Left atrial ligation alters intracardiac flow patterns and the biomechanical landscape in the chick embryo

Recasting Current Knowledge of Human Fetal Circulation: The Importance of Computational Models

Computational hemodynamic simulations are becoming increasingly important for cardiovascular research and clinical practice, yet incorporating numerical simulations of human fetal circulation is relatively underutilized and underdeveloped. The fetus possesses unique vascular shunts to appropriately distribute oxygen and nutrients acquired from the placenta, adding complexity and adaptability to blood flow patterns within the fetal vascular network. Perturbations to fetal circulation compromise fetal growth and trigger the abnormal cardiovascular remodeling that underlies congenital heart defects. Computational modeling can be used to elucidate complex blood flow patterns in the fetal circulatory system for normal versus abnormal development. We present an overview of fetal cardiovascular physiology and its evolution from being investigated with invasive experiments and primitive imaging techniques to advanced imaging (4D MRI and ultrasound) and computational modeling. We introduce the theoretical backgrounds of both lumped-parameter networks and three-dimensional computational fluid dynamic simulations of the cardiovascular system. We subsequently summarize existing modeling studies of human fetal circulation along with their limitations and challenges. Finally, we highlight opportunities for improved fetal circulation models.

Flow-Mediated Factors in the Pathogenesis of Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is a life-threatening congenital heart disease that is characterized by severe underdevelopment of left heart structures. Currently, there is no cure, and affected individuals require surgical palliation or cardiac transplantation to survive. Despite these resource-intensive measures, only about half of individuals reach adulthood, often with significant comorbidities such as liver disease and neurodevelopmental disorders. A major barrier in developing effective treatments is that the etiology of HLHS is largely unknown. Here, we discuss how intracardiac blood flow disturbances are an important causal factor in the pathogenesis of impaired left heart growth. Specifically, we highlight results from a recently developed mouse model in which surgically reducing blood flow through the mitral valve after cardiogenesis led to the development of HLHS. In addition, we discuss the role of interventional procedures that are based on improving blood flow through the left heart, such as fetal aortic valvuloplasty. Lastly, using the surgically-induced mouse model, we suggest investigations that can be undertaken to identify the currently unknown biological pathways in left heart growth failure and their associated therapeutic targets.

Numerical Investigation of the Fetal Left Heart Hemodynamics During Gestational Stages

Flow-driven hemodynamic forces on the cardiac tissues have critical importance, and have a significant role in the proper development of the heart. These mechanobiological mechanisms govern the cellular responses for the growth and remodeling of the heart, where the altered hemodynamic environment is believed to be a major factor that is leading to congenital heart defects (CHDs). In order to investigate the mechanobiological development of the normal and diseased hearts, identification of the blood flow patterns and wall shear stresses (WSS) on these tissues are required for an accurate hemodynamic assessment. In this study, we focus on the left heart hemodynamics of the human fetuses throughout the gestational stages. Computational fetal left heart models are created for the healthy fetuses using the ultrasound images at various gestational weeks. Realistic inflow boundary conditions are implemented in the models using the Doppler ultrasound measurements for resolving the specific blood flow waveforms in the mitral valve. Obtained results indicate that WSS and vorticity levels in the fetal left heart decrease with the development of the fetus. The maximum WSS around the mitral valve is determined around 36 Pa at the gestational week of 16. This maximum WSS decreases to 11 Pa at the gestational week of 27, indicating nearly three-times reduction in the peak shear stress. These findings reveal the highly dynamic nature of the left heart hemodynamics throughout the development of the human fetus and shed light into the relevance of hemodynamic environment and development of CHDs.

Fluid mechanics of the left atrial ligation chick embryonic model of hypoplastic left heart syndrome

Left atrial ligation (LAL) of the chick embryonic heart at HH21 is a model of the hypoplastic left heart syndrome (HLHS) disease, demonstrating morphological and hemodynamic features similar to human HLHS cases. Since it relies on mechanical intervention without genetic or pharmacological manipulations, it is a good model for understanding the biomechanics origins of such HLHS malformations. To date, however, the fluid mechanical environment of this model is poorly understood. In the current study, we performed 4D ultrasound imaging of LAL and normal chick embryonic hearts and 4D cardiac flow simulations to help shed light on the mechanical environment that may lead to the HLHS morphology. Results showed that the HH25 LAL atrial function was compromised, and velocities in the ventricle were reduced. The HH25 LAL ventricles developed a more triangular shape with a sharper apex, and in some cases, the atrioventricular junction shifted medially. These changes led to more sluggish flow near the ventricular free wall and apex, where more fluid particles moved in an oscillatory manner with the motion of the ventricular wall, while slowly being washed out, resulting in lower wall shear stresses and higher oscillatory indices. Consequent to these flow conditions, at HH28, even before septation is complete, the left ventricle was found to be hypoplastic while the right ventricle was found to be larger in compensation. Our results suggest that the low and oscillatory flow near the left side of the heart may play a role in causing the HLHS morphology in the LAL model.

