Mechanics of ventricular torsion

Evaluation of left ventricular rotation in obese subjects by velocity vector imaging

AIMS

Obesity is a well-known risk factor in the development of cardiovascular disease. We hypothesize that early left ventricular (LV) dysfunction secondary to obesity could be signalled by abnormal LV rotation.

METHODS AND RESULTS

This prospective study examined 60 subjects divided into two groups: obese group with body mass index (BMI) >or=30 and control group with BMI <25. The peak rotation, twist, and torsion of the left ventricle were studied in obese and control subjects, using velocity vector imaging. Age and gender were comparable between the two groups. Obese subjects had higher BMI, waist circumference, fasting glucose, triglycerides, systolic and diastolic blood pressure, low-density lipoprotein cholesterol, and lower high-density lipoprotein cholesterol (P < 0.05). In obese subjects, LV mass and LV mass index were increased, and the ratio of mitral early and late diastolic filling velocity was decreased (P < 0.05). In obese subjects, the peak twist and torsion of the left ventricle displayed a lower trend, and the peak rotation of the left ventricle apex decreased significantly (3.81 +/- 2.09 degrees vs. 5.77 +/- 3.27 degrees , P < 0.001).

CONCLUSION

Obesity was associated with changes in LV rotation. Velocity vector imaging is a feasible and reproducible echocardiographic technique for the detection of early subclinical LV dysfunction.

Ventricular rotation is independent of cardiac looping: a study in mice with situs inversus totalis using speckle-tracking echocardiography

BACKGROUND

The authors conducted an ultrasound interrogation of a mutant mouse model with a Dnah5 mutation to determine whether cardiac mechanics may be affected by reversal of cardiac situs. This mutant is a bona fide model of primary ciliary dyskinesia, with surviving homozygous mice showing either situs solitus (SS) or situs inversus totalis (SI).

METHODS

High-frequency ultrasound interrogations of 27 neonatal and infant Dnah5 mutant mice, 16 with SS and 11 with SI, were conducted using an ultra-high-frequency biomicroscope. Electrocardiographic and respiratory gating were used to reconstruct high-resolution two-dimensional cines at 1,000 Hz, with speckle-tracking echocardiography used to further analyze midchamber and apical rotation.

RESULTS

All SS mice exhibited the expected counterclockwise apical rotation as viewed caudocranially, and surprisingly, the same counterclockwise motion was also observed in SI mice. Speckle-tracking analysis confirmed counterclockwise systolic rotation in both SS and SI mice, and this increased in magnitude from the subepicardium to the endocardium and from the papillary muscles to the apex. The magnitude of apical endocardial rotation was not different for SS and SI mice (5.64+/-0.75 degrees and 5.76+/-1.90 degrees, respectively, P=.93). The anatomic segments responsible for the largest components of apical endocardial systolic rotation differed between the SS and SI hearts (P=.004). In both, the two largest contributors to rotation were offset 180 degrees from each other, but the anatomic regions differed between them. In SS hearts, maximal regional rotation occurred at the anterior mid-septum and posterolateral free wall, while in SI hearts, it was derived from the posterior septum and the anterolateral free wall. Analysis by episcopic fluorescence image capture histology of representative SI and SS mice showed normal intracardiac and segmental anatomy ({S,D,S} or {I,L,I}) without intracardiac defects.

CONCLUSIONS

These results show that mirror-image cardiac looping did not result in mirror-image rotation of the morphologic left ventricle. These findings suggest that further studies are warranted to evaluate whether fiber orientation and cardiac mechanics may be abnormal in individuals with reversal of cardiac situs. The results of this study indicate that cardiac looping and myofiber orientation may be independently regulated.

Influence of cardiac shape on left ventricular twist

The dynamic interaction between subendocardial and subepicardial fibre helices in the left ventricle (LV) leads to a twisting deformation, which has an important role in LV function. This study sought to assess the influence of cardiac shape on LV twist in the normal and dilated human heart. The study comprised 45 dilated cardiomyopathy (DCM) patients and 60 for age- and gender-matched healthy volunteers. Speckle tracking echocardiography was used to determine basal and apical LV peak systolic rotation (Rotmax) and instantaneous LV peak systolic twist (Twistmax). LV sphericity index was calculated by dividing the LV maximal long-axis internal dimension by the maximal short-axis internal dimension at end-diastole. A parabolic relation between the sphericity index and apical Rotmaxor Twistmaxwas identified in the total study population ( R2= 0.56 and R2= 0.54, respectively; both P < 0.001) and healthy volunteers ( R2= 0.39 and R2= 0.25, respectively; both P < 0.001), whereas these relations were linear in DCM patients ( R2= 0.40 and R2= 0.43, respectively; both P < 0.001). In a multivariate analysis, LV sphericity index was the strongest independent predictor of apical Rotmaxand Twistmax. In conclusion, LV apical rotation and twist are significantly influenced by LV configuration. Taking the important function of LV twist into account, this finding highlights the vital influence of cardiac shape on LV systolic function.

