Regional and Epi- to Endocardial Differences in Transmural Angles of Left Ventricular Cardiomyocytes Measured inEx VivoPig Hearts: Functional Implications

Antagonism of contractile forces in left ventricular hypertrophy: a diagnostic challenge for better pathophysiological and clinical understanding

Cardiac function is characterised by haemodynamic parameters in the clinical scenario. Due to recent development in imaging techniques, the clinicians focus on the quantitative assessment of left ventricular size, shape and motion patterns mostly analysed by echocardiography and cardiac magnetic resonance. Because of the physiologically known antagonistic structure and function of the heart muscle, the effective performance of the heart remains hidden behind haemodynamic parameters. In fact, a smaller component of oblique transmural netting of cardiac muscle fibres simultaneously engenders contracting and dilating force vectors, while the predominant mass of the tangentially aligned fibres only acts in one direction. In case of hypertrophy, an increased influence of the dilating transmural fibre component might counteract systolic wall thickening, thereby counteract cardiac output. A further important aspect is the response to inotropic stimulation that is different for the tangentially aligned fibre component in comparison to the transmural component. Both aspects highlight the importance to integrate the analysis of intramural fibre architecture into the clinical cardiac diagnostics.

Assessing Myocardial Architecture: The Challenges and Controversies

In recent decades, investigators have strived to describe and quantify the orientation of the cardiac myocytes in an attempt to classify their arrangement in healthy and diseased hearts. There are, however, striking differences between the investigations from both a technical and methodological standpoint, thus limiting their comparability and impeding the drawing of appropriate physiological conclusions from the structural assessments. This review aims to elucidate these differences, and to propose guidance to establish methodological consensus in the field. The review outlines the theory behind myocyte orientation analysis, and importantly has identified pronounced differences in the definitions of otherwise widely accepted concepts of myocytic orientation. Based on the findings, recommendations are made for the future design of studies in the field of myocardial morphology. It is emphasised that projection of myocyte orientations, before quantification of their angulation, introduces considerable bias, and that angles should be assessed relative to the epicardial curvature. The transmural orientation of the cardiomyocytes should also not be neglected, as it is an important determinant of cardiac function. Finally, there is considerable disagreement in the literature as to how the orientation of myocardial aggregates should be assessed, but to do so in a mathematically meaningful way, the normal vector of the aggregate plane should be utilised.

Cardiac multi-scale investigation of the right and left ventricle ex vivo: a review

The heart is a complex multi-scale system composed of components integrated at the subcellular, cellular, tissue and organ levels. The myocytes, the contractile elements of the heart, form a complex three-dimensional (3D) network which enables propagation of the electrical signal that triggers the contraction to efficiently pump blood towards the whole body. Cardiovascular diseases (CVDs), a major cause of mortality in developed countries, often lead to cardiovascular remodeling affecting cardiac structure and function at all scales, from myocytes and their surrounding collagen matrix to the 3D organization of the whole heart. As yet, there is no consensus as to how the myocytes are arranged and packed within their connective tissue matrix, nor how best to image them at multiple scales. Cardiovascular imaging is routinely used to investigate cardiac structure and function as well as for the evaluation of cardiac remodeling in CVDs. For a complete understanding of the relationship between structural remodeling and cardiac dysfunction in CVDs, multi-scale imaging approaches are necessary to achieve a detailed description of ventricular architecture along with cardiac function. In this context, ventricular architecture has been extensively studied using a wide variety of imaging techniques: ultrasound (US), optical coherence tomography (OCT), microscopy (confocal, episcopic, light sheet, polarized light), magnetic resonance imaging (MRI), micro-computed tomography (micro-CT) and, more recently, synchrotron X-ray phase contrast imaging (SR X-PCI). Each of these techniques have their own set of strengths and weaknesses, relating to sample size, preparation, resolution, 2D/3D capabilities, use of contrast agents and possibility of performing together with in vivo studies. Therefore, the combination of different imaging techniques to investigate the same sample, thus taking advantage of the strengths of each method, could help us to extract the maximum information about ventricular architecture and function. In this review, we provide an overview of available and emerging cardiovascular imaging techniques for assessing myocardial architecture ex vivo and discuss their utility in being able to quantify cardiac remodeling, in CVDs, from myocyte to whole organ.

