The forces generated within the musculature of the left ventricular wall

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.

Vascular cells improve functionality of human cardiac organoids

Crosstalk between cardiac cells is critical for heart performance. Here we show that vascular cells within human cardiac organoids (hCOs) enhance their maturation, force of contraction, and utility in disease modeling. Herein we optimize our protocol to generate vascular populations in addition to epicardial, fibroblast, and cardiomyocyte cells that self-organize into in-vivo-like structures in hCOs. We identify mechanisms of communication between endothelial cells, pericytes, fibroblasts, and cardiomyocytes that ultimately contribute to cardiac organoid maturation. In particular, (1) endothelial-derived LAMA5 regulates expression of mature sarcomeric proteins and contractility, and (2) paracrine platelet-derived growth factor receptor β (PDGFRβ) signaling from vascular cells upregulates matrix deposition to augment hCO contractile force. Finally, we demonstrate that vascular cells determine the magnitude of diastolic dysfunction caused by inflammatory factors and identify a paracrine role of endothelin driving dysfunction. Together this study highlights the importance and role of vascular cells in organoid models.

Myocardial mesostructure and mesofunction

The complex and highly organized structural arrangement of some five billion cardiomyocytes directs the coordinated electrical activity and mechanical contraction of the human heart. The characteristic transmural change in cardiomyocyte orientation underlies base-to-apex shortening, circumferential shortening, and left ventricular torsion during contraction. Individual cardiomyocytes shorten ∼15% and increase in diameter ∼8%. Remarkably, however, the left ventricular wall thickens by up to 30-40%. To accommodate this, the myocardium must undergo significant structural rearrangement during contraction. At the mesoscale, collections of cardiomyocytes are organized into sheetlets, and sheetlet shear is the fundamental mechanism of rearrangement that produces wall thickening. Herein, we review the histological and physiological studies of myocardial mesostructure that have established the sheetlet shear model of wall thickening. Recent developments in tissue clearing techniques allow for imaging of whole hearts at the cellular scale, whereas magnetic resonance imaging (MRI) and computed tomography (CT) can image the myocardium at the mesoscale (100 µm to 1 mm) to resolve cardiomyocyte orientation and organization. Through histology, cardiac diffusion tensor imaging (DTI), and other modalities, mesostructural sheetlets have been confirmed in both animal and human hearts. Recent in vivo cardiac DTI methods have measured reorientation of sheetlets during the cardiac cycle. We also examine the role of pathological cardiac remodeling on sheetlet organization and reorientation, and the impact this has on ventricular function and dysfunction. We also review the unresolved mesostructural questions and challenges that may direct future work in the field.

Tissue motion annular displacement to assess the left ventricular systolic function in healthy cats

The tissue motion annular displacement (TMAD) measures the longitudinal displacement of the mitral annulus during systole, using speckle-tracking echocardiography (STE). The main objective was to determine the TMAD means in healthy cats, exploring the correlations with systolic surrogates. The influence of age, body surface area (BSA), heart rate, and systemic blood pressure on the indices was also analyzed. One hundred ninety-three healthy, client-owned cats participated in this prospective, cross-sectional observational study undergoing conventional and STE. Apical four-chamber (AP4) and two-chamber (AP2) images were recorded for offline calculations. Mean TMAD values were similar to mitral annulus plane systolic excursion (MAPSE), varying between 4 to 4.8 mm depending on the annulus and image used. No significant differences between age and BSA categories were detected, except for AP4 MP%, reduced in the heavier group. TMAD variables showed moderate correlation with longitudinal strain (LSt) and MAPSE, but not with fraction shortening (FS) and ejection fraction (EF). The median time required for the offline calculation was 12.2 s for AP4 and 11.8 s for AP2. The technique showed moderate inter and intraobserver variation, proving a reliable tool for assessing left ventricular longitudinal systolic function in cats.