Mending a broken heart: In vitro, in vivo and in silico models of congenital heart disease

Birth defects contribute to ∼0.3% of global infant mortality in the first month of life, and congenital heart disease (CHD) is the most common birth defect among newborns worldwide. Despite the significant impact on human health, most treatments available for this heterogenous group of disorders are palliative at best. For this reason, the complex process of cardiogenesis, governed by multiple interlinked and dose-dependent pathways, is well investigated. Tissue, animal and, more recently, computerized models of the developing heart have facilitated important discoveries that are helping us to understand the genetic, epigenetic and mechanobiological contributors to CHD aetiology. In this Review, we discuss the strengths and limitations of different models of normal and abnormal cardiogenesis, ranging from single-cell systems and 3D cardiac organoids, to small and large animals and organ-level computational models. These investigative tools have revealed a diversity of pathogenic mechanisms that contribute to CHD, including genetic pathways, epigenetic regulators and shear wall stresses, paving the way for new strategies for screening and non-surgical treatment of CHD. As we discuss in this Review, one of the most-valuable advances in recent years has been the creation of highly personalized platforms with which to study individual diseases in clinically relevant settings.

Mechanosensitive Pathways in Heart Development: Findings from Chick Embryo Studies

The heart is the first organ that starts to function in a developing embryo. It continues to undergo dramatic morphological changes while pumping blood to the rest of the body. Genetic regulation of heart development is partly governed by hemodynamics. Chick embryo is a major animal model that has been used extensively in cardiogenesis research. To reveal mechanosensitive pathways, a variety of surgical interferences and chemical treatments can be applied to the chick embryo to manipulate the blood flow. Such manipulations alter expressions of mechanosensitive genes which may anticipate induction of morphological changes in the developing heart. This paper aims to present different approaches for generating clinically relevant disturbed hemodynamics conditions using this embryonic chick model and to summarize identified mechanosensitive genes using the model, providing insights into embryonic origins of congenital heart defects.

Computational Modeling of Blood Flow Hemodynamics for Biomechanical Investigation of Cardiac Development and Disease

The heart is the first functional organ in a developing embryo. Cardiac development continues throughout developmental stages while the heart goes through a serious of drastic morphological changes. Previous animal experiments as well as clinical observations showed that disturbed hemodynamics interfere with the development of the heart and leads to the formation of a variety of defects in heart valves, heart chambers, and blood vessels, suggesting that hemodynamics is a governing factor for cardiogenesis, and disturbed hemodynamics is an important source of congenital heart defects. Therefore, there is an interest to image and quantify the flowing blood through a developing heart. Flow measurement in embryonic fetal heart can be performed using advanced techniques such as magnetic resonance imaging (MRI) or echocardiography. Computational fluid dynamics (CFD) modeling is another approach especially useful when the other imaging modalities are not available and in-depth flow assessment is needed. The approach is based on numerically solving relevant physical equations to approximate the flow hemodynamics and tissue behavior. This approach is becoming widely adapted to simulate cardiac flows during the embryonic development. While there are few studies for human fetal cardiac flows, many groups used zebrafish and chicken embryos as useful models for elucidating normal and diseased cardiogenesis. In this paper, we explain the major steps to generate CFD models for simulating cardiac hemodynamics in vivo and summarize the latest findings on chicken and zebrafish embryos as well as human fetal hearts.

Effect of left atrial ligation-driven altered inflow hemodynamics on embryonic heart development: clues for prenatal progression of hypoplastic left heart syndrome

Congenital heart defects (CHDs) are abnormalities in the heart structure present at birth. One important condition is hypoplastic left heart syndrome (HLHS) where severely underdeveloped left ventricle (LV) cannot support systemic circulation. HLHS usually initiates as localized tissue malformations with no underlying genetic cause, suggesting that disturbed hemodynamics contribute to the embryonic development of these defects. Left atrial ligation (LAL) is a surgical procedure on embryonic chick resulting in a phenotype resembling clinical HLHS. In this study, we investigated disturbed hemodynamics and deteriorated cardiac growth following LAL to investigate possible mechanobiological mechanisms for the embryonic development of HLHS. We integrated techniques such as echocardiography, micro-CT and computational fluid dynamics (CFD) for these analyses. Specifically, LAL procedure causes an immediate flow disturbance over atrioventricular (AV) cushions. At later stages after the heart septation, it causes hemodynamic disturbances in LV. As a consequence of the LAL procedure, the left-AV canal and LV volume decrease in size, and in the opposite way, the right-AV canal and right ventricle volume increase. According to our CFD analysis, LAL results in an immediate decrease in the left AV canal WSS levels for 3.5-day (HH21) pre-septated hearts. For 7-day post-septated hearts (HH30), LAL leads to further reduction in WSS levels in the left AV canal, and relatively increased WSS levels in the right AV canal. This study demonstrates the critical importance of the disturbed hemodynamics during the heart valve and ventricle development.