Anatomically correct assessment of the orientation of the cardiomyocytes using diffusion tensor imaging

Diffusion tensor imaging has been used for assessing the orientation of cardiac myocytes for decades. Striking methodological differences exist between studies when quantifying these orientations. This limits the comparability between studies, and impedes collaboration and the drawing of appropriate physiological conclusions. We have sought to elucidate these differences, permitting us to propose a standardised "tool set" that might better establish consensus in future studies. We fixed hearts from seven 25 kg pigs in formalin, and scanned them using diffusion tensor imaging. Using various angle definitions as found in literature, we assessed the orientations of cardiomyocytes, comparing them in terms of helical and intrusion angles, along with the orientation of their aggregations. The difference between assessment of the helical angle with and without relation to the epicardial curvature was 25.2° (SD: 7.9) at the base, 5.8° (1.9) at the equatorial level, and 28.0° (7.0) at the apex, ANOVA P = 0.001. In comparable fashion, the intrusion angle differed by 25.9° (12.9), 7.6° (0.98) and 17.5° (4.7), P = 0.01, and the angle of the aggregates (E3-angle) differed by 25.0° (13.5) at the base, 9.4° (1.7) at the equator, and 23.1° (6.2) apically, P = 0.003. When assessing 14 definitions used in literature to calculate the orientation of aggregates, only 4 rendered identical results. The findings show that any attempt to use projection of eigenvectors introduces considerable bias. The epicardial curvature of the ventricular cone needs to be taken into account when seeking to provide accurate quantification of the orientation of the aggregated cardiomyocytes, especially in the apical and basal regions. This means that projection of eigenvectors should be avoided prior to quantifying myocyte orientation, especially when assessing radial orientation. Based on our results, we suggest appropriate methods for valid assessment of myocyte orientation using diffusion tensor imaging.

Resolving the natural myocardial remodelling brought upon by cardiac contraction; a porcine ex-vivo cardiovascular magnetic resonance study of the left and right ventricle

BACKGROUND

The three-dimensional rearrangement of the right ventricular (RV) myocardium during cardiac deformation is unknown. Previous in-vivo studies have shown that myocardial left ventricular (LV) deformation is driven by rearrangement of aggregations of cardiomyocytes that can be characterised by changes in the so-called E3-angle. Ex-vivo imaging offers superior spatial resolution compared with in-vivo measurements, and can thus provide novel insight into the deformation of the myocardial microstructure in both ventricles. This study sought to describe the dynamic changes of the orientations of the cardiomyocytes in both ventricles brought upon by cardiac contraction, with particular interest in the thin-walled RV, which has not previously been described in terms of its micro-architecture.

METHODS

The hearts of 14 healthy 20 kg swine were excised and preserved in either a relaxed state or a contracted state. Myocardial architecture was assessed and compared between the two contractional states by quantification of the helical, transmural and E3-angles of the cardiomyocytes using high-resolution diffusion tensor imaging.

RESULTS

The differences between the two states of contraction were most pronounced in the endocardium where the E3-angle decreased from 78.6° to 24.8° in the LV and from 82.6° to 68.6° in the RV. No significant change in neither the helical nor the transmural angle was found in the cardiomyocytes of the RV. In the endocardium of the LV, however, the helical angle increased from 35.4° to 47.8° and the transmural angle increased from 3.1° to 10.4°.

CONCLUSION

The entire myocardium rearranges through the cardiac cycle with the change in the orientation of the aggregations of cardiomyocytes being the predominant mediator of myocardial wall thickening. Interestingly, differences also exist between the RV and LV, which helps in the explanation of the different physiological capabilities of the ventricles.

What is the clinical significance of ventricular mural antagonism?

Recent morphological studies provide evidence that the ventricular walls are arranged as a 3D meshwork of aggregated cardiomyocyte chains, exhibiting marked local structural variations. In contrary to previous findings, up to two-fifths of the chains are found to have a partially transmural alignment, thus deviating from the prevailing tangential orientation. Upon contraction, they produce, in addition to a tangential force, a radial force component that counteracts ventricular constriction and aids widening of the ventricular cavity. In experimental studies, we have provided evidence for the existence of such forces, which are auxotonic in nature. This is in contrast to the tangentially aligned myocytes that produce constrictive forces, which are unloading in nature. The ventricular myocardium is, therefore, able to function in an antagonistic fashion, with the prevailing constrictive forces acting simultaneously with a dilatory force component. The ratio of constrictive to dilating force varies locally according to the specific mural architecture. Such antagonism acts according to local demands to preserve the ventricular shape, store the elastic energy that drives the fast late systolic dilation and apportion mural motion to facilitate the spiralling nature of intracavitary flow. Intracavitary pressure and flow dynamics are thus governed concurrently by ventricular constrictive and dilative force components. Antagonistic activity, however, increases deleteriously in states of cardiac disease, such as hypertrophy and fibrosis. ß-blockade at low dosage acts selectively to temper the auxotonic forces.