Tools for studying and modulating (cardiac muscle) cell mechanics and mechanosensing across the scales

Cardiomyocytes generate force for the contraction of the heart to pump blood into the lungs and body. At the same time, they are exquisitely tuned to the mechanical environment and react to e.g. changes in cell and extracellular matrix stiffness or altered stretching due to reduced ejection fraction in heart disease, by adapting their cytoskeleton, force generation and cell mechanics. Both mechanical sensing and cell mechanical adaptations are multiscale processes. Receptor interactions with the extracellular matrix at the nanoscale will lead to clustering of receptors and modification of the cytoskeleton. This in turn alters mechanosensing, force generation, cell and nuclear stiffness and viscoelasticity at the microscale. Further, this affects cell shape, orientation, maturation and tissue integration at the microscale to macroscale. A variety of tools have been developed and adapted to measure cardiomyocyte receptor-ligand interactions and forces or mechanics at the different ranges, resulting in a wealth of new information about cardiomyocyte mechanobiology. Here, we take stock at the different tools for exploring cardiomyocyte mechanosensing and cell mechanics at the different scales from the nanoscale to microscale and macroscale.

Cell proliferation fate mapping reveals regional cardiomyocyte cell-cycle activity in subendocardial muscle of left ventricle

Cardiac regeneration involves the generation of new cardiomyocytes from cycling cardiomyocytes. Understanding cell-cycle activity of pre-existing cardiomyocytes provides valuable information to heart repair and regeneration. However, the anatomical locations and in situ dynamics of cycling cardiomyocytes remain unclear. Here we develop a genetic approach for a temporally seamless recording of cardiomyocyte-specific cell-cycle activity in vivo. We find that the majority of cycling cardiomyocytes are positioned in the subendocardial muscle of the left ventricle, especially in the papillary muscles. Clonal analysis revealed that a subset of cycling cardiomyocytes have undergone cell division. Myocardial infarction and cardiac pressure overload induce regional patterns of cycling cardiomyocytes. Mechanistically, cardiomyocyte cell cycle activity requires the Hippo pathway effector YAP. These genetic fate-mapping studies advance our basic understanding of cardiomyocyte cell cycle activity and generation in cardiac homeostasis, repair, and regeneration.

The adult mammalian heart exhibits stubbornly low levels of cardiomyocyte proliferation, leading to high morbidity after injury or heart attack. Here the authors develop an approach for tracking cardiomyocyte cell cycling and show that the majority are located adjacent to the endocardium.

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.

Bionic women and men - Part 3: Right ventricular dysfunction in patients implanted with left ventricular assist devices

NEW FINDINGS

What is the topic of this review? Right heart dysfunction remains a major adverse event in patients with end stage heart failure undergoing left ventricular assist device placement. This article reviews the pathophysiology and clinical considerations of right heart failure in this patient population. What advances does it highlight? This review highlights the anatomic and physiological peculiarities of the interplay between left and right heart function in patients undergoing LVAD therapy. These would allow us to further advance our understanding of right ventricular function.

ABSTRACT

The adaptation of the right ventricular (RV) output to a left ventricular assist device (LVAD) often determines the fate of patients with pulmonary hypertension secondary to left heart failure. Pre-existing right heart dysfunction in patients with advanced left heart failure is the consequence of increased (arterial) afterload and not simply the consequence of myocardial disease. If unaccounted for, it has the potential of accelerating into clinical right heart failure after LVAD, leading to significant morbidity and mortality. After LVAD implantation, the RV has to face increased flow generated by the LVAD, cardiac arrhythmias and exaggerated functional interactions between both ventricles. Understanding the key physiological mechanisms of RV dysfunction in patients with end-stage heart failure will allow us to predict and therefore prevent RV failure after LVAD implantation.

How are the cardiomyocytes aggregated together within the walls of the left ventricular cone?

The manner of packing together of the cardiomyocytes within the walls of the cardiac ventricles has now been investigated for over half a millennium. In 1669, Lower dissected the ventricular mass, likening the arrangement to skeletal musculature, in the form of a myocardial band extending between the right and left atrioventricular junctions. Pettigrew subsequently showed obvious helical arrangements to be evident within the ventricular walls, but emphasised that the cardiomyocytes were attached to each other, and could not justifiably be compared with skeletal cardiomyocytes. Torrent-Guasp then reactivated the notion that the ventricular mass was formed of a solitary band. Unlike Lower, he dissected the band as extending between the pulmonary to the aortic roots. Multiple investigations conducted using gross dissection and histology, and more recently diffusion tensor magnetic resonance imaging and computed tomographic analysis, have shown an absence of any anatomical boundaries within the walls that might permit the mass uniformly to be dissected so as to reveal the band. A response to a recent letter to the Journal, nonetheless, claimed that the dissections had been validated by clinicians interpreting the findings so as to provide an explanation for ventricular cardiodynamics, arguing that the findings provided a suitable anatomical model for this purpose. Anatomical models, however, are of no value unless they are anatomically correct. In this review, therefore, we summarise the evidence showing that the cardiomyocytes making up the ventricular walls, rather than forming a ventricular myocardial band, are instead aggregated together to form a three-dimensional myocardial mesh.