Angiogenesis in the Avian Embryo Chorioallantoic Membrane: A Perspective on Research Trends and a Case Study on Toxicant Vascular Effects

The chorioallantoic membrane (CAM) of the avian embryo is an intrinsically interesting gas exchange and osmoregulation organ. Beyond study by comparative biologists, however, the CAM vascular bed has been the focus of translational studies by cardiovascular life scientists interested in the CAM as a model for probing angiogenesis, heart development, and physiological functions. In this perspective article, we consider areas of cardiovascular research that have benefited from studies of the CAM, including the themes of investigation of the CAM's hemodynamic influence on heart and central vessel development, use of the CAM as a model vascular bed for studying angiogenesis, and the CAM as an assay tool. A case study on CAM vascularization effects of very low doses of crude oil as a toxicant is also presented that embraces some of these themes, showing the induction of subtle changes in the pattern of the CAM vasculature growth that are not readily observed by standard vascular assessment methodologies. We conclude by raising several questions in the area of CAM research, including the following: (1) Do changes in patterns of CAM growth, as opposed to absolute CAM growth, have biological significance?; (2) How does the relative amount of CAM vascularization compared to the embryo per se change during development?; and (3) Is the CAM actually representative of the mammalian systemic vascular beds that it is presumed to model?

Validating the Paradigm That Biomechanical Forces Regulate Embryonic Cardiovascular Morphogenesis and Are Fundamental in the Etiology of Congenital Heart Disease

The goal of this review is to provide a broad overview of the biomechanical maturation and regulation of vertebrate cardiovascular (CV) morphogenesis and the evidence for mechanistic relationships between function and form relevant to the origins of congenital heart disease (CHD). The embryonic heart has been investigated for over a century, initially focusing on the chick embryo due to the opportunity to isolate and investigate myocardial electromechanical maturation, the ability to directly instrument and measure normal cardiac function, intervene to alter ventricular loading conditions, and then investigate changes in functional and structural maturation to deduce mechanism. The paradigm of "Develop and validate quantitative techniques, describe normal, perturb the system, describe abnormal, then deduce mechanisms" was taught to many young investigators by Dr. Edward B. Clark and then validated by a rapidly expanding number of teams dedicated to investigate CV morphogenesis, structure-function relationships, and pathogenic mechanisms of CHD. Pioneering studies using the chick embryo model rapidly expanded into a broad range of model systems, particularly the mouse and zebrafish, to investigate the interdependent genetic and biomechanical regulation of CV morphogenesis. Several central morphogenic themes have emerged. First, CV morphogenesis is inherently dependent upon the biomechanical forces that influence cell and tissue growth and remodeling. Second, embryonic CV systems dynamically adapt to changes in biomechanical loading conditions similar to mature systems. Third, biomechanical loading conditions dynamically impact and are regulated by genetic morphogenic systems. Fourth, advanced imaging techniques coupled with computational modeling provide novel insights to validate regulatory mechanisms. Finally, insights regarding the genetic and biomechanical regulation of CV morphogenesis and adaptation are relevant to current regenerative strategies for patients with CHD.

Microstructure of early embryonic aortic arch and its reversibility following mechanically altered hemodynamic load release