High-resolution diffusion tensor MR imaging for evaluating myocardial anisotropy and fiber tracking at 3T: the effect of the number of diffusion-sensitizing gradient directions

Objective

We wanted to evaluate the effect of the number of diffusion-sensitizing gradient directions on the image quality for evaluating myocardial anisotropy and fiber tracking by using in vitro diffusion tensor MR imaging (DT-MRI).

Materials and Methods

The DT-MR images, using a SENSE-based echoplanar imaging technique, were acquired from ten excised porcine hearts by using a 3T MR scanner. With a b-value of 800 s/mm², the diffusion tensor images were obtained for 6, 15 and 32 diffusion-sensitizing gradient directions at the midventricular level. The number of tracked fibers, the fractional anisotropy (FA), and the length of the tracked fibers were measured for the quantitative analysis. Two radiologists assessed the image quality of the fiber tractography for the qualitative analysis.

Results

By increasing the number of diffusion-sensitizing gradient directions from 6 to 15, and then to 32, the FA and standard deviation were significantly reduced (p < 0.01), and the number of tracked fibers and the length of the tracked fibers were significantly increased (p < 0.01). The image quality of the fiber tractography was significantly increased with the increased number of diffusion-sensitizing gradient directions (p < 0.01).

Conclusion

The image quality of in vitro DT-MRI is significantly improved as the number of diffusion-sensitizing gradient directions is increased.

Left ventricular twist and untwist in aortic stenosis

BACKGROUND

To optimally exploit the potential added diagnostic and prognostic value of new left ventricular (LV) deformation parameters, better understanding of LV mechanics in aortic stenosis (AS) is warranted. We sought to determine a broad spectrum of LV rotation parameters in AS patients and age-matched healthy controls, in order to gain insight into the mechanical properties of the LV in AS.

METHODS

The study comprised 48 AS patients with an aortic valve area<2.0 cm2 and LV ejection fraction>50%, and 24 healthy--for age and gender matched--control subjects. LV peak systolic rotation (Rotmax), LV peak systolic twist (Twistmax), untwisting rate (mean diastolic untwisting velocity from Twistmax to mitral valve opening), peak diastolic untwisting velocity, and time-to-peak diastolic untwisting velocity were determined by speckle tracking echocardiography.

RESULTS

AS patients had normal basal Rotmax and increased apical Rotmax, resulting in increased Twistmax (13.4±4.0° vs. 11.4±2.7°, P<0.05). Apical Rotmax and Twistmax correlated significantly to echo-Doppler indicators of AS severity. Time-to-peak diastolic untwisting velocity was increased (20±10% vs. 15±9%, P<0.05) and untwisting rate was decreased (-38±21°/s vs. -50±28°/s, P<0.01) in AS patients.

CONCLUSIONS

Twistmax increases proportionally to the severity of AS, which might serve as a compensatory mechanism to maintain systolic LV function. LV diastolic untwisting is delayed and the untwisting rate is reduced in AS.

Alterations in left ventricular untwisting with ageing

BACKGROUND

In order to gain further insight into age-associated changes of left ventricular (LV) diastolic function, the purpose of the current study was to investigate alterations in LV untwisting with ageing.

METHODS AND RESULTS

The study comprised 75 healthy volunteers, classified into 3 groups: age 16-35 (n=25), 36-55 (n=25) and 56-75 (n=25) years. LV untwisting (as a percentage of peak systolic twist) at 5%, 10%, 15% and 50% of diastole, peak diastolic untwisting velocity, time-to-peak diastolic untwisting velocity and untwisting rate (mean untwisting velocity during the time interval from peak systolic twist to mitral valve opening) were assessed using speckle-tracking echocardiography. Untwisting at 5%, 10%, 15% and 50% of diastole decreased with ageing. Although the peak diastolic untwisting velocity and untwisting rate were not significantly different between the age groups, when normalized for LV peak systolic twist, these parameters decreased with advancing age (both P<0.01). Time-to-peak diastolic untwisting velocity increased with ageing (P<0.01).