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.

[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.

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.

Effects of incremental beta-blocker dosing on myocardial mechanics of the human left ventricle: MRI 3D-tagging insight into pharmacodynamics supports theory of inner antagonism

Beta-blockers contribute to treatment of heart failure. Their mechanism of action, however, is incompletely understood. Gradients in beta-blocker sensitivity of helically aligned cardiomyocytes compared with counteracting transversely intruding cardiomyocytes seem crucial. We hypothesize that selective blockade of transversely intruding cardiomyocytes by low-dose beta-blockade unloads ventricular performance. Cardiac magnetic resonance imaging (MRI) 3D tagging delivers parameters of myocardial performance. We studied 13 healthy volunteers by MRI 3D tagging during escalated intravenous administration of esmolol. The circumferential, longitudinal, and radial myocardial shortening was determined for each dose. The curves were analyzed for peak value, time-to-peak, upslope, and area-under-the-curve. At low doses, from 5 to 25 μg·kg−1·min−1, peak contraction increased while time-to-peak decreased yielding a steeper upslope. Combining the values revealed a left shift of the curves at low doses compared with baseline without esmolol. At doses of 50 to 150 μg·kg−1·min−1, a right shift with flattening occurred. In healthy volunteers we found more pronounced myocardial shortening at low compared with clinical dosage of beta-blockers. In patients with ventricular hypertrophy and higher prevalence of transversely intruding cardiomyocytes selective low-dose beta-blockade could be even more effective. MRI 3D tagging could help to determine optimal individual beta-blocker dosing avoiding undesirable side effects.

The length-force behavior and operating length range of squid muscle vary as a function of position in the mantle wall

Hollow cylindrical muscular organs are widespread in animals and are effective in providing support for locomotion and movement, yet are subject to significant non-uniformities in circumferential muscle strain. During contraction of the mantle of squid, the circular muscle fibers along the inner (lumen) surface of the mantle experience circumferential strains 1.3 to 1.6 times greater than fibers along the outer surface of the mantle. This transmural gradient of strain may require the circular muscle fibers near the inner and outer surfaces of the mantle to operate in different regions of the length-tension curve during a given mantle contraction cycle. We tested the hypothesis that circular muscle contractile properties vary transmurally in the mantle of the Atlantic longfin squid, Doryteuthis pealeii. We found that both the length-twitch force and length-tetanic force relationships of the obliquely striated, central mitochondria-poor (CMP) circular muscle fibers varied with radial position in the mantle wall. CMP circular fibers near the inner surface of the mantle produced higher force relative to maximum isometric tetanic force, P0, at all points along the ascending limb of the length-tension curve than CMP circular fibers near the outer surface of the mantle. The mean ± s.d. maximum isometric tetanic stresses at L₀ (the preparation length that produced the maximum isometric tetanic force) of 212 ± 105 and 290 ± 166 kN m(-2) for the fibers from the outer and inner surfaces of the mantle, respectively, did not differ significantly (P=0.29). The mean twitch:tetanus ratios for the outer and inner preparations, 0.60 ± 0.085 and 0.58 ± 0.10, respectively, did not differ significantly (P=0.67). The circular fibers did not exhibit length-dependent changes in contraction kinetics when given a twitch stimulus. As the stimulation frequency increased, L₀ was approximately 1.06 times longer than LTW, the mean preparation length that yielded maximum isometric twitch force. Sonomicrometry experiments revealed that the CMP circular muscle fibers operated in vivo primarily along the ascending limb of the length-tension curve. The CMP fibers functioned routinely over muscle lengths at which force output ranged from only 85% to 40% of P₀, and during escape jets from 100% to 30% of P₀. Our work shows that the functional diversity of obliquely striated muscles is much greater than previously recognized.

