Mechanics of the mitral annulus in chronic ischemic cardiomyopathy

Bayesian Optimization-Based Inverse Finite Element Analysis for Atrioventricular Heart Valves

Inverse finite element analysis (iFEA) of the atrioventricular heart valves (AHVs) can provide insights into the in-vivo valvular function, such as in-vivo tissue strains; however, there are several limitations in the current state-of-the-art that iFEA has not been widely employed to predict the in-vivo, patient-specific AHV leaflet mechanical responses. In this exploratory study, we propose the use of Bayesian optimization (BO) to study the AHV functional behaviors in-vivo. We analyzed the efficacy of Bayesian optimization to estimate the isotropic Lee-Sacks material coefficients in three benchmark problems: (i) an inflation test, (ii) a simplified leaflet contact model, and (iii) an idealized AHV model. Then, we applied the developed BO-iFEA framework to predict the leaflet properties for a patient-specific tricuspid valve under a congenital heart defect condition. We found that the BO could accurately construct the objective function surface compared to the one from a [Formula: see text] grid search analysis. Additionally, in all cases the proposed BO-iFEA framework yielded material parameter predictions with average element errors less than 0.02 mm/mm (normalized by the simulation-specific characteristic length). Nonetheless, the solutions were not unique due to the presence of a long-valley minima region in the objective function surfaces. Parameter sets along this valley can yield functionally equivalent outcomes (i.e., closing behavior) and are typically observed in the inverse analysis or parameter estimation for the nonlinear mechanical responses of the AHV. In this study, our key contributions include: (i) a first-of-its-kind demonstration of the BO method used for the AHV iFEA; and (ii) the evaluation of a candidate AHV in-silico modeling approach wherein the chordae could be substituted with equivalent displacement boundary conditions, rendering the better iFEA convergence and a smoother objective surface.

Geometric data of commercially available tricuspid valve annuloplasty devices

Tricuspid valve annuloplasty is the gold standard surgical treatment for functional tricuspid valve regurgitation. During this procedure, ring-like devices are implanted to reshape the diseased tricuspid valve annulus and to restore function. For the procedure, surgeons can choose from multiple available device options varying in shape and size. In this article, we provide the three-dimensional (3D) scanned geometry (*.stl) and reduced midline (*.vtk) of five different annuloplasty devices of all commercially available sizes. Three-dimensional images were captured using a 3D scanner. After extracting the surface geometry from these images, the images were converted to 3D point clouds and skeletonized to generate a 3D midline of each device. In total, we provide 30 data sets comprising the Edwards Classic, Edwards MC3, Edwards Physio, Medtronic TriAd, and Medtronic Contour 3D of sizes 26-36. This dataset can be used in computational models of tricuspid valve annuloplasty repair to inform accurate repair geometry and boundary conditions. Additionally, others can use these data to compare and inspire new device shapes and sizes.

An analytical, mathematical annuloplasty ring curvature model for planning of valve-in-ring transcatheter mitral valve replacement

OBJECTIVES

An increasing number of high-risk patients with previous mitral valve annuloplasty require transcatheter mitral valve replacement due to recurrent regurgitation. Annulus dilation with a transcatheter balloon is often performed before valve-in-ring transcatheter mitral valve replacement, which is believed to reduce misalignment and paravalvular leakage, yet little evidence exists to support this practice. Our objective was to generate intuitive annuloplasty ring analyses for improved valve-in-ring transcatheter mitral valve replacement planning.

METHODS

We generated a mathematical model that calculates image-tracked differential ring curvature to build quantifications for improved planning for valve-in-ring procedures. Carpentier-Edwards Physio M24 and M30 (n = 2 each), Physio II M24 and M26 (n = 3 each), LivaNova AnnuloFlex M26 (n = 2), and Edwards Geoform M28 (n = 2) rings were tested with a 30-mm Toray Inoue balloon inflated to maximum rated pressures.

RESULTS

Curvature variance reduces with larger ring sizes, indicating that larger rings are initially more circular than smaller ones. Evaluated semi-rigid and rigid rings showed little to no difference between pre- and post-dilation states. Annuloflex rings (flexible band) showed a postdilation variance reduction of 32.83% (P < .001) followed by an increase after 10 minutes of relaxation that was still reduced by 19.62% relative to the initial state (P < .001).