In the embryonic heart, blood flow is distributed through a bilaterally paired artery system composed of the aortic arches (AAs). The purpose of this study is to establish an understanding of the governing mechanism of microstructural maturation of the AA matrix and its reversibility, toward the desired macroscopic vessel lumen diameter and thickness for healthy, abnormal, and in ovo repaired abnormal mechanical loading. While matrix-remodeling mechanisms were significantly different for normal versus conotruncal banding (CTB), both led to an increase in vessel lumen. Correlated with right-sided flow increase at Hamburger & Hamilton stages 21, intermittent load switching between collagen I and III with elastin and collagen-IV defines the normal process. However, decreases in collagen I, elastin, vascular endothelial growth factor-A, and fibrillin-1 in CTB were recovered almost fully following the CTB-release model, primarily because of the pressure load changes. The complex temporal changes in matrix proteins are illustrated through a predictive finite-element model based on elastin and collagen load-sharing mechanism to achieve lumen area increase and thickness increase resulting from wall shear stress and tissue strain, respectively. The effect of embryonic timing in cardiac interventions on AA microstructure was established where abnormal mechanical loading was selectively restored at the key stage of development. Recovery of the normal mechanical loading via early fetal intervention resulted in delayed microstructural maturation. Temporal elastin increase, correlated with wall shear stress, is required for continuous lumen area growth.NEW & NOTEWORTHY The present study undertakes comparative analyses of the mechanistic differences of the arterial matrix microstructure and dynamics in the three fundamental processes of control, conotruncal banded, and released conotruncal band in avian embryo. Among other findings, this study provides specific evidence on the restorative role of elastin during the early lumen growth process. During vascular development, a novel intermittent load-switching mechanism between elastin and collagen, triggered by a step increase in wall shear stress, governs the chronic vessel lumen cross-sectional area increase. Mimicking the fetal cardiovascular interventions currently performed in humans, the early release of the abnormal mechanical load rescues the arterial microstructure with time lag.

Engineering Heart Morphogenesis

Recent advances in stem cell biology and tissue engineering have laid the groundwork for building complex tissues in a dish. We propose that these technologies are ready for a new challenge: recapitulating cardiac morphogenesis in vitro. In development, the heart transforms from a simple linear tube to a four-chambered organ through a complex process called looping. Here, we re-examine heart tube looping through the lens of an engineer and argue that the linear heart tube is an advantageous starting point for tissue engineering. We summarize the structures, signaling pathways, and stresses in the looping heart, and evaluate approaches that could be used to build a linear heart tube and guide it through the process of looping.

Organ Dynamics and Hemodynamic of the Whole HH25 Avian Embryonic Heart, Revealed by Ultrasound Biomicroscopy, Boundary Tracking, and Flow Simulations

Congenital heart malformations occur to substantial number of pregnancies. Studies showed that abnormal flow biomechanical environments could lead to malformations, making it important to understand the biomechanical environment of the developing heart. We performed 4D high-frequency ultrasound scans of chick embryonic hearts at HH25 to study the biomechanics of the whole heart (atria and ventricle). A novel and high-fidelity motion estimation technique, based on temporal motion model and non-rigid image registration algorithm, allowed automatic tracking of fluid-structure boundaries from scan images, and supported flow simulations. Results demonstrated that atrial appendages were the most contractile portion of the atria, having disproportionately high contribution to atrial blood pumping for its volume in the atria. However, the atria played a small role in blood pumping compared to the ventricle, as it had much lower ejection energy expenditure, and as the ventricle appeared to be able to draw inflow from the veins directly during late diastole. Spatially and temporally averaged wall shear stresses (WSS) for various cardiac structures were 0.062–0.068 Pa, but spatial-averaged WSS could be as high as 0.54 Pa in the RV. WSS was especially elevated at the atrial inlet, atrioventricular junction, regions near to the outflow tract, and at dividing lines between the left and right atrium and left and right side of the ventricle, where septation had begun and the lumen had narrowed. Elevated WSS could serve as biomechanics stimulation for proper growth and development.

Fluid dynamics and forces in the HH25 avian embryonic outflow tract

The embryonic outflow tract (OFT) eventually undergoes aorticopulmonary septation to form the aorta and pulmonary artery, and it is hypothesized that blood flow mechanical forces guide this process. We performed detailed studies of the geometry, wall motions, and fluid dynamics of the HH25 chick embryonic OFT just before septation, using noninvasive 4D high-frequency ultrasound and computational flow simulations. The OFT exhibited expansion and contraction waves propagating from proximal to distal end, with periods of luminal collapse at locations of the two endocardial cushions. This, combined with periods of reversed flow, resulted in the OFT cushions experiencing wall shear stresses (WSS or flow drag forces) with elevated oscillatory characteristics, which could be important to signal for further development of cushions into valves and septum. Furthermore, the OFT exhibits interesting double-helical flow during systole, where a pair of helical flow structures twisted about each other from the proximal to distal end. This coincided with the location of the future aorticopulmonary septum, which also twisted from the proximal to distal end, suggesting that this flow pattern may be guiding OFT septation.