CONCLUSIONS

Impairment of the relative peak diastolic untwisting velocity and untwisting rate, resulting in delayed LV untwisting, may help to explain diastolic dysfunction in the elderly. (Circ J 2010; 74: 101 - 108).

A tissue-level electromechanical model of the left ventricle: application to the analysis of intraventricular pressure

The ventricular pressure profile is characteristic of the cardiac contraction progress and is useful to evaluate the cardiac performance. In this contribution, a tissue-level electromechanical model of the left ventricle is proposed, to assist the interpretation of left ventricular pressure waveforms. The left ventricle has been modeled as an ellipsoid composed of twelve mechano-hydraulic sub-systems. The asynchronous contraction of these twelve myocardial segments has been represented in order to reproduce a realistic pressure profiles. To take into account the different energy domains involved, the tissue-level scale and to facilitate the building of a modular model, multiple formalisms have been used: Bond Graph formalism for the mechano-hydraulic aspects and cellular automata for the electrical activation. An experimental protocol has been defined to acquire ventricular pressure signals from three pigs, with different afterload conditions. Evolutionary Algorithms have been used to identify the model parameters in order to minimize the error between experimental and simulated ventricular pressure signals. Simulation results show that the model is able to reproduce experimental ventricular pressure. In addition, electro-mechanical activation times have been determined in the identification process. For example, the maximum electrical activation time is reached, respectively, 96.5, 139.3 and 131.5 ms for the first, second, and third pigs. These preliminary results are encouraging for the application of the model on non-invasive data like ECG, arterial pressure or myocardial strain.

Left ventricular torsion in paced patients

BACKGROUND

In healthy people the left ventricle presents a counter-clockwise apical rotation and a clockwise basal rotation ending in late systole. In early systole (during isovolumic contraction) there is a fast and inverse rotation (counter-clockwise at the base and clockwise at the apex). This opposite rotation between apex and base produces the systolic torsion of the left ventricle. The effect of permanent conventional pacing on this torsion is little known.

OBJECTIVES

The aim of this study was to assess, by speckle tracking echocardiography, left ventricular rotation and torsion in patients conventionally paced at the apex of the right ventricle.

METHODS

Left ventricular apical and basal rotation and the consequent torsion were evaluated by means of speckle tracking echocardiography, in 13 paced patients, without ischemic or valvular disease, and in 17 healthy participants. Left ventricular dyssynchrony was evaluated by means of temporal uniformity of strain.

RESULTS

In the paced group there was a significant reduction in early-systolic clockwise torsion (-0.4 degrees +/- 1.2 vs. -1.5 degrees +/- 1.6; P = 0.04), and in late-systolic counter-clockwise torsion (15.1 degrees +/- 4.3 vs. 19.1 degrees +/- 5.5; P = 0.03). Circumferential temporal uniformity of strain averaged significantly lower in paced patients.

CONCLUSIONS

Conventional pacing from the apex of the right ventricle alters both the torsional mechanic and the synchrony of the left ventricle.

A new surgical ventricular restoration technique to reset residual myocardium's fiber orientation: the "KISS" procedure

BACKGROUND

The history of surgical reconstruction of the left ventricle after an anterior myocardial infarction shows an evolution of techniques which tend to a more and more physiologic restoration of ventricular shape and volume, with increasing attention to the orientation of myocardial fibers.

METHODS

We set a new surgical procedure for endoventricular patch reconstruction technique with the aim to rebuild a physiologic shape and volume of the left ventricle caring about realignment of myocardial fibers orientation. Peculiarities of this reconstruction are the shape of the patch (reduction of minor axis compared with currently used oval-shaped patch) and the asymmetrical way of suturing it inside the ventricle.

RESULTS

We present a detailed description of operative steps of this procedure, and we add some relevant surgical hints to clarify its peculiarities. Most of the patients operated on with this technique showed the original renewal of apical rotation and left ventricular torsion as specific index of the restoration of physiologic fiber orientation: we report an exemplary case of at-sight recovery of apical rotation in the operating room.