The end of the unique myocardial band: Part II. Clinical and functional considerations

Two of the leading concepts of mural ventricular architecture are the unique myocardial band and the myocardial mesh model. We have described, in an accompanying article published in this journal, how the anatomical, histological and high-resolution computed tomographic studies strongly favour the latter concept. We now extend the argument to describe the linkage between mural architecture and ventricular function in both health and disease. We show that clinical imaging by echocardiography and magnetic resonance imaging, and electrophysiological studies, all support the myocardial mesh model. We also provide evidence that the unique myocardial band model is not compatible with much of scientific research.

[Antagonistic function of the heart muscle : Part II: Clinical implications]

In the hypertrophic heart the myostructural afterload in the form of endoepicardial networks is predominant, which enhances myocardial hypertrophy. The intrinsic antagonism is derailed. Likewise, the connective tissue scaffold, i.e. the stromatogenic afterload, is enriched in the response to the derailment of antagonism in a hypertrophic heart up to regional captivation of the heart musculature. Due to the selective susceptibility of the auxotonic, contracting oblique transmural myocardial network for low dose negative inotropic medication, this promises to attenuate progress in myocardial hypertrophy. Volume reduction surgery is most effective in reducing wall stress as long as the myocardium is not critically fettered by fibrosis. The use of external mechanical circulatory support is then effective if the heart is supported in its resting mode, which means around a middle width and at minimal amplitude of motion. The takotsubo cardiomyopathy might possibly reflect an isolated, extreme stimulation of the intrinsic antagonism as a response to hormonally induced sensitization of the myocardium to catecholamine. A particular significant conclusion with respect to the diseased heart is that clinical diagnostics need new impulses with a focus on the analysis of local motion patterns and on myocardial stiffness reflecting disease-dependent antagonistic intensity. This would become a relevant diagnostic marker if corresponding (noninvasive) measurement techniques would become available.

[The antagonistic function of the heart muscle sustains the autoregulation according to Frank and Starling : Part I: Structure and function of heart muscle]

In the tradition of Harvey and according to Otto Frank the heart muscle structure is arranged in a strictly tangential fashion hence all contractile forces act in the direction of ventricular ejection. In contrast, morphology confirms that the heart consists of a 3-dimensional network of muscle fibers with up to two fifths of the chains of aggregated myocytes deviating from a tangential alignment at variable angles. Accordingly, the myocardial systolic forces contain, in addition to a constrictive also a (albeit smaller) radially acting component. Using needle force probes we have correspondingly measured an unloading type of force in a tangential direction and an auxotonic type in dilatative transversal direction of the ventricular walls to show that the myocardial body contracts actively in a 3-dimensional pattern. This antagonism supports the autoregulation of heart muscle function according to Frank and Starling, preserving ventricular shape, enhances late systolic fast dilation and attenuates systolic constriction of the ventricle wall. Auxotonic dilating forces are particularly sensitive to inotropic medication. Low dose beta-blocker is able to attenuate the antagonistic activity. All myocardial components act against four components of afterload, the hemodynamic, the myostructural, the stromatogenic and the hydraulic component. This complex interplay critically complicates clinical diagnostics. Clinical implications are far-reaching (see Part II, https://doi.org/10.1007/s00059-018-4735-x).

Resolving the True Ventricular Mural Architecture

The precise nature of packing together of the cardiomyocytes within the ventricular walls has still to be determined. The spiraling nature of the chains of interconnected cardiomyocytes has long been recognized. As long ago as the end of the nineteenth century, Pettigrew had emphasized that the ventricular cone was not arranged on the basis of skeletal muscle. Despite this guidance, subsequent anatomists described entities such as “bulbo-spiral muscles”, with this notion of subunits culminating in the suggestion that the ventricular cone could be unwrapped so as to produce a “ventricular myocardial band”. Others, in contrast, had suggested that the ventricular walls were arranged on the basis of “sheets”, or more recently “sheetlets”, with investigators seeking to establishing the angulation of these entities using techniques such as magnetic resonance imaging. Our own investigations, in contrast, have shown that the cardiomyocytes are aggregated together within the supporting fibrous matrix so as to produce a three-dimensional myocardial mesh. In this review, we summarize the previous accounts, and provide the anatomical evidence we have thus far accumulated to support the model of the myocardial mesh. We show how these anatomic findings underscore the concept of the myocardial mesh functioning in antagonistic fashion. They lend evidence to support the notion that the ventricular myocardium works as a muscular hydrostat.

Will the real ventricular architecture please stand up?