Hemodynamics in Cardiac Development

The beating heart is subject to intrinsic mechanical factors, exerted by contraction of the myocardium (stretch and strain) and fluid forces of the enclosed blood (wall shear stress). The earliest contractions of the heart occur already in the 10-somite stage in the tubular as yet unsegmented heart. With development, the looping heart becomes asymmetric providing varying diameters and curvatures resulting in unequal flow profiles. These flow profiles exert various wall shear stresses and as a consequence different expression patterns of shear responsive genes. In this paper we investigate the morphological alterations of the heart after changing the blood flow by ligation of the right vitelline vein in a model chicken embryo and analyze the extended expression in the endocardial cushions of the shear responsive gene Tgfbeta receptor III. A major phenomenon is the diminished endocardial-mesenchymal transition resulting in hypoplastic (even absence of) atrioventricular and outflow tract endocardial cushions, which might be lethal in early phases. The surviving embryos exhibit several cardiac malformations including ventricular septal defects and malformed semilunar valves related to abnormal development of the aortopulmonary septal complex and the enclosed neural crest cells. We discuss the results in the light of the interactions between several shear stress responsive signaling pathways including an extended review of the involved Vegf, Notch, Pdgf, Klf2, eNos, Endothelin and Tgfβ/Bmp/Smad networks.

The effects of reduced hemodynamic loading on morphogenesis of the mouse embryonic heart

Development of the embryonic heart involves an intricate network of biochemical and genetic cues to ensure its proper growth and morphogenesis. However, studies from avian and teleost models reveal that biomechanical force, namely hemodynamic loading (blood pressure and shear stress), plays a significant role in regulating heart development. To study how hemodynamic loading impacts development of the mammalian embryonic heart, we utilized mouse embryo culture and manipulation techniques and performed optical projection tomography imaging followed by morphometric analysis to determine how reduced-loading affects heart volume, myocardial thickness, trabeculation and looping. Our results reveal that hemodynamic loading can regulate these features at different thresholds. Intermediate levels of hemodynamic loading are sufficient to promote proper myocardial growth and heart size, but insufficient to promote looping and trabeculation. Whereas, low levels of hemodynamic loading fails to promote proper growth of the myocardium and heart size. These results reveal that the regulation of heart development by biomechanical force is conserved across many vertebrate classes, and this study begins to elucidate how these specific forces regulate development of the mammalian heart.

Histone deacetylase adaptation in single ventricle heart disease and a young animal model of right ventricular hypertrophy

BackgroundHistone deacetylase (HDAC) inhibitors are promising therapeutics for various forms of cardiac diseases. The purpose of this study was to assess cardiac HDAC catalytic activity and expression in children with single ventricle (SV) heart disease of right ventricular morphology, as well as in a rodent model of right ventricular hypertrophy (RVH).MethodsHomogenates of right ventricle (RV) explants from non-failing controls and children born with a SV were assayed for HDAC catalytic activity and HDAC isoform expression. Postnatal 1-day-old rat pups were placed in hypoxic conditions, and echocardiographic analysis, gene expression, HDAC catalytic activity, and isoform expression studies of the RV were performed.ResultsClass I, IIa, and IIb HDAC catalytic activity and protein expression were elevated in the hearts of children born with a SV. Hypoxic neonatal rats demonstrated RVH, abnormal gene expression, elevated class I and class IIb HDAC catalytic activity, and protein expression in the RV compared with those in the control.ConclusionsThese data suggest that myocardial HDAC adaptations occur in the SV heart and could represent a novel therapeutic target. Although further characterization of the hypoxic neonatal rat is needed, this animal model may be suitable for preclinical investigations of pediatric RV disease and could serve as a useful model for future mechanistic studies.

A method to quantify mechanobiologic forces during zebrafish cardiac development using 4-D light sheet imaging and computational modeling

Blood flow and mechanical forces in the ventricle are implicated in cardiac development and trabeculation. However, the mechanisms of mechanotransduction remain elusive. This is due in part to the challenges associated with accurately quantifying mechanical forces in the developing heart. We present a novel computational framework to simulate cardiac hemodynamics in developing zebrafish embryos by coupling 4-D light sheet imaging with a stabilized finite element flow solver, and extract time-dependent mechanical stimuli data. We employ deformable image registration methods to segment the motion of the ventricle from high resolution 4-D light sheet image data. This results in a robust and efficient workflow, as segmentation need only be performed at one cardiac phase, while wall position in the other cardiac phases is found by image registration. Ventricular hemodynamics are then quantified by numerically solving the Navier-Stokes equations in the moving wall domain with our validated flow solver. We demonstrate the applicability of the workflow in wild type zebrafish and three treated fish types that disrupt trabeculation: (a) chemical treatment using AG1478, an ErbB2 signaling inhibitor that inhibits proliferation and differentiation of cardiac trabeculation; (b) injection of gata1a morpholino oligomer (gata1aMO) suppressing hematopoiesis and resulting in attenuated trabeculation; (c) weak-atriumm58 mutant (wea) with inhibited atrial contraction leading to a highly undeveloped ventricle and poor cardiac function. Our simulations reveal elevated wall shear stress (WSS) in wild type and AG1478 compared to gata1aMO and wea. High oscillatory shear index (OSI) in the grooves between trabeculae, compared to lower values on the ridges, in the wild type suggest oscillatory forces as a possible regulatory mechanism of cardiac trabeculation development. The framework has broad applicability for future cardiac developmental studies focused on quantitatively investigating the role of hemodynamic forces and mechanotransduction during morphogenesis.