CONCLUSION

This technique can represent a reproducible new way to realign myocardial fibers in a near-normal setting, improving the physiological restoration of ischemically injured left ventricle. It could be also the basis to reconsider surgical treatment for heart failure.

A dynamic double helical band as a model for cardiac pumping

We address here, by means of finite-element computational modeling, two features of heart mechanics and, most importantly, their timing relationship: one of them is the ejected volume and the other is the twist of the heart. The corner stone of our approach is to take the double helical muscle fiber band as the dominant active macrostructure behind the pumping function. We show that this double helical model easily reproduces a physiological maximal ejection fraction of up to 60% without exceeding the limit on local muscle fiber contraction of 15%. Moreover, a physiological ejection fraction can be achieved independently of the excitation pattern. The left ventricular twist is also largely independent of the type of excitation. However, the physiological relationship between the ejection fraction and twist can only be reproduced with Purkinje-type excitation schemes. Our results indicate that the proper timing coordination between twist and ejection dynamics can be reproduced only if the excitation front originates in the septum region near the apex. This shows that the timing of the excitation is directly related to the productive pumping operation of the heart and illustrates the direction for possible bioinspired pump design.

Age-related changes in the biomechanics of left ventricular twist measured by speckle tracking echocardiography

The increasing number and proportion of aged individuals in the population warrants knowledge of normal physiological changes of left ventricular (LV) biomechanics with advancing age. LV twist describes the instantaneous circumferential motion of the apex with respect to the base of the heart and has an important role in LV ejection and filling. This study sought to investigate the biomechanics behind age-related changes in LV twist by determining a broad spectrum of LV rotation parameters in different age groups, using speckle tracking echocardiography (STE). The final study population consisted of 61 healthy volunteers (16–35 yr, n = 25; 36–55 yr, n = 23; 56–75 yr, n = 13; 31 men). LV peak systolic rotation during the isovolumic contraction phase (Rotearly), LV peak systolic rotation during ejection (Rotmax), instantaneous LV peak systolic twist (Twistmax), the time to Rotearly, Rotmax, and Twistmax, and rotational deformation delay (defined as the difference of time to basal Rotmax and apical Rotmax) were determined by STE using QLAB Advanced Quantification Software (version 6.0; Philips, Best, The Netherlands). With increasing age, apical Rotmax ( P < 0.05), time to apical Rotmax ( P < 0.01), and Twistmax ( P < 0.01) increased, whereas basal Rotearly ( P < 0.001), time to basal Rotearly ( P < 0.01), and rotational deformation delay ( P < 0.05) decreased. Rotational deformation delay was significantly correlated to Twistmax ( R2 = 0.20, P < 0.05). In conclusion, Twistmax increased with aging, resulting from both increased apical Rotmax and decreased rotational deformation delay between the apex and the base of the LV. This may explain the preservation of LV ejection fraction in the elderly.

Transmural variation in myosin heavy chain isoform expression modulates the timing of myocardial force generation in porcine left ventricle

Recent studies have shown that the sequence and timing of mechanical activation of myocardium vary across the ventricular wall. However, the contributions of variable expression of myofilament protein isoforms in mediating the timing of myocardial activation in ventricular systole are not well understood. To assess the functional consequences of transmural differences in myofilament protein expression, we studied the dynamic mechanical properties of multicellular skinned preparations isolated from the sub-endocardial and sub-epicardial regions of the porcine ventricular midwall. Compared to endocardial fibres, epicardial fibres exhibited significantly faster rates of stretch activation and force redevelopment (k(tr)), although the amount of force produced at a given [Ca2+] was not significantly different. Consistent with these results, SDS-PAGE analysis revealed significantly elevated expression of alpha myosin heavy chain (MHC) isoform in epicardial fibres (13 +/- 1%) versus endocardial fibres (3 +/- 1%). Linear regression analysis revealed that the apparent rates of delayed force development and force decay following stretch correlated with MHC isoform expression (r2 = 0.80 and r2 = 0.73, respectively, P < 0.05). No differences in the relative abundance or phosphorylation status of other myofilament proteins were detected. These data show that transmural differences in MHC isoform expression contribute to regional differences in dynamic mechanical function of porcine left ventricles, which in turn modulate the timing of force generation across the ventricular wall and work production during systole.