Models of ventricular structure and function reviewed for clinical cardiologists

The architectural arrangement of cardiomyocytes aggregated together within the ventricular walls remains controversial. Two models currently attract clinical attention, with neither model standing rigorous anatomical scrutiny. The first is based on the notion that ventricular mass can be unraveled consistently to produce a unique myocardial band. The second model was initially based on the notion that cardiomyocytes were bundled together in uniform fashion, with fibrous shelves interposed in transmural fashion. This concept was subsequently modified to accept the fact that the fibrous matrix supporting the cardiomyocytes within the ventricular walls does not form transmural sheets. Current observations demonstrate that not all cardiomyocytes are aggregated together in tangential fashion. A significant netting component is aligned in obliquely intruding and transversal fashion. The interaction between the tangential and transversal chains of cardiomyocytes with the fibrous matrix produces antagonistic forces, with both unloading and auxotonic forces necessary to explain normal and abnormal cardiodynamics. This article is part of a JCTR special issue on Cardiac Anatomy.

Three-dimensional alignment of the aggregated myocytes in the normal and hypertrophic murine heart

Several observations suggest that the transmission of myocardial forces is influenced in part by the spatial arrangement of the myocytes aggregated together within ventricular mass. Our aim was to assess, using diffusion tensor magnetic resonance imaging (DT-MRI), any differences in the three-dimensional arrangement of these myocytes in the normal heart compared with the hypertrophic murine myocardium. We induced ventricular hypertrophy in seven mice by infusion of angiotensin II through a subcutaneous pump, with seven other mice serving as controls. DT-MRI of explanted hearts was performed at 3.0 Tesla. We used the primary eigenvector in each voxel to determine the three-dimensional orientation of aggregated myocytes in respect to their helical angles and their transmural courses (intruding angles). Compared with controls, the hypertrophic hearts showed significant increases in myocardial mass and the outer radius of the left ventricular chamber ( P < 0.05). In both groups, a significant change was noted from positive intruding angles at the base to negative angles at the ventricular apex ( P < 0.01). Compared with controls, the hypertrophied hearts had significantly larger intruding angles of the aggregated myocytes, notably in the apical and basal slices ( P < 0.001). In both groups, the helical angles were greatest in midventricular sections, albeit with significantly smaller angles in the mice with hypertrophied myocardium ( P < 0.01). The use of DT-MRI revealed significant differences in helix and intruding angles of the myocytes in the mice with hypertrophied myocardium.

Models versus established knowledge in describing the functional morphology of the ventricular myocardium

The myocytes comprising the ventricular mass are arranged so as to function in antagonistic fashion, the walls having the capacity to generate both constrictive and dilatory forces. This dualistic activity is organized on the basis of a site-specific morphologic pattern, permitting marked regional specificity for mural motion and providing a target for regional therapy. Diseased regions can be removed surgically without danger of jeopardizing the remaining healthy mural segments. The sensitivity of the intruding population of myocytes to positive and negative inotropic medication is markedly more pronounced than that of the prevailing tangentially aligned myocytes. This asymmetrical action of inotropes in the setting of global ventricular imbalance promotes the potential to restore constrictive as opposed to dilatory actions.

Statistical analysis of the angle of intrusion of porcine ventricular myocytes from epicardium to endocardium using diffusion tensor magnetic resonance imaging