CONCLUSIONS

We discovered that balloon dilation does not significantly deform evaluated semi-rigid or rigid rings at maximum rated balloon pressures. This may mean that dilation for these conditions before valve-in-ring transcatheter mitral valve replacement is unnecessary. Our mathematical approach creates a foundation for extended classification of this practice, providing meaningful quantification of ring geometry.

Utilization of Engineering Advances for Detailed Biomechanical Characterization of the Mitral-Ventricular Relationship to Optimize Repair Strategies: A Comprehensive Review

The geometrical details and biomechanical relationships of the mitral valve-left ventricular apparatus are very complex and have posed as an area of research interest for decades. These characteristics play a major role in identifying and perfecting the optimal approaches to treat diseases of this system when the restoration of biomechanical and mechano-biological conditions becomes the main target. Over the years, engineering approaches have helped to revolutionize the field in this regard. Furthermore, advanced modelling modalities have contributed greatly to the development of novel devices and less invasive strategies. This article provides an overview and narrative of the evolution of mitral valve therapy with special focus on two diseases frequently encountered by cardiac surgeons and interventional cardiologists: ischemic and degenerative mitral regurgitation.

A pilot investigation of the tricuspid valve annulus in newborns with hypoplastic left heart syndrome

Objective

Hypoplastic left heart syndrome (HLHS) is a congenital disease characterized by an underdevelopment of the anatomical components inside the left heart. Approximately 30% of newborns with HLHS will develop tricuspid regurgitation, and it is currently unknown how the valve annulus mechanics and geometry are associated with regurgitation. Thus, we present an engineering mechanics-based analysis approach to quantify the mechanics and geometry of the HLHS-afflicted tricuspid valve (TV), using 4-dimensional echocardiograms.

Methods

Infants born with HLHS (n = 8) and healthy newborns (n = 4) had their TVs imaged, and the data were imported to 3D Slicer. The annular curves were defined at 5 points in the cardiac cycle. The geometry and deformation (strain) of the TV annulus were calculated to elucidate the mechanics of this critical structure and to compare them between neonates with and without HLHS.

Results

For the annular geometry, HLHS-afflicted newborns had significantly larger annular circumferences (20%-30%) and anteroposterior diameters (35%-45%) than the healthy patients. From a biomechanics' perspective, the HLHS patients had significantly smaller strains in the anterior segments (–0.1 ± 2.6%) during end-diastolic and end-isovolumetric relaxation (1.7 ± 3.0%) compared with the healthy counterparts (–13.3 ± 2.9% and 6.8 ± 0.9%, respectively).

Conclusions

The image-based analysis presented in this study may provide novel insights into the geometric and mechanistic differences in the TV annulus between the healthy and HLHS newborns. Future longitudinal studies of the biomechanics of TV annulus and other subvalvular structures may inform our understanding of the initiation and development of tricuspid regurgitation and the design of optimal repairs in this challenging population.

The role of surgery for secondary mitral regurgitation and heart failure in the era of transcatheter mitral valve therapies

The approach to the management of mitral valve (MV) disease and heart failure (HF) has dramatically changed over the last decades. It is well recognized that severe mitral regurgitation secondary to ischemic or non-ischemic cardiomyopathy is associated with an excess risk of mortality. Understanding the impact of the surgical treatment modality on mortality outcomes has been difficult due to the broad spectrum of secondary mitral regurgitation (SMR) phenotypes and lack of randomized surgical clinical trials. Over the last 30 years, surgeons have failed to provide compelling evidence to convince the medical community of the need to treat SMR in patients with severe HF. Therefore, the surgical treatment of SMR has never gained uniform acceptance as a significant option among patients suffering from SMR. Recent evidence from randomized trials in a non-surgical eligible patients treated with transcatheter therapies, has provided a new perspective on SMR treatment. Recently published European and American guidelines confirm the key role of percutaneous treatment of SMR and in parallel, these guidelines reinforce the role of mitral valve surgery in patients who require surgical revascularization. Complex mitral valve repair combining subvalvular apparatus repair along with annuloplasty seems to be a promising approach in selected patients in selected centers. Meanwhile, mitral valve replacement has become the preferred surgical strategy in most patients with advanced heart failure and severe LV remodeling or high risk of recurrent mitral regurgitation. In this comprehensive review, we aimed to discuss the role of mitral surgery for SMR in patients with heart failure in the contemporary era and to provide a practical approach for its surgical management.