Model-based analysis of myocardial strain data acquired by tissue Doppler imaging

OBJECTIVE

Tissue Doppler imaging (TDI) is commonly used to evaluate regional ventricular contraction properties through the analysis of myocardial strain. During the clinical examination, a set of strain signals is acquired concurrently at different locations. However, the joint interpretation of these signals remains difficult. This paper proposes a model-based approach in order to assist the clinician in making an analysis of myocardial strain.

METHODS AND MATERIALS

The proposed method couples a model of the left ventricle, which takes into account cardiac electrical, mechanical and hydraulic activities with an adapted identification algorithm, in order to obtain patient-specific model representations. The proposed model presents a tissue-level resolution, adapted to TDI strain analysis. The method is applied in order to reproduce TDI strain signals acquired from two healthy subjects and a patient presenting with dilated cardiomyopathy (DCM).

RESULTS

The comparison between simulated and experimental strains for the three subjects reflects a satisfying adaptation of the model on different strain morphologies. The mean error between real and synthesized signals is equal to 2.34% and 2.09%, for the two healthy subjects and 1.30% for the patient suffering from DCM. Identified parameters show significant electrical conduction and mechanical activation delays for the pathologic case and have shown to be useful for the localization of the failing myocardial segments, which are situated on the anterior and lateral walls of the ventricular base.

CONCLUSION

The present study shows the feasibility of a model-based method for the analysis of TDI strain signals. The identification of delayed segments in the pathologic case produces encouraging results and may represent a way to better utilize the information included in strain signals and to improve the therapy assistance.

Effect of pregnancy on left ventricular motion (twist) in women with aortic stenosis

The combination of fixed outflow obstruction from aortic stenosis (AS) and the hemodynamic changes of pregnancy increased the risk of maternal or fetal deterioration. Left ventricular (LV) response in patients with AS to the hemodynamic changes of pregnancy has not been examined. We studied and compared myocardial mechanics with echocardiography in 3 groups of 10 women each, including (1) pregnant with bicuspid aortic valve (BAV; peak aortic gradient 59 +/-7 mm Hg, aortic valve area 0.9 +/- 0.04 cm2), (2) pregnant without BAV, and (3) nonpregnant, healthy volunteer. Measurements in the pregnant BAV group were made on 3 occasions, within a year before pregnancy (baseline), in the antepartum period, and at least 6 weeks postpartum. Tissue tracking ultrasound was used to assess longitudinal strain and LV twist. During pregnancy, peak AS gradient rose from 59 +/- 7 to 70 +/- 9 mm Hg (p = 0.004) whereas valve area remained unchanged 0.9 +/- 0.04 to 0.8 +/- 0.04 cm2 (p = 0.48) as compared with baseline (before pregnancy). Overall, in all patients, there was no significant change in the longitudinal strain (-22 +/- 1, -21 +/- 0.6, -20 +/- 0.6 percent, p = 0.21)] at baseline, during pregnancy, or after pregnancy, respectively. Patients with AS had a higher baseline LV twist compared with both control groups (5.4 +/- 0.3, pregnant, with AS; 4.1 +/- 0.8, pregnant, without AS; 3.6 +/- 0.3, nonpregnant volunteer; expressed in degrees; p = 0.023). Additionally, all but 2 patients had a significant increase in LV twist during pregnancy compared with baseline. These 2 women had symptomatic deterioration requiring urgent aortic balloon valvuloplasty. Postpartum, in all AS patients, LV twist returned to antepartum values. In conclusion, we found that LV twist was significantly increased in women with congenital AS. During pregnancy, LV twist further increased in the antepartum period, except in those women who experienced functional deterioration.

Contribution of the Pericardium to Left Ventricular Torsion and Regional Myocardial Function in Patients with Total Absence of the Left Pericardium

OBJECTIVE

The relationship between left ventricular (LV) torsional deformation and myocardial function has recently been recognized. However, little is known about whether the pericardium affects this relationship. Our aim was to identify the contribution of the pericardium to LV torsion and regional myocardial function in the clinical setting.

METHODS

We examined LV torsion in basal and apical LV short-axis views, and regional LV myocardial function, such as longitudinal strain in apical 4-chamber view, and circumferential and radial strains in parasternal LV short-axis views using 2-dimensional speckle-tracking imaging method in 5 patients with congenital total absence of the left pericardium and systolic paradoxical ventricular septal motion on M-mode echocardiogram and in 10 control subjects. Diagnosis of the pericardial defect was based on chest radiograph, computed tomography, jugular phlebogram, and M-mode and 2-dimensional echocardiogram. LV torsion was defined as the net difference in LV rotation in the basal and apical planes.