Pairs of cylindrical knives were used to punch semicircular slices from the left basal, sub-basal, equatorial, and apical ventricular wall of porcine hearts. The sections extended from the epicardium to the endocardium. Their semicircular shape compensated for the depth-related changing orientation of the myocytes relative to the equatorial plane. The slices were analyzed by diffusion tensor magnetic resonance imaging. The primary eigenvector of the diffusion tensor was determined in each pixel to calculate the number and angle of intrusion of the long axis of the aggregated myocytes relative to the epicardial surface. Arrays of axially sectioned aggregates were found in which 53% of the approximately two million segments evaluated intruded up to +/-15 degrees , 40% exhibited an angle of intrusion between +/-15 degrees and +/-45 degrees , and 7% exceeded an angle of +/-45 degrees , the positive sign thereby denoting an epi- to endocardial spiral in clockwise direction seen from the apex, while a negative sign denotes an anticlockwise spiral from the epicardium to the endocardium. In the basal and apical slices, the greater number of segments intruded in positive direction, while in the sub-basal and equatorial slices, negative angles of intrusion prevailed. The sampling of the primary eigenvectors was insensitive to postmortem decomposition of the tissue. In a previous histological study, we also documented the presence of large numbers of myocytes aggregated with their long axis intruding obliquely from the epicardial to the endocardial ventricular surfaces. We used magnetic resonance diffusion tensor imaging in this study to provide a comprehensive statistical analysis.

Diastolic ventricular aspiration: a mechanism supporting the rapid filling phase of the human ventricles

During the rapid filling phase of the heart cycle, the internal volumes of the two ventricular cavities approximately double, while the intraventricular pressures rise typically only by an amount of less than 1 kPa. Such a small pressure increase cannot be the sole driving mechanism for the large inflow of blood associated with ventricular expansion during this period. Instead, the rapid filling phase is to be interpreted as being mediated primarily by the heart recoiling elastically from its contracted state, causing blood to be aspirated rapidly into the ventricles. In order to study the role of this mechanism, elastic finite element (FE) simulations of ventricular expansion were performed, taking into account the large deformations occurring during this period and the effective compressibility of the myocardium due to intramural fluid flow. Thereby, a realistic three-dimensional geometry derived from magnetic resonance imaging (MRI) measurements of both human ventricles was used. To validate our FE analyses, the results were compared with published measurements relating to the rapid filling phase of the human left ventricle. Our study shows that, under normal physiological conditions, ventricular aspiration plays a key role in the ventricular filling process.

Beta-blockade at low doses restoring the physiological balance in myocytic antagonism

OBJECTIVE

The ventricular mass is organized in the form of meshwork, with populations of myocytes aggregated in a supporting matrix of fibrous tissue, with some myocytes aligned obliquely across the wall so as to work in an antagonistic fashion compared to the majority of myocytes, which are aggregated together in tangential alignment. Prompted by results from animal experiments, which showed a disparate response of the two populations of aggregated myocytes to negative inotropic medication, we sought to establish whether those myocytes that aggregated so as to extend obliquely across the thickness of the ventricular walls are more sensitive to beta-blockade than the prevailing population in which the myocytes are aggregated together with tangential alignment. If the two populations respond in similar differing fashion in the clinical situation, we hypothesize that this might help to explain why drugs blocking the beta-receptors improve function of the ventricular pump in the setting of congestive cardiac failure.

METHODS

We implanted needle probes in 13 patients studied during open heart surgery, measuring the forces generated in the ventricular wall and seeking to couple the probes either to myocytes aggregated together with tangential alignment or to those aggregated in oblique fashion across the ventricular walls. In a first series of patients, we injected probatory doses intravenously, amounting to a total bolus of 40-100mg Esmolol, while in a second series, we gave fixed yet rising doses of 5, 10, and 20mg Esmolol in three separate boluses.

RESULTS

Forces recorded in the aggregated myocytes with tangential alignment decreased insignificantly upon administration of low doses (57.1+/-12.4 mN-->56.6+/-7.6 mN), while forces recorded in the myocytes aggregated obliquely across the ventricular wall showed a significant decrease in the mean (59.3+/-11.6 mN-->47.4+/-6.4 mN).

CONCLUSIONS

The markedly disparate action of drugs blocking beta-receptors at low dosage seems to be related to the heterogeneous extent, and time course, of systolic loading of the myocytes. This, in turn, depends on whether the myocytes themselves are aggregated together with tangential or oblique alignments relative to the thickness of the ventricular walls.

How are the myocytes aggregated so as to make up the ventricular mass?