Ex Vivo Model of Ischemic Mitral Regurgitation and Analysis of Adjunctive Papillary Muscle Repair

Ischemic mitral regurgitation (IMR) is particularly challenging to repair with lasting durability due to the complex valvular and subvalvular pathologies resulting from left ventricular dysfunction. Ex vivo simulation is uniquely suited to quantitatively analyze the repair biomechanics, but advancements are needed to model the nuanced IMR disease state. Here we present a novel IMR model featuring a dilation device with precise dilatation control that preserves annular elasticity to enable accurate ex vivo analysis of surgical repair. Coupled with augmented papillary muscle head positioning, the enhanced heart simulator system successfully modeled IMR pre- and post-surgical intervention and enabled the analysis of adjunctive subvalvular papillary muscle repair to alleviate regurgitation recurrence. The model resulted in an increase in regurgitant fraction: 11.6 ± 1.7% to 36.1 ± 4.4% (p < 0.001). Adjunctive papillary muscle head fusion was analyzed relative to a simple restrictive ring annuloplasty repair and, while both repairs successfully eliminated regurgitation initially, the addition of the adjunctive subvalvular repair reduced regurgitation recurrence: 30.4 ± 5.7% vs. 12.5 ± 2.6% (p = 0.002). Ultimately, this system demonstrates the success of adjunctive papillary muscle head fusion in repairing IMR as well as provides a platform to optimize surgical techniques for increased repair durability.

Collagen denaturation in the infarcted myocardium involves temporally distinct effects of MT1-MMP-dependent proteolysis and mechanical tension

Tissue injury results in profound alterations in the collagen network, associated with unfolding of the collagen triple helix, proteolytic degradation and generation of fragments. In the infarcted myocardium, changes in the collagen network are critically involved in the pathogenesis of left ventricular rupture, adverse remodeling and chronic dysfunction. We hypothesized that myocardial infarction is associated with temporally and spatially restricted patterns of collagen denaturation that may reflect distinct molecular mechanisms of collagen unfolding. We used a mouse model of non-reperfused myocardial infarction, and in vitro assays in fibroblast-populated collagen lattices. In healing infarcts, labeling with collagen hybridizing peptide (CHP) revealed two distinct patterns of collagen denaturation. During the inflammatory and proliferative phases of infarct healing, collagen denaturation was pericellular, localized in close proximity to macrophages and myofibroblasts. qPCR array analysis of genes associated with matrix remodeling showed that Membrane Type 1-Matrix Metalloproteinase (MT1-MMP) is markedly upregulated in infarct macrophages and fibroblasts, suggesting its involvement in pericellular collagen denaturation. In vitro, MT1-MMP-mediated pericellular collagen denaturation is involved in cardiac fibroblast migration. The effects of MT1-MMP on collagen denaturation and fibroblast migration involve the catalytic site, and require hemopexin domain-mediated actions. In contrast, during the maturation phase of infarct healing, extensive collagen denaturation was noted in the hypocellular infarct, in the infarct border zone and in the mitral valve annulus, in the absence of MT1-MMP. In vitro, mechanical tension in attached collagen lattices was sufficient to induce peripheral collagen denaturation. Our study suggests that in healing infarcts, early pericellular collagen denaturation may be important for migration of macrophages and reparative myofibroblasts in the infarct. Extensive denaturation of collagen fibers is noted in mature scars, likely reflecting mechanical tension. Chronic collagen denaturation may increase susceptibility of the matrix to proteolysis, thus contributing to progressive cardiac dilation and post-infarction heart failure.