A method to study the hemodynamics of chicken embryo's aortic arches using optical coherence tomography

Congenital cardiovascular defects are the leading cause of birth defect related death. It has been hypothesized that fluid mechanical forces of embryonic blood flow affect cardiovascular development and play a role in congenital malformations. Studies in small animal embryos can improve our understanding of congenital malformations and can lead to better treatment. We present a feasibility study in which high-resolution optical coherence tomography (OCT) and computational fluid dynamics (CFD) are combined to provide quantitative analysis of the embryonic flow mechanics and the associated anatomy in a small animal model.

Model of Human Fetal Growth in Hypoplastic Left Heart Syndrome: Reduced Ventricular Growth Due to Decreased Ventricular Filling and Altered Shape

INTRODUCTION

Hypoplastic left heart syndrome (HLHS) is a congenital condition with an underdeveloped left ventricle (LV) that provides inadequate systemic blood flow postnatally. The development of HLHS is postulated to be due to altered biomechanical stimuli during gestation. Predicting LV size at birth using mid-gestation fetal echocardiography is a clinical challenge critical to prognostic counseling.

HYPOTHESIS

We hypothesized that decreased ventricular filling in utero due to mitral stenosis may reduce LV growth in the fetal heart via mechanical growth signaling.

METHODS

We developed a novel finite element model of the human fetal heart in which cardiac myocyte growth rates are a function of fiber and cross-fiber strains, which is affected by altered ventricular filling, to simulate alterations in LV growth and remodeling. Model results were tested with echocardiogram measurements from normal and HLHS fetal hearts.

RESULTS

A strain-based fetal growth model with a normal 22-week ventricular filling (1.04 mL) was able to replicate published measurements of changes between mid-gestation to birth of mean LV end-diastolic volume (EDV) (1.1-8.3 mL) and dimensions (long-axis, 18-35 mm; short-axis, 9-18 mm) within 15% root mean squared deviation error. By decreasing volumetric load (-25%) at mid-gestation in the model, which emulates mitral stenosis in utero, a 65% reduction in LV EDV and a 46% reduction in LV wall volume were predicted at birth, similar to observations in HLHS patients. In retrospective blinded case studies for HLHS, using mid-gestation echocardiographic data, the model predicted a borderline and severe hypoplastic LV, consistent with the patients' late-gestation data in both cases. Notably, the model prediction was validated by testing for changes in LV shape in the model against clinical data for each HLHS case study.

CONCLUSION

Reduced ventricular filling and altered shape may lead to reduced LV growth and a hypoplastic phenotype by reducing myocardial strains that serve as a myocyte growth stimulus. The human fetal growth model presented here may lead to a clinical tool that can help predict LV size and shape at birth based on mid-gestation LV echocardiographic measurements.

Pulling on my heartstrings: mechanotransduction in cardiac development and function

PURPOSE OF REVIEW

Endothelial cells line the surface of the cardiovascular system and display a large degree of heterogeneity due to developmental origin and location. Despite this heterogeneity, all endothelial cells are exposed to wall shear stress (WSS) imparted by the frictional force of flowing blood, which plays an important role in determining the endothelial cell phenotype. Although the effects of WSS have been greatly studied in vascular endothelial cells, less is known about the role of WSS in regulating cardiac function and cardiac endothelial cells.

RECENT FINDINGS

Recent advances in genetic and imaging technologies have enabled a more thorough investigation of cardiac hemodynamics. Using developmental models, shear stress sensing by endocardial endothelial cells has been shown to play an integral role in proper cardiac development including morphogenesis and formation of the conduction system. In the adult, less is known about hemodynamics and endocardial endothelial cells, but a clear role for WSS in the development of coronary and valvular disease is increasingly appreciated.

SUMMARY

Future research will further elucidate a role for WSS in the developing and adult heart, and understanding this dynamic relationship may represent a potential therapeutic target for the treatment of cardiomyopathies.