Of late, it has become fashionable in the surgical literature to describe the ventricular mass as though arranged in the form of a continuous myocardial band, which starts at the aorta and ends at the pulmonary trunk. On the basis of this concept, its supporters have produced revisionist accounts of cardiac development and ventricular function, as well as using it as the basis for proposed surgical maneuvers. They seem unaware, however, that the original concept itself has never been supported by independent anatomic studies, while, to the best of our knowledge, they have not themselves performed anatomic investigations to prove its substance. Furthermore, the current proponents of the "unique myocardial band" ignore a large body of previous anatomic study which showed that the ventricular mass is arranged in the form of a modified blood vessel, with each myocyte anchored to its neighbor within a 3-dimensional myocardial mesh, rather than being arranged in a fashion analogous to skeletal muscles, with discrete origins and insertions of myocardial bands or tracts. In this review, we summarize the evidence showing that there are no anatomic structures within the ventricular myocardium that permit it to be unraveled in systematic fashion so as to produce the purported myocardial band. We also re-visit our own previous investigations, which supported the conventional approach, namely that the myocytes are aggregated together within a supporting fibrous matrix in the form of a 3-dimensional meshwork.

Cross-Talk Between Cardiac Muscle and Coronary Vasculature

The cardiac muscle and the coronary vasculature are in close proximity to each other, and a two-way interaction, called cross-talk, exists. Here we focus on the mechanical aspects of cross-talk including the role of the extracellular matrix. Cardiac muscle affects the coronary vasculature. In diastole, the effect of the cardiac muscle on the coronary vasculature depends on the (changes in) muscle length but appears to be small. In systole, coronary artery inflow is impeded, or even reversed, and venous outflow is augmented. These systolic effects are explained by two mechanisms. The waterfall model and the intramyocardial pump model are based on an intramyocardial pressure, assumed to be proportional to ventricular pressure. They explain the global effects of contraction on coronary flow and the effects of contraction in the layers of the heart wall. The varying elastance model, the muscle shortening and thickening model, and the vascular deformation model are based on direct contact between muscles and vessels. They predict global effects as well as differences on flow in layers and flow heterogeneity due to contraction. The relative contributions of these two mechanisms depend on the wall layer (epi- or endocardial) and type of contraction (isovolumic or shortening). Intramyocardial pressure results from (local) muscle contraction and to what extent the interstitial cavity contracts isovolumically. This explains why small arterioles and venules do not collapse in systole. Coronary vasculature affects the cardiac muscle. In diastole, at physiological ventricular volumes, an increase in coronary perfusion pressure increases ventricular stiffness, but the effect is small. In systole, there are two mechanisms by which coronary perfusion affects cardiac contractility. Increased perfusion pressure increases microvascular volume, thereby opening stretch-activated ion channels, resulting in an increased intracellular Ca2+transient, which is followed by an increase in Ca2+sensitivity and higher muscle contractility (Gregg effect). Thickening of the shortening cardiac muscle takes place at the expense of the vascular volume, which causes build-up of intracellular pressure. The intracellular pressure counteracts the tension generated by the contractile apparatus, leading to lower net force. Therefore, cardiac muscle contraction is augmented when vascular emptying is facilitated. During autoregulation, the microvasculature is protected against volume changes, and the Gregg effect is negligible. However, the effect is present in the right ventricle, as well as in pathological conditions with ineffective autoregulation. The beneficial effect of vascular emptying may be reduced in the presence of a stenosis. Thus cardiac contraction affects vascular diameters thereby reducing coronary inflow and enhancing venous outflow. Emptying of the vasculature, however, enhances muscle contraction. The extracellular matrix exerts its effect mainly on cardiac properties rather than on the cross-talk between cardiac muscle and coronary circulation.