RESULTS

There was no significant difference in LV ejection fraction determined by 2-dimensional echocardiography between the pericardial defect and control groups. LV torsion was markedly decreased in the pericardial defect group compared with the control group. There were no significant differences in longitudinal, radial, and circumferential systolic strains and systolic and early diastolic strain rates in the LV walls and in longitudinal systolic strains and systolic and early diastolic strain rates in the left atrial walls between the two groups.

CONCLUSIONS

Pericardial defects cause a lack of LV torsion while maintaining LV regional myocardial function in patients with systolic paradoxical ventricular septal motion. Therefore, pericardium plays an important role in LV torsion.

MR diffusion tensor imaging study of postinfarct myocardium structural remodeling in a porcine model

This study aimed to investigate postinfarct left ventricular (LV) fiber structural alterations by ex vivo diffusion tensor imaging (DTI) in a porcine heart model. In vivo cardiac MR imaging was first performed to measure ventricular function in six adult pigs with septal infarction near apex induced by the LAD ligation 13 weeks earlier. Hearts were then excised from the infarct pigs (n = 6) and six intact controls (n = 6) and fixed in formalin. High-resolution DTI was employed to examine changes in fractional anisotropy (FA), apparent diffusion coefficient (ADC), and transmural helix angle distribution in the infarct, adjacent and remote regions as compared to the sham regions in the controls. FA values were found to decrease in the infarct and differ between the adjacent and remote regions. ADC increase in the infarct region was substantial, while changes in the adjacent and remote regions were insignificant. Structurally, the double-helix myocardial structure shifted toward more left-handed around the infarcted myocardium. Accordingly, the histological analysis revealed clear fiber structural degradation in the adjacent region. These findings confirmed the subtle alterations in the myocardial fiber quality and structure not only in the infarcted but also in the surrounding noninfarcted myocardium or borderzone.

Assessment of training-dependent changes in the left ventricle torsion dynamics of professional soccer players using speckle-tracking echocardiography

Recently, it has been proposed the use of speckle-tracking echography (STE) to study the left ventricle (LV) torsion dynamics, which would make LV torsion assessment more available in clinical and research cardiology. LV torsion has been described during exercise and in some sportsmen, but so far, its dynamics has not been studied in soccer players. The aims were to characterize and to compare LV apical and basal rotation, and to analyze LV torsion in professional soccer players using STE, and to determine the main differences in torsion between soccer players and age-matched non-trained individuals. The STE allowed characterizing LV rotation and torsion in both groups. LV torsion level and velocities were lesser in soccer players than in non-trained individuals. Changes in torsion in soccer players could represent physiological adaptations to training.

Rewind the heart: a novel technique to reset heart fibers' orientation in surgery for ischemic cardiomyopathy

Ischemic cardiomyopathy is the most common cause of dilated cardiomyopathy and congestive heart failure. It affects approximately 1 out of 100 people, most often middle-aged to elderly men. Left ventricular restoration surgery is a challenging therapeutic approach to this pathology: it aims to rebuild a near-normal ventricular chamber in a heart damaged by a myocardial infarction, reducing its volume and improving the fraction of blood ejected by each systole. This is obtained by eliminating the akinetic/dyskinetic part of the cardiac muscle and closing the final defect with or without a synthetic patch. Optimization of surgical repair is mandatory as far as ischemic cardiomyopathy is a worldwide disease responsible for many cardiac deaths and because of its potential use as an alternative to heart transplantation in selected patients. Until now, this surgery has been performed without caring for myocardial fibers' disposition but recent evidences clarified the key role of fibers' alignment in heart physiology. The myocardium of the left ventricle has a unique three-dimensional, multilayered structure: it constitutes the anatomical basis for the cardiac function and for left ventricular torsion, a key movement of normal heart. Myocardial infarction alters myocardial structure in the site of the necrosis and subsequent cardiomyopathy eliminates left ventricular torsion. On the other hand, histological evidences show that myofibers' orientation in the thickness of residual normal myocardium is not changed and that transmural courses of fiber orientation angles near infarct zones were similar to those of normal myocardium. We hypothesize that, with a particular surgical technique, it could be possible to realign the anatomically normal fibers of the residual myocardium in order to rebuild a physiologic setting. We planned a novel surgical technique of left ventricular restoration using a very narrow, string-shaped patch and a particular suturing sequence and technique, whose aim is to near normally oriented residual myocardial fibers. The renewal of left ventricular torsion was evident at sight just at the end of this kind of ventricular restoration, still in the operating room, then confirmed by 2D speckle tracking echocardiography. These observations are indirect proofs of fibers' realignment, as the torsion movement of the left ventricle is due to the interlaced, oblique orientation of myocardial fibers. We herein propose a theoretical explanation of this outcome, drawing a geometrical modeling of the surgical procedure.