4D subject-specific inverse modeling of the chick embryonic heart outflow tract hemodynamics

Blood flow plays a critical role in regulating embryonic cardiac growth and development, with altered flow leading to congenital heart disease. Progress in the field, however, is hindered by a lack of quantification of hemodynamic conditions in the developing heart. In this study, we present a methodology to quantify blood flow dynamics in the embryonic heart using subject-specific computational fluid dynamics (CFD) models. While the methodology is general, we focused on a model of the chick embryonic heart outflow tract (OFT), which distally connects the heart to the arterial system, and is the region of origin of many congenital cardiac defects. Using structural and Doppler velocity data collected from optical coherence tomography, we generated 4D ([Formula: see text]) embryo-specific CFD models of the heart OFT. To replicate the blood flow dynamics over time during the cardiac cycle, we developed an iterative inverse-method optimization algorithm, which determines the CFD model boundary conditions such that differences between computed velocities and measured velocities at one point within the OFT lumen are minimized. Results from our developed CFD model agree with previously measured hemodynamics in the OFT. Further, computed velocities and measured velocities differ by [Formula: see text]15 % at locations that were not used in the optimization, validating the model. The presented methodology can be used in quantifications of embryonic cardiac hemodynamics under normal and altered blood flow conditions, enabling an in-depth quantitative study of how blood flow influences cardiac development.

Noninvasive in vivo determination of residual strains and stresses

Vascular growth and remodeling during embryonic development are associated with blood flow and pressure induced stress distribution, in which residual strains and stresses play a central role. Residual strains are typically measured by performing in vitro tests on the excised vascular tissue. In this paper, we investigated the possibility of estimating residual strains and stresses using physiological pressure-radius data obtained through in vivo noninvasive measurement techniques, such as optical coherence tomography or ultrasound modalities. This analytical approach first tested with in vitro results using experimental data sets for three different arteries such as rabbit carotid artery, rabbit thoracic artery, and human carotid artery based on Fung's pseudostrain energy function and Delfino's exponential strain energy function (SEF). We also examined residual strains and stresses in the human swine iliac artery using the in vivo experimental ultrasound data sets corresponding to the systolic-to-diastolic region only. This allowed computation of the in vivo residual stress information for loading and unloading states separately. Residual strain parameters as well as the material parameters were successfully computed with high accuracy, where the relative errors are introduced in the range of 0-7.5%. Corresponding residual stress distributions demonstrated global errors all in acceptable ranges. A slight discrepancy was observed in the computed reduced axial force. Results of computations performed based on in vivo experimental data obtained from loading and unloading states of the artery exhibited alterations in material properties and residual strain parameters as well. Emerging noninvasive measurement techniques combined with the present analytical approach can be used to estimate residual strains and stresses in vascular tissues as a precursor for growth estimates. This approach is also validated with a finite element model of a general two-layered artery, where the material remodeling states and residual strain generation are investigated.

Calcium signaling regulates ventricular hypertrophy during development independent of contraction or blood flow

In utero interventions aimed at restoring left ventricular hemodynamic forces in fetuses with prenatally diagnosed hypoplastic left heart syndrome failed to stimulate ventricular myocardial growth during gestation, suggesting chamber growth during development may not rely upon fluid forces. We therefore hypothesized that ventricular hypertrophy during development may depend upon fundamental Ca(2+)-dependent growth pathways that function independent of hemodynamic forces. To test this hypothesis, zebrafish embryos were treated with inhibitors or activators of Ca(2+) signaling in the presence or absence of contraction during the period of chamber development. Abolishment of contractile function alone in the setting of preserved Ca(2+) signaling did not impair ventricular hypertrophy. In contrast, inhibition of L-type voltage-gated Ca(2+) influx abolished contraction and led to reduced ventricular hypertrophy, whereas increasing L-type voltage-gated Ca(2+) influx led to enhanced ventricular hypertrophy in either the presence or absence of contraction. Similarly, inhibition of the downstream Ca(2+)-sensitive phosphatase calcineurin, a known regulator of adult cardiac hypertrophy, led to reduced ventricular hypertrophy in the presence or absence of contraction, whereas hypertrophy was rescued in the absence of L-type voltage-gated Ca(2+) influx and contraction by expression of a constitutively active calcineurin. These data suggest that ventricular cardiomyocyte hypertrophy during chamber formation is dependent upon Ca(2+) signaling pathways that are unaffected by heart function or hemodynamic forces. Disruption of Ca(2+)-dependent hypertrophy during heart development may therefore represent one mechanism for impaired chamber formation that is not related to impaired blood flow.