Tricuspid Valve Annuloplasty Alters Leaflet Mechanics

Tricuspid valve regurgitation is associated with significant morbidity and mortality. Its most common treatment option, tricuspid valve annuloplasty, is not optimally effective in the long-term. Toward identifying the causes for annuloplasty's ineffectiveness, we have previously investigated the technique's impact on the tricuspid annulus and the right ventricular epicardium. In our current work, we are extending our analysis to the anterior tricuspid valve leaflet. To this end, we adopted our previous strategy of performing DeVega suture annuloplasty as an experimental methodology that allows us to externally control the degree of cinching during annuloplasty. Thus, in ten sheep we successively cinched the annulus and quantified changes to leaflet motion, dynamics, and strain in the beating heart by combining sonomicrometry with our well-established mechanical framework. We found that successive cinching of the valve enforced earlier coaptation and thus reduced leaflet range of motion. Additionally, leaflet angular velocity during opening and closing decreased. Finally, we found that leaflet strains were also reduced. Specifically, radial and areal strains decreased as a function of annular cinching. Our findings are critical as they suggest that suture annuloplasty alters the mechanics of the tricuspid valve leaflets which may disrupt their resident cells' mechanobiological equilibrium. Long-term, such disruption may stimulate tissue maladaptation which could contribute to annuloplasty's sub-optimal effectiveness. Additionally, our data suggest that the extent to which annuloplasty alters leaflet mechanics can be controlled via degree of cinching. Hence, our data may provide direct surgical guidelines.

An efficient and accurate method for modeling nonlinear fractional viscoelastic biomaterials

Computational biomechanics plays an important role in biomedical engineering: using modeling to understand pathophysiology, treatment and device design. While experimental evidence indicates that the mechanical response of most tissues is viscoelastic, current biomechanical models in the computational community often assume hyperelastic material models. Fractional viscoelastic constitutive models have been successfully used in literature to capture viscoelastic material response; however, the translation of these models into computational platforms remains limited. Many experimentally derived viscoelastic constitutive models are not suitable for three-dimensional simulations. Furthermore, the use of fractional derivatives can be computationally prohibitive, with a number of current numerical approximations having a computational cost that is 𝒪 ( N T 2 ) and a storage cost that is 𝒪(N_T ) (N_T denotes the number of time steps). In this paper, we present a novel numerical approximation to the Caputo derivative which exploits a recurrence relation similar to those used to discretize classic temporal derivatives, giving a computational cost that is 𝒪(N_T ) and a storage cost that is fixed over time. The approximation is optimized for numerical applications, and an error estimate is presented to demonstrate the efficacy of the method. The method, integrated into a finite element solid mechanics framework, is shown to be unconditionally stable in the linear viscoelastic case. It was then integrated into a computational biomechanical framework, with several numerical examples verifying the accuracy and computational efficiency of the method, including in an analytic test, in an analytic fractional differential equation, as well as in a computational biomechanical model problem.

The Effect of Downsizing on the Normal Tricuspid Annulus

Tricuspid annuloplasty is a surgical procedure that cinches the valve's annulus in order to reduce regurgitant blood flow. One of its critical parameters is the degree of downsizing. To provide insight into the effect of downsizing, we studied the annulus of healthy sheep during suture annuloplasty. To this end, we implanted fiduciary markers along the annulus of sheep and subsequently performed a DeVega suture annuloplasty. We performed five downsizing steps in each animal while recording hemodynamic and sonomicrometry data in beating hearts. Subsequently, we used splines to approximate the annulus at baseline and at each downsizing step. Based on these approximations we computed clinical metrics of annular shape and dynamics, and the continuous field metrics height, strain, and curvature. With these data, we demonstrated that annular area reduction during downsizing was primarily driven by compression of the anterior annulus. Similarly, reduction in annular dynamics was driven by reduced contractility in the anterior annulus. Finally, changes in global height and eccentricity of the annulus could be explained by focal changes in the continuous height profile and changes in annular curvature. Our findings are important as they provide insight into a regularly performed surgical procedure and may inform the design of transcatheter devices that mimic suture annuloplasty.