Study of myocardial fiber pathway using magnetic resonance diffusion tensor imaging

The purpose of this study was to investigate myocardial fiber pathway distribution in order to provide supplemental information on myocardial fiber architecture and cardiac mechanics. Diffusion tensor imaging (DTI) with medium diffusion resolution (15 directions) was performed on normal canine heart samples (N=6) fixed in formalin. With the use of diffusion tensor fiber tracking, left ventricle (LV) myocardial fiber pathways and helix angles were computed pixel by pixel at short-axis slices from base to apex. Distribution of DTI-tracked fiber pathway length and number was analyzed quantitatively as a function of fiber helix angle in step of 9 degrees . The long fiber pathways were found to have small helix angles. They are mostly distributed in the middle myocardium and run circumferentially. Fiber pathways tracked at the middle and upper LV are generally longer than those near the apex. Majority of fiber pathways have small helix angles between -20 degrees and 20 degrees , dominating the fiber architecture in myocardium. Likely, such myocardial fiber pathway measurement by DTI may reflect the spatial connectiveness or connectivity of elastic myofiber bundles along their preferential pathway of electromechanical activation. The dominance of the long and circumferentially running fiber pathways found in the study may explain the circumferential predominance in left ventricular contraction.

An improved numerical method for strong coupling of excitation and contraction models in the heart

Quantifying the interactions between excitation and contraction is fundamental to furthering our understanding of cardiac physiology. To date simulating these effects in strongly coupled excitation and contraction tissue models has proved computationally challenging. This is in part due to the numerical methods implemented to maintain numerical stability in previous simulations, which produced computationally intensive problems. In this study, we analytically identify and quantify the velocity and length dependent sources of instability in the current established coupling method and propose a new method which addresses these issues. Specifically, we account for the length and velocity dependence of active tension within the finite deformation equations, such that the active tension is updated at each intermediate Newton iteration, within the mechanics solution step. We then demonstrate that the model is stable and converges in a three-dimensional rod under isometric contraction. Subsequently, we show that the coupling method can produce stable solutions in a cube with many of the attributes present in the heart, including asymmetrical activation, an inhomogeneous fibre field and a nonlinear constitutive law. The results show no instabilities and quantify the error introduced by discrete length updates. This validates our proposed coupling framework, demonstrating significant improvement in the stability of excitation and contraction simulations.

Transmural heterogeneity of diffusion anisotropy in the sheep myocardium characterized by MR diffusion tensor imaging

The orientation of MRI-measured diffusion tensor in the myocardium has been directly correlated to the tissue fiber direction and widely characterized. However, the scalar anisotropy indexes have mostly been assumed to be uniform throughout the myocardial wall. The present study examines the fractional anisotropy (FA) as a function of transmural depth and circumferential and longitudinal locations in the normal sheep cardiac left ventricle. Results indicate that FA remains relatively constant from the epicardium to the midwall and then decreases (25.7%) steadily toward the endocardium. The decrease of FA corresponds to 7.9% and 12.9% increases in the secondary and tertiary diffusion tensor diffusivities, respectively. The transmural location of the FA transition coincides with the location where myocardial fibers run exactly circumferentially. There is also a significant difference in the midwall-endocardium FA slope between the septum and the posterior or lateral left ventricular free wall. These findings are consistent with the cellular microstructure from histological studies of the myocardium and suggest a role for MR diffusion tensor imaging in characterization of not only fiber orientation but, also, other tissue parameters, such as the extracellular volume fraction.