Ventricular twisting, essential for cardiac function, is attributed to the contraction of myocardial helical fibers. The exact relationship between ventricular anatomy and function remains to be determined, but one commonly used explanatory model is the helical ventricular myocardial band (HVMB) model of Torrent‐Guasp. This model has been successful in explaining many aspects of ventricular function, (Torrent‐Guasp et al. Eur. J. Cardiothorac. Surg., 25, 376, 2004; Buckberg et al. Eur. J. Cardiothorac. Surg., 47, 587, 2015; Buckberg et al. Eur. J. Cardiothorac. Surg. 47, 778, 2015) but the model ignores important aspects of ventricular anatomy and should probably be replaced. The purpose of this review is to compare the HVMB model with a different model (nested layers). A complication when interpreting experimental observations that relate anatomy to function is that, in the myocardium, shortening does not always imply activation and lengthening does not always imply inactivation.

High resolution 3-Dimensional imaging of the human cardiac conduction system from microanatomy to mathematical modeling

Cardiac arrhythmias and conduction disturbances are accompanied by structural remodelling of the specialised cardiomyocytes known collectively as the cardiac conduction system. Here, using contrast enhanced micro-computed tomography, we present, in attitudinally appropriate fashion, the first 3-dimensional representations of the cardiac conduction system within the intact human heart. We show that cardiomyocyte orientation can be extracted from these datasets at spatial resolutions approaching the single cell. These data show that commonly accepted anatomical representations are oversimplified. We have incorporated the high-resolution anatomical data into mathematical simulations of cardiac electrical depolarisation. The data presented should have multidisciplinary impact. Since the rate of depolarisation is dictated by cardiac microstructure, and the precise orientation of the cardiomyocytes, our data should improve the fidelity of mathematical models. By showing the precise 3-dimensional relationships between the cardiac conduction system and surrounding structures, we provide new insights relevant to valvar replacement surgery and ablation therapies. We also offer a practical method for investigation of remodelling in disease, and thus, virtual pathology and archiving. Such data presented as 3D images or 3D printed models, will inform discussions between medical teams and their patients, and aid the education of medical and surgical trainees.

The myocardial architecture changes in persistent pulmonary hypertension of the newborn in an ovine animal model

BACKGROUND

Persistent pulmonary hypertension in the newborn remains a syndrome with high mortality. Knowledge of changes in myocardial architecture in the setting of heart failure in persistent pulmonary hypertension is lacking, and could aid in the explanation of the prevailing high mortality.

METHODS

Persistent pulmonary hypertension was induced by antenatal ligation of the arterial duct in six ovine fetuses. The hearts were compared ex vivo with five matched control hearts, using diffusion tensor imaging to provide the overall anatomical arrangement, and assessment of the angulations and course of the cardiomyocytes. Fibrosis was assessed with histology.

RESULTS

We found an overall increase in heart size in pulmonary hypertension, with myocardial thickening confined to the interventricular septum. An increase of 3.5° in angulation of myocyte aggregations was found in hypertensive hearts. In addition, we observed a 2.2% increase in collagen content in the right ventricular free wall. Finally, we found a previously undescribed subepicardial layer of strictly longitudinally oriented cardiomyocytes confined to the right ventricle in all hearts.

CONCLUSION

Myocardial fibrosis and possibly changes in angulations of myocytes seem to play a part in the etiology of persistent pulmonary hypertension. Moreover, a new anatomical arrangement of right ventricular mural architecture is described.

Adaptive anisotropic gaussian filtering to reduce acquisition time in cardiac diffusion tensor imaging

Diffusion tensor imaging (DTI) is used to quantify myocardial fiber orientation based on helical angles (HA). Accurate HA measurements require multiple excitations (NEX) and/or several diffusion encoding directions (DED). However, increasing NEX and/or DED increases acquisition time (TA). Therefore, in this study, we propose to reduce TA by implementing a 3D adaptive anisotropic Gaussian filter (AAGF) on the DTI data acquired from ex-vivo healthy and infarcted porcine hearts. DTI was performed on ex-vivo hearts [9-healthy, 3-myocardial infarction (MI)] with several combinations of DED and NEX. AAGF, mean (AVF) and median filters (MF) were applied on the primary eigenvectors of the diffusion tensor prior to HA estimation. The performance of AAGF was compared against AVF and MF. Root mean square error (RMSE), concordance correlation-coefficients and Bland-Altman's technique was used to determine optimal combination of DED and NEX that generated the best HA maps in the least possible TA. Lastly, the effect of implementing AAGF on the infarcted porcine hearts was also investigated. RMSE in HA estimation for AAGF was lower compared to AVF or MF. Post-filtering (AAGF) fewer DED and NEX were required to achieve HA maps with similar integrity as those obtained from higher NEX and/or DED. Pathological alterations caused in HA orientation in the MI model were preserved post-filtering (AAGF). Our results demonstrate that AAGF reduces TA without affecting the integrity of the myocardial microstructure.