Biomechanics of the Tricuspid Annulus: A Review of the Annulus' In Vivo Dynamics With Emphasis on Ovine Data

The tricuspid annulus forms the boundary between the tricuspid valve leaflets and their surrounding perivalvular tissue of the right atrioventricular junction. Its shape changes throughout the cardiac cycle in response to the forces from the contracting right heart myocardium and the blood-valve interaction. Alterations to annular shape and dynamics in disease lead to valvular dysfunctions such as tricuspid regurgitation from which millions of patients suffer. Successful treatment of such dysfunction requires an in-depth understanding of the normal shape and dynamics of the tricuspid annulus and of the changes following disease and subsequent repair. In this manuscript we review what we know about the shape and dynamics of the normal tricuspid annulus and about the effects of both disease and repair based on non-invasive imaging studies and invasive fiduciary marker-based studies. We further show, by means of ovine data, that detailed engineering analyses of the tricuspid annulus provide regionally-resolved insight into the kinematics of the annulus which would remain hidden if limiting analyses to simple geometric metrics.

Fatigue exhaustion of the mitral valve tissue

Sudden failure and rupture of the tissue is a rare but serious short-term complication after the mitral valve surgical repair. Excessive cyclic loading on the suture line of the repair can progressively damage the surrounding tissue and finally cause tissue rupture. Moreover, mechanical over-tension, which occurs in a diseased mitral valve, gradually leads to tissue floppiness, mitral annular dilation, and leaflet rupture. In this work, the rupture mechanics of mitral valve is studied by characterizing the fracture toughness exhaustion of healthy tissue. Results of this study show that fracture toughness of the posterior mitral valve is lower than its anterior counterpart, indicating that posterior tissue is more prone to failure. Moreover, the decrease in fracture toughness by increasing the number of fatigue cycles shows that excessive mechanical loading leads to progressive failure and rupture of mitral valve tissue within a damage accumulative process.

Effect of glutaraldehyde based cross-linking on the viscoelasticity of mitral valve basal chordae tendineae

BACKGROUND

Mitral valve failure can require repair or replacement. Replacement bioprosthetic valves are treated with glutaraldehyde prior to implantation. The aim of this study was to determine the changes in mechanical properties following glutaraldehyde fixation of mitral valve chordae.

METHODS

To investigate the impact of glutaraldehyde on mitral valve chordae, 24 basal chordae were dissected from four porcine hearts. Anterior and posterior basal (including strut) chordae were used. All 24 chordae were subjected to a sinusoidally varying load (mean level 2N, dynamic amplitude 2N) over a frequency range of 0.5-10 Hz before and after glutaraldehyde treatment.

RESULTS

The storage and loss modulus of all chordal types decreased following glutaraldehyde fixation. The storage modulus ranged from: 108 to 119 MPa before fixation and 67.3-87.4 MPa following fixation for basal chordae; 52.3-58.4 MPa before fixation and 47.9-53.5 MPa following fixation for strut chordae. Similarly, the loss modulus ranged from: 5.47 to 6.25 MPa before fixation and 3.63-4.94 MPa following fixation for basal chordae; 2.60-2.97 MPa before fixation and 2.31-2.93 MPa following fixation for strut chordae.

CONCLUSION

The viscoelastic properties of mitral valve chordae are affected by glutaraldehyde fixation; in particular, the reduction in storage moduli decreased with an increase in chordal diameter.

Interpreting Activation Mapping of Atrial Fibrillation: A Hybrid Computational/Physiological Study

Atrial fibrillation is the most common rhythm disorder of the heart associated with a rapid and irregular beating of the upper chambers. Activation mapping remains the gold standard to diagnose and interpret atrial fibrillation. However, fibrillatory activation maps are highly sensitive to far-field effects, and often disagree with other optical mapping modalities. Here we show that computational modeling can identify spurious non-local components of atrial fibrillation electrograms and improve activation mapping. We motivate our approach with a cohort of patients with potential drivers of persistent atrial fibrillation. In a computational study using a monodomain Maleckar model, we demonstrate that in organized rhythms, electrograms successfully track local activation, whereas in atrial fibrillation, electrograms are sensitive to spiral wave distance and number, spiral tip trajectories, and effects of fibrosis. In a clinical study, we analyzed n = 15 patients with persistent atrial fibrillation that was terminated by limited ablation. In five cases, traditional activation maps revealed a spiral wave at sites of termination; in ten cases, electrogram timings were ambiguous and activation maps showed incomplete reentry. By adjusting electrogram timing through computational modeling, we found rotational activation, which was undetectable with conventional methods. Our results demonstrate that computational modeling can identify non-local deflections to improve activation mapping and explain how and where ablation can terminate persistent atrial fibrillation. Our hybrid computational/physiological approach has the potential to optimize map-guided ablation and improve ablation therapy in atrial fibrillation.