Effect of right ventricular pacing on cardiac apex rotation assessed by a gyroscopic sensor

To quantify cardiac apex rotation (CAR), the authors recently proposed the use of a Coriolis force sensor (gyroscope) as an alternative to other complex techniques. The aim of this study was to evaluate the effects of right ventricular (RV) pacing on CAR. A sheep heart was initially paced from the right atrium to induce a normal activation sequence at a fixed heart rate (AAI mode) and then an atrioventricular pacing was performed (DOO mode, AV delay = 60 ms). A small gyroscope was epicardially glued on the cardiac apex to measure the angular velocity (Ang V). From AAI to DOO pacing mode, an increase (+9.2%, p < 0.05) of the maximum systolic twisting velocity (Ang VMAX) and a marked decrease (-19.9%, p < 0.05) of the maximum diastolic untwisting velocity (Ang VMIN) resulted. RV pacing had negligible effects (-3.1%, p = 0.09) on the maximum angle of CAR, obtained by integrating Ang V. The hemodynamic parameters of systolic (LVdP/dtMAX) and diastolic (LVdP/dtMIN) cardiac function showed slight variations (-3.8%, p < 0.05 and +3.9%, p < 0.05, respectively). Results suggest that cardiac dyssynchrony induced by RV pacing can alter the normal physiological ventricular twist patterns, particularly affecting diastolic untwisting velocity.

Left ventricular form and function revisited: applied translational science to cardiovascular ultrasound imaging

Doppler tissue imaging (DTI) and DTI-derived strain imaging are robust physiologic tools used for the noninvasive assessment of regional myocardial function. As a result of high temporal and spatial resolution, regional function can be assessed for each phase of the cardiac cycle and within the transmural layers of the myocardial wall. Newer techniques that measure myocardial motion by speckle tracking in gray-scale images have overcome the angle dependence of DTI strain, allowing for measurement of 2-dimensional strain and cardiac rotation. DTI, DTI strain, and speckle tracking may provide unique information that deciphers the deformation sequence of complexly oriented myofibers in the left ventricular wall. The data are, however, limited. This review examines the structure and function of the left ventricle relative to the potential clinical application of DTI and speckle tracking in assessing the global mechanical sequence of the left ventricle in vivo.

MR tagging demonstrates quantitative differences in regional ventricular wall motion in mice, rats, and men

Rats and genetically manipulated mouse models have played an important role in the exploration of molecular causes of cardiovascular diseases. However, it has not been fully investigated whether mice or rats and humans manifest similar patterns of ventricular wall motion. Although similarities in anatomy and myofiber architecture suggest that fundamental patterns of ventricular wall motion may be similar, the considerable differences in heart size, heart rate, and sarcomeric protein isoforms may yield quantitative differences in ventricular wall mechanics. To further our understanding of the basic mechanisms of myofiber contractile performance, we quantified regional and global indexes of ventricular wall motion in mice, rats, and men using magnetic resonance (MR) imaging. Both regular cine and tagged MR images at apical, midventricular, and basal levels were acquired from six male volunteers, six Fischer 344 rats, and seven C57BL/6 mice. Morphological parameters and ejection fraction were computed directly from cine images. Myocardial twist (rotation angle), torsion (net twist per unit length), circumferential strain, and normalized radial shortening were calculated by homogeneous strain analysis from tagged images. Our data show that ventricular twist was conserved among the three species, leading to a significantly smaller torsion, measured as net twist per unit length, in men. However, both circumferential strain and normalized radial shortening were the largest in male subjects. Although other parameters, such as circumferential-longitudinal shear strain, need to be evaluated, and the causes of these differences in contractile mechanics remain to be elucidated, the preservation of twist appears fundamental to cardiac function and should be considered in studies that extrapolate data from animals to humans.

Left ventricular structure and function: basic science for cardiac imaging

The myofiber geometry of the left ventricle (LV) changes gradually from a right-handed helix in the subendocardium to a left-handed helix in the subepicardium. In this review, we associate the LV myofiber architecture with emerging concepts of the electromechanical sequence in a beating heart. We discuss: 1) the morphogenesis and anatomical arrangement of muscle fibers in the adult LV; 2) the sequence of depolarization and repolarization; 3) the physiological inhomogeneity of transmural myocardial mechanics and the apex-to-base sequence of longitudinal and circumferential deformation; 4) the sequence of LV rotation; and 5) the link between LV deformation and the intracavitary flow direction observed during each phase of the cardiac cycle. Integrating the LV structure with electrical activation and motion sequences observed in vivo provides an understanding about the spatiotemporal sequence of regional myocardial performance that is essential for noninvasive cardiac imaging.