A finite element model of the human left ventricular systole

Local wall stress is the pivotal determinant of the heart muscle's systolic function. Under in vivo conditions, however, such stresses cannot be measured systematically and quantitatively. In contrast, imaging techniques based on magnetic resonance (MR) allow the determination of the deformation pattern of the left ventricle (LV) in vivo with high accuracy. The question arises to what extent deformation measurements are significant and might provide a possibility for future diagnostic purposes. The contractile forces cause deformation of LV myocardial tissue in terms of wall thickening, longitudinal shortening, twisting rotation and radial constriction. The myocardium is thereby understood to act as a densely interlaced mesh. Yet, whole cycle image sequences display a distribution of wall strains as function of space and time heralding a significant amount of inhomogeneity even under healthy conditions. We made similar observations previously by direct measurement of local contractile activity. The major reasons for these inhomogeneities derive from regional deviations of the ventricular walls from an ideal spheroidal shape along with marked disparities in focal fibre orientation. In response to a lack of diagnostic tools able to measure wall stress in clinical routine, this communication is aimed at an analysis and functional interpretation of the deformation pattern of an exemplary human heart at end-systole. To this end, the finite element (FE) method was used to simulate the three-dimensional deformations of the left ventricular myocardium due to contractile fibre forces at end-systole. The anisotropy associated with the fibre structure of the myocardial tissue was included in the form of a fibre orientation vector field which was reconstructed from the measured fibre trajectories in a post mortem human heart. Contraction was modelled by an additive second Piola-Kirchhoff active stress tensor. As a first conclusion, it became evident that longitudinal fibre forces, cross-fibre forces and shear along with systolic fibre rearrangement have to be taken into account for a useful modelling of systolic deformation. Second, a realistic geometry and fibre architecture lead to typical and substantially inhomogeneous deformation patterns as they are recorded in real hearts. We therefore, expect that the measurement of systolic deformation might provide useful diagnostic information.

Three-dimensional architecture of the left ventricular myocardium

Concepts for ventricular function tend to assume that the majority of the myocardial cells are aligned with their long axes parallel to the epicardial ventricular surface. We aimed to validate the existence of aggregates of myocardial cells orientated with their long axis intruding obliquely between the ventricular epicardial and endocardial surfaces and to quantitate their amount and angulation. To compensate for the changing angle of the long axis of the myocytes relative to the equatorial plane of the ventricles with varying depths within the ventricular walls, the so-called helical angle, we used pairs of cylindrical knives of different diameters to punch semicircular slices from the left ventricular wall of pigs, the slices extending from the epicardium to the endocardium. The slices were pinned flat, fixed in formaldehyde, embedded in paraffin, sectioned, stained with azan or hematoxilin and eosin, and analyzed by a new semiautomatic procedure. We made use of new techniques in informatics to determine the number and angulation of the aggregates of myocardial cells cut in their long axis. The alignment of the myocytes cut longitudinally varied markedly between the epicardium and the endocardium. Populations of myocytes, arranged in strands, diverge by varying angles from the epicardial surface. When paired knives of decreasing diameter were used to cut the slices, the inclination of the diagonal created by the arrays increases, while the lengths of the array of cells cut axially decreases. The visualization of the size, shape, and alignment of the myocytic arrays at any side of the ventricular wall is determined by the radius of the knives used, the range of helical angles subtended by the alignment of the myocytes throughout the thickness of the wall, and their angulation relative to the epicardial surface. Far from the majority of the ventricular myocytes being aligned at angles more or less tangential to the epicardial lining, we found that three-fifths of the myocardial cells had their long axes diverging at angles between 7.5 and 37.5 degrees from an alignment parallel to the epicardium. This arrangement, with the individual myocytes supported by connective tissue, might control the cyclic rearrangement of the myocardial fibers. This could serve as an important control of both ventricular mural thickening and intracavitary shape.

A new floating sensor array to detect electric near fields of beating heart preparations

A new flexible sensor for in vitro experiments was developed to measure the surface potential, Phi, and its gradient, E (electric near field), at given sites of the heart. During depolarisation, E describes a vector loop from which direction and magnitude of local conduction velocity theta can be computed. Four recording silver electrodes (14 microm x 14 microm) separated by 50 microm, conducting leads, and solderable pads were patterned on a 50 microm thick polyimide film. The conductive structures, except the electrodes, were isolated with polyimide, and electrodes were chlorided. Spacer pillars mounted on the tip fulfil two functions: they keep the electrodes 70 microm from the tissue allowing non-contact recording of Phi and prevent lateral slipping. The low mass (9.1 mg) and flexibility (6.33 N/m) of the sensor let it easily follow the movement of the beating heart without notable displacement. We examined the electrodes on criteria like rms-noise of Phi, signal-to-noise ratio of Phi and E, maximum peak-slope recording dPhi/dt, and deviation of local activation time (LAT) from a common signal and obtained values of 24-28 microV, 46 and 41 dB, 497-561 V/s and no differences, respectively. With appropriate data acquisition (sampling rate 100 kHz, 24-bit), we were able to record Phi and to monitor E and theta on-line from beat-to-beat even at heart rates of 600 beats/min. Moreover, this technique can discriminate between uncoupled cardiac activations (as occur in fibrotic tissue) separated by less than 1 mm and 1 ms.