Engineering Analysis of Tricuspid Annular Dynamics in the Beating Ovine Heart

Functional tricuspid regurgitation is a significant source of morbidity and mortality in the US. Furthermore, treatment of functional tricuspid regurgitation is suboptimal with significant recurrence rates, which may, at least in part, be due to our limited knowledge of the relationship between valvular shape and function. Here we study the dynamics of the healthy in vivo ovine tricuspid annulus to improve our understanding of normal annular deformations throughout the cardiac cycle. To this end, we determine both clinical as well as engineering metrics of in vivo annular dynamics based on sonomicrometry crystals surgically attached to the annulus. We confirm that the tricuspid annulus undergoes large dynamic changes in area, perimeter, height, and eccentricity throughout the cardiac cycle. This deformation may be described as asymmetric in-plane motion of the annulus with minor out-of-plane motion. In addition, we employ strain and curvature to provide mechanistic insight into the origin of this deformation. Specifically, we find that strain and curvature vary considerable across the annulus with highly localized minima and maxima resulting in aforementioned configurational changes throughout the cardiac cycle. It is our hope that these data provide valuable information for clinicians and engineers alike and ultimately help us improve treatment of functional tricuspid regurgitation.

A virtual sizing tool for mitral valve annuloplasty

Functional mitral regurgitation, a backward leakage of the mitral valve, is a result of left ventricular growth and mitral annular dilatation. Its gold standard treatment is mitral annuloplasty, the surgical reduction in mitral annular area through the implantation of annuloplasty rings. Recurrent regurgitation rates may, however, be as high as 30% and more. While the degree of annular downsizing has been linked to improved long-term outcomes, too aggressive downsizing increases the risk of ring dehiscences and significantly impairs repair durability. Here, we prototype a virtual sizing tool to quantify changes in annular dimensions, surgically induced tissue strains, mitral annular stretches, and suture forces in response to mitral annuloplasty. We create a computational model of dilated cardiomyopathy onto which we virtually implant annuloplasty rings of different sizes. Our simulations confirm the common intuition that smaller rings are more invasive to the surrounding tissue, induce higher strains, and require larger suture forces than larger rings: The total suture force was 2.2 N for a 24-mm ring, 1.9 N for a 28-mm ring, and 0.8 N for a 32-mm ring. Our model predicts the highest risk of dehiscence in the septal and postero-lateral annulus where suture forces are maximal. These regions co-localize with regional peaks in myocardial strain and annular stretch. Our study illustrates the potential of realistic predictive simulations in cardiac surgery to identify areas at risk for dehiscence, guide the selection of ring size and shape, rationalize the design of smart annuloplasty rings and, ultimately, improve long-term outcomes after surgical mitral annuloplasty. Copyright © 2016 John Wiley & Sons, Ltd.

Numerical prediction of thrombus risk in an anatomically dilated left ventricle: the effect of inflow cannula designs

BACKGROUND

Implantation of a rotary blood pump (RBP) can cause non-physiological flow fields in the left ventricle (LV) which may trigger thrombosis. Different inflow cannula geometry can affect LV flow fields. The aim of this study was to determine the effect of inflow cannula geometry on intraventricular flow under full LV support in a patient specific model.

METHODS

Computed tomography angiography imaging of the LV was performed on a RBP candidate to develop a patient-specific model. Five inflow cannulae were evaluated, which were modelled on those used clinically or under development. The inflow cannulae are described as a crown like tip, thin walled tubular tip, large filleted tip, trumpet like tip and an inferiorly flared cannula. Placement of the inflow cannula was at the LV apex with the central axis intersecting the centre of the mitral valve. Full support was simulated by prescribing 5 l/min across the mitral valve. Thrombus risk was evaluated by identifying regions of stagnation. Rate of LV washout was assessed using a volume of fluid model. Relative haemolysis index and blood residence time was calculated using an Eulerian approach.

