volume relationships (ESPVR and EDPVR), and efficiency. EF increases following volume reduction in both simulation but, concurrently, SV shows modest improvement (dilated ventricle) or reduction (healthy ventricle) at progressive degrees of resection. The ESPVR and EDPVR slope increases and shifts leftward with the resection extent, but the increase of the ESPVR slope is more pronounced in dilated ventricle. Efficiency is improved in the dilated heart after resections, while does not improve when the healthy-heart volume is reduced. The simulation of partial left ventriculectomy suggests an improvement of systolic performance, counterbalanced by increased diastolic stiffness following inverse remodelling. Efficiency of simulated dilated ventricles is enhanced by volume reduction, suggesting a favourable effect of reduction of the metabolic demand of the failing heart.

Remodeling of cardiac fiber structure after infarction in rats quantified with diffusion tensor MRI

Structural remodeling of myocardium after infarction plays a critical role in functional adaptation. Diffusion tensor magnetic resonance imaging (DTMRI) provides a means for rapid and nondestructive characterization of the three-dimensional fiber architecture of cardiac tissues. In this study, microscopic structural changes caused by MI were evaluated in Fischer 344 rats 4 wk after infarct surgery. DTMRI studies were performed on 15 excised, formalin-fixed rat hearts of both infarct (left anterior descending coronary artery occlusion, n = 8) and control (sham, n = 7) rats. Infarct myocardium exhibited increased water diffusivity (41% increase in trace values) and decreased diffusion anisotropy (37% decrease in relative anisotropy index). The reduced diffusion anisotropy correlated negatively with microscopic fiber disarray determined by histological analysis (R = 0.81). Transmural courses of fiber orientation angles in infarct zones were similar to those of normal myocardium. However, regional angular deviation of the diffusion tensor increased significantly in the infarct myocardium and correlated strongly with microscopic fiber disarray (R = 0.86). These results suggest that DTMRI may provide a valuable tool for defining structural remodeling in diseased myocardium at the cellular and tissue level.

Alterations in left ventricular torsion in tachycardia-induced dilated cardiomyopathy

OBJECTIVE

Left ventricular torsion reduces transmural systolic gradients of fiber strain, and torsional recoil in early diastole is thought to enhance left ventricular filling. Left ventricular remodeling in dilated cardiomyopathy may result in changes in torsion dynamics, but these effects are not yet characterized. Tachycardia-induced cardiomyopathy is accompanied by systolic and diastolic heart failure and left ventricular remodeling. We hypothesized that cardiomyopathy would alter systolic and diastolic left ventricular torsion mechanics, and this hypothesis was tested by studying sheep before and after the development of tachycardia-induced cardiomyopathy.

METHODS

Implanted miniature radiopaque markers were used in 8 sheep to measure left ventricular geometry and function, maximal torsional deformation, and early diastolic recoil before and after rapid ventricular pacing was used to create tachycardia-induced cardiomyopathy.

RESULTS

All animals had significant heart failure with ventricular dilatation and remodeling. With tachycardia-induced cardiomyopathy, maximum torsion relative to control conditions decreased (1.69 degrees +/- 0.61 degrees vs 4.25 degrees +/- 2.33 degrees ), and early diastolic recoil was completely abolished (0.53 degrees +/- 1.19 degrees vs -1.17 degrees +/- 0.94 degrees ).

CONCLUSIONS

Cardiomyopathy is accompanied by decreased and delayed systolic left ventricular torsional deformation and loss of early diastolic recoil, which may contribute to left ventricular dysfunction by increasing systolic transmural strain gradients and impairing diastolic filling. Analysis of left ventricular torsion with radiofrequency-tagging magnetic resonance imaging should be explored to elucidate the role of torsion in patients with cardiomyopathy.

Muscle LIM protein deficiency leads to alterations in passive ventricular mechanics

Accumulating evidence indicates that cytoskeletal defects may be an important pathway for dilated cardiomyopathy and eventual heart failure. Targeted disruption of muscle LIM protein (MLP) has previously been shown to result in dilated cardiomyopathy with many of the clinical signs of heart failure, although the effects of MLP disruption on passive ventricular mechanics and myocyte architecture are not known. We used the MLP knockout model to examine changes in passive ventricular mechanics and laminar myofiber sheet architecture. Pressure-volume and pressure-strain relations were altered in MLP knockout mice, in general suggesting a less compliant tissue in the dilated hearts. Transmural laminar myocyte structure was also altered in this mouse model, especially near the epicardium. A mathematical model of the heart showed a likely increase in passive tissue stiffness in the MLP-deficient (-/-) heart. These results suggest that the disruption of the cytoskeletal protein MLP results in less compliant passive tissue and concomitant structural alterations in the three-dimensional myocyte architecture that may in part explain the ventricular dysfunction in the dilated heart.

Regional septal dysfunction in a three-dimensional computational model of focal myofiber disarray

MLC2v/ras transgenic mice display a phenotype characteristic of hypertrophic cardiomyopathy, with septal hypertrophy and focal myocyte disarray. Experimental measurements of septal wall mechanics in ras transgenic mice have previously shown that regions of myocyte disarray have reduced principal systolic shortening, torsional systolic shear, and sarcomere length. To investigate the mechanisms of this regional dysfunction, a three-dimensional prolate spheroidal finite-element model was used to simulate filling and ejection in the hypertrophied mouse left ventricle with septal disarray. Focally disarrayed septal myocardium was modeled by randomly distributed three-dimensional regions of altered material properties based on measured statistical distributions of muscle fiber angular dispersion. Material properties in disarrayed regions were modeled by decreased systolic anisotropy derived from increased fiber angle dispersion and decreased systolic tension development associated with reduced sarcomere lengths. Compared with measurements in ras transgenic mice, the model showed similar heterogeneity of septal systolic strain with the largest reductions in principal shortening and torsional shear in regions of greatest disarray. Average systolic principal shortening on the right ventricular septal surface of the model was -0.114 for normal regions and -0.065 for disarrayed regions; for torsional shear, these values were 0.047 and 0.019, respectively. These model results suggest that regional dysfunction in ras transgenic mice may be explained in part by the observed structural defects, including myofiber dispersion and reduced sarcomere length, which contributed about equally to predicted dysfunction in the disarrayed myocardium.

Regional dysfunction correlates with myofiber disarray in transgenic mice with ventricular expression of ras

A hallmark of certain cardiac diseases such as familial hypertrophic cardiomyopathy is focal myofiber disarray. Regional ventricular dysfunction occurs in human subjects with hypertrophic cardiomyopathy; however, no direct evidence exists to correlate regional dysfunction with myofiber disarray. We used a transgenic mouse, which exhibits regional myofiber disarray via ventricular expression of the human oncogene ras, to investigate the relationship between myofiber disarray and septal surface strain. An isolated ejecting mouse heart preparation was used to record deformation of markers on the septal surface and to determine nonhomogeneous septal surface strain maps. Myofiber disarray made in histological tissue sections was correlated with gradients in surface systolic shortening. Significantly smaller maximum principal shortening was associated with disarray located near the right ventricle (RV) septal surface. There was also significantly smaller surface shear strain associated with disarray located either near the RV surface or at the midwall. Because surface shear is a local indicator of torsion, we conclude that myofiber disarray is associated with reduced septal torsion and reduced surface shortening.