Heuristic problems in defining the three-dimensional arrangement of the ventricular myocytes

There is lack of consensus concerning the three-dimensional arrangement of the myocytes within the ventricular muscle masses. Bioengineers are seeking to model the structure of the heart. Although the success of such models depends on the accuracy of the anatomic evidence, most of them have been based on concepts that are far from anatomical reality, which ignore many significant previous accounts of anatomy presented over the past 400 years. During the 19th century, Pettigrew emphasized that the heart was built on the basis of a modified blood vessel rather than in the form of skeletal muscles. This fact was reemphasized by Lev and Simkins as well as Grant in the 20th century, but the caveats listed by these authors have been ignored by proponents of two current concepts, which state either that the myocardium is arranged in the form of a "unique myocardial band," or that the walls of the ventricles are sequestrated in uniform fashion by laminar sheets of fibrous tissue extending from epicardium to endocardium. These two concepts are themselves incompatible and are further at variance with the majority of anatomic studies, which have emphasized the regional heterogeneity to be found in the three-dimensional packing of the myocytes within a supporting matrix of fibrous tissue. We reemphasize the significance of this three-dimensional muscular mesh, showing how the presence of intruding aggregates of myocytes extending in oblique transmural fashion also contends against the notion that all myocytes are orientated with their long axes parallel to the epicardial and enodcardial surfaces.

A finite element study relating to the rapid filling phase of the human ventricles

During the rapid diastolic filling phase at rest, the ventricles of the human heart double approximately in volume. In order to investigate whether the ventricular filling pressures measured under physiological conditions can give rise to such an extensive augmentation in ventricular volumes, a finite element model of the human right and left ventricles has been developed, taking into account the nonlinear mechanical behavior and effective compressibility of the myocardial tissue. The results were compared with the filling phase of the human left ventricle as extrapolated from measurements documented in the literature. We arrived at the conclusion that the ventricular pressures measured during the rapid filling phase cannot be the sole cause of the rise of the observed ventricular volumes. We rather advocate the assumption that further dilating mechanisms might be part of ventricular activity thus heralding a multiple function of the ventricular muscle body. A further result indicates that under normal conditions the influence of the viscoelasticity of the tissue should not be disregarded in ventricular mechanics.

New aspects of the ventricular septum and its function: an echocardiographic study

OBJECTIVES

To examine whether the line dividing the septum into two layers is found consistently by conventional echocardiography and to evaluate functional differences in the right and left side of the septum in terms of wall thickening, strain rate, and strain imaging.

DESIGN

In a systematic study in 30 normal subjects, M mode and Doppler myocardial imaging data from the interventricular septum (IVS) were recorded. Velocity curves, regional strain rate, and strain profiles were obtained. Systolic deformation (wall thickening, radial and longitudinal strain rate, and strain) of both sides were assessed. Furthermore, three patients with one sided abnormalities were studied.

RESULTS

A bright echo consistently segmented the IVS into a left and right part. In this normal population radial deformation was different for the left and right side of the septum (mean (SD) wall thickening on the left, 49 (46)%, and on the right, 17 (38)%; strain rate on the left, 3.8 (0.6) 1/s, and on the right, 2.1 (1.9) 1/s; strain on the left, 41 (17)%, and on the right, 22 (14)%), whereas longitudinal deformation was found to be similar (strain rate on the left, -2.2 (0.7) 1/s, and on the right, -2.0 (0.6) 1/s; strain on the left, -28 (12)%, and on the right, -25 (12)%). The presented clinical examples show that abnormalities can be strictly limited to one layer.

CONCLUSIONS

Differential radial deformation and knowledge of fibre architecture showing an abrupt change in the middle of the septum, together with the clinical cases, suggest the septum to be a morphologically and functionally bilayered structure potentially supplied by different coronary arteries.