Investigating developmental cardiovascular biomechanics and the origins of congenital heart defects

Innovative research on the interactions between biomechanical load and cardiovascular (CV) morphogenesis by multiple investigators over the past 3 decades, including the application of bioengineering approaches, has shown that the embryonic heart adapts both structure and function in order to maintain cardiac output to the rapidly growing embryo. Acute adaptive hemodynamic mechanisms in the embryo include the redistribution of blood flow within the heart, dynamic adjustments in heart rate and developed pressure, and beat to beat variations in blood flow and vascular resistance. These biomechanically relevant events occur coincident with adaptive changes in gene expression and trigger adaptive mechanisms that include alterations in myocardial cell growth and death, regional and global changes in myocardial architecture, and alterations in central vascular morphogenesis and remodeling. These adaptive mechanisms allow the embryo to survive these biomechanical stresses (environmental, maternal) and to compensate for developmental errors (genetic). Recent work from numerous laboratories shows that a subset of these adaptive mechanisms is present in every developing multicellular organism with a “heart” equivalent structure. This chapter will provide the reader with an overview of some of the approaches used to quantify embryonic CV functional maturation and performance, provide several illustrations of experimental interventions that explore the role of biomechanics in the regulation of CV morphogenesis including the role of computational modeling, and identify several critical areas for future investigation as available experimental models and methods expand.

Femtosecond laser photodisruption of vitelline vessels of avian embryos as a technique to study embryonic vascular remodeling

During cardiogenesis, congenital heart defects (CHDs), generally start as local tissue abnormalities without underlying genetic causes, suggesting abnormal hemodynamics may be an important source. Due to the scarcity of experimental techniques that permits the formation of minimally-invasive and well-controlled cardiac perturbations, experimental investigation of embryonic development of CHD via in-vivo models is difficult. In this study, in order to investigate the relationship between abnormal mechanical signaling and embryonic CHD development, a previously developed laser-based technique was adopted to alter chicken embryonic cardiovascular development. The technique incorporates two-photon fluorescence microscopy to visualize deep tissue while femtosecond-pulsed laser photodisruption is used to ablate targeted tissue. Vitelline vessel remodeling under abnormal hemodynamics was the prime concern of the study. In order to alter the hemodynamics, blood flowing inside 50–300 µm diameter Hamburger–Hamilton 24 embryonic vessels was selectively ablated. Red blood cells in the blood and endothelial cells of the vessel walls were damaged as a result of ablation. Cellular injuries led to micro-occlusions in the vessels. Several micro-occlusions formed stable clots, resulting in a complete cessation of blood flow in the targeted vessels. By measuring blood velocities in the surrounding vessels via line scanning technique, the subsequent redistribution of blood flow in the immediate upstream and downstream vessels was revealed. The network was analyzed after 24 h, and it was found to be degraded. Degradation of the entire network can be attributed to the abnormalities in hemodynamics within the vessels. For studying embryonic development of heart defects under disturbed flow conditions, the present study can be extended to clot a blood vessel inside the embryo or a vitelline vessel in the vicinity of the heart. These results demonstrate that, laser-based noninvasive tools should be considered as powerful techniques to analyze hemodynamic signals encountered in embryonic development of CHD.

Mechanical regulation of cardiac development

Mechanical forces are essential contributors to and unavoidable components of cardiac formation, both inducing and orchestrating local and global molecular and cellular changes. Experimental animal studies have contributed substantially to understanding the mechanobiology of heart development. More recent integration of high-resolution imaging modalities with computational modeling has greatly improved our quantitative understanding of hemodynamic flow in heart development. Merging these latest experimental technologies with molecular and genetic signaling analysis will accelerate our understanding of the relationships integrating mechanical and biological signaling for proper cardiac formation. These advances will likely be essential for clinically translatable guidance for targeted interventions to rescue malforming hearts and/or reconfigure malformed circulations for optimal performance. This review summarizes our current understanding on the levels of mechanical signaling in the heart and their roles in orchestrating cardiac development.

Mechanical regulation of cardiac development

Mechanical forces are essential contributors to and unavoidable components of cardiac formation, both inducing and orchestrating local and global molecular and cellular changes. Experimental animal studies have contributed substantially to understanding the mechanobiology of heart development. More recent integration of high-resolution imaging modalities with computational modeling has greatly improved our quantitative understanding of hemodynamic flow in heart development. Merging these latest experimental technologies with molecular and genetic signaling analysis will accelerate our understanding of the relationships integrating mechanical and biological signaling for proper cardiac formation. These advances will likely be essential for clinically translatable guidance for targeted interventions to rescue malforming hearts and/or reconfigure malformed circulations for optimal performance. This review summarizes our current understanding on the levels of mechanical signaling in the heart and their roles in orchestrating cardiac development.