The functional architecture of skeletal compared to cardiac musculature: Myocyte orientation, lamellar unit morphology, and the helical ventricular myocardial band

How the cardiomyocytes are aggregated within the heart walls remains contentious. We still do not fully understand how the end-to-end longitudinal myocytic chains are arranged, nor the true extent and shape of the lamellar units they aggregate to form. In this article, we show that an understanding of the complex arrangement of cardiac musculature requires knowledge of three-dimensional myocyte orientation (helical and intrusion angle), and appreciation of myocyte packing within the connective tissue matrix. We show how visualization and segmentation of high-resolution three-dimensional image data can accurately identify the morphology and orientation of the myocytic chains, and the lamellar units. Some maintain that the ventricles can be unwrapped in the form of a "helical ventricular myocardial band," that is, as a compartmentalized band with selective regional innervation and deformation, and a defined origin and insertion like most skeletal muscles. In contrast to the simpler interpretation of the helical ventricular myocardial band, we provide insight as to how the complex myocytic chains, the heterogeneous lamellar units, and connective tissue matrix form an interconnected meshwork, which facilitates the complex internal deformations of the ventricular wall. We highlight the dangers of disregarding the intruding cardiomyocytes. Preparation of the band destroys intruding myocytic chains, and thus disregards the functional implications of the antagonistic auxotonic forces they produce. We conclude that the ventricular myocardium is not analogous to skeletal muscle, but is a complex three-dimensional meshwork, with a heterogeneous branching lamellar architecture.

Second-order motion-compensated spin echo diffusion tensor imaging of the human heart

PURPOSE

Myocardial microstructure has been challenging to probe in vivo. Spin echo-based diffusion-weighted sequences allow for single-shot acquisitions but are highly sensitive to cardiac motion. In this study, the use of second-order motion-compensated diffusion encoding was compared with first-order motion-compensated diffusion-weighted imaging during systolic contraction of the heart.

METHODS

First- and second-order motion-compensated diffusion encoding gradients were incorporated into a triggered single-shot spin echo sequence. The effect of contractile motion on the apparent diffusion coefficients and tensor orientations was investigated in vivo from basal to apical level of the heart.

RESULTS

Second-order motion compensation was found to increase the range of systolic trigger delays from 30%-55% to 15%-77% peak systole at the apex and from 25%-50% to 15%-79% peak systole at the base. Diffusion tensor analysis yielded more physiological transmural distributions when using second-order motion-compensated diffusion tensor imaging.

CONCLUSION

Higher-order motion-compensated diffusion encoding decreases the sensitivity to cardiac motion, thereby enabling cardiac DTI over a wider range of time points during systolic contraction of the heart.

Dual-phase cardiac diffusion tensor imaging with strain correction

Purpose

In this work we present a dual-phase diffusion tensor imaging (DTI) technique that incorporates a correction scheme for the cardiac material strain, based on 3D myocardial tagging.

Methods

In vivo dual-phase cardiac DTI with a stimulated echo approach and 3D tagging was performed in 10 healthy volunteers. The time course of material strain was estimated from the tagging data and used to correct for strain effects in the diffusion weighted acquisition. Mean diffusivity, fractional anisotropy, helix, transverse and sheet angles were calculated and compared between systole and diastole, with and without strain correction. Data acquired at the systolic sweet spot, where the effects of strain are eliminated, served as a reference.

Results

The impact of strain correction on helix angle was small. However, large differences were observed in the transverse and sheet angle values, with and without strain correction. The standard deviation of systolic transverse angles was significantly reduced from 35.9±3.9° to 27.8°±3.5° (p<0.001) upon strain-correction indicating more coherent fiber tracks after correction. Myocyte aggregate structure was aligned more longitudinally in systole compared to diastole as reflected by an increased transmural range of helix angles (71.8°±3.9° systole vs. 55.6°±5.6°, p<0.001 diastole). While diastolic sheet angle histograms had dominant counts at high sheet angle values, systolic histograms showed lower sheet angle values indicating a reorientation of myocyte sheets during contraction.

Conclusion

An approach for dual-phase cardiac DTI with correction for material strain has been successfully implemented. This technique allows assessing dynamic changes in myofiber architecture between systole and diastole, and emphasizes the need for strain correction when sheet architecture in the heart is imaged with a stimulated echo approach.