RESULTS

The inferiorly flared inflow cannula had the lowest thrombus risk due to low stagnation volumes. All cannulae had similar rates of LV washout and blood residence time. The crown like tip and thin walled tubular tip resulted in relatively higher blood damage indices within the LV.

CONCLUSION

Changes in intraventricular flow due to variances in cannula geometry resulted in different stagnation volumes. Cannula geometry does not appreciably affect LV washout rates and blood residence time. The patient specific, full support computational fluid dynamic model provided a repeatable platform to investigate the effects of inflow cannula geometry on intraventricular flow.

Increased systolic load causes adverse remodeling of fetal aortic and mitral valves

While abnormal hemodynamic forces alter fetal myocardial growth, little is known about whether such insults affect fetal cardiac valve development. We hypothesized that chronically elevated systolic load would detrimentally alter fetal valve growth. Chronically instrumented fetal sheep received either a continuous infusion of adult sheep plasma to increase fetal blood pressure, or a lactated Ringer's infusion as a volume control beginning on day 126 ± 4 of gestation. After 8 days, mean arterial pressure was higher in the plasma infusion group (63.0 mmHg vs. 41.8 mmHg, P < 0.05). Mitral annular septal-lateral diameter (11.9 mm vs. 9.1 mm, P < 0.05), anterior leaflet length (7.7 mm vs. 6.4 mm, P < 0.05), and posterior leaflet length (P2; 4.0 mm vs. 3.0 mm, P < 0.05) were greater in the elevated load group. mRNA levels of Notch-1, TGF-β2, Wnt-2b, BMP-1, and versican were suppressed in aortic and mitral valve leaflets; elastin and α1 type I collagen mRNA levels were suppressed in the aortic valves only. We conclude that sustained elevated arterial pressure load on the fetal heart valve leads to anatomic remodeling and, surprisingly, suppression of signaling and extracellular matrix genes that are important to valve development. These novel findings have important implications on the developmental origins of valve disease and may have long-term consequences on valve function and durability.

Cellular and Extracellular Matrix Basis for Heterogeneity in Mitral Annular Contraction

PURPOSE

Regional heterogeneity in mitral annular contraction, which is generally ascribed to the fibrous vs. muscular annular composition, ensures proper leaflet motion and timing of coaptation. It is unknown whether the fibroblast-like cells in the annulus modulate this heterogeneity, even though valvular interstitial cells (VICs) can be mechanically "activated."

METHODS

Fourteen sheep underwent implantation of radiopaque markers around the mitral annulus defining four segments: septal (SEPT), lateral (LAT), and anterior (ANT-C) and posterior (POST-C) commissures. Segmental annular contraction was calculated using biplane videofluoroscopy. Immunohistochemistry of annular cross sections assessed regional matrix content, matrix turnover, and cell phenotype. Micropipette aspiration measured the Young's modulus of the leaflets adjacent to the myocardial border.

RESULTS

Whereas SEPT contained more collagen I and III, LAT demonstrated more collagen and elastin turnover as shown by greater decorin, lysyl oxidase, and matrix metalloprotease (MMP)-13 and smooth muscle alpha-actin (SMaA). This greater matrix turnover paralleled greater annular contraction in LAT vs. SEPT (22.5% vs. 4.1%). Similarly, POST-C had more SMaA and MMP13 than ANT-C, consistent with greater annular contraction in POST-C (18.8% vs. 11.1%). Interestingly, POST-C had the greatest effective modulus, significantly higher than LAT.

CONCLUSIONS

These data suggest that matrix turnover by activated VICs relates to annular motion heterogeneity, maintains steady-state mechanical properties in the annulus, and could be a therapeutic target when annular motion is impaired. Conversely, alterations in this heterogeneous annular contraction, whether through disease or secondary to ring annuloplasty, could disrupt this normal pattern of cell-mediated matrix remodeling and further adversely impact mitral valve function.

Modeling Pathologies of Diastolic and Systolic Heart Failure

Chronic heart failure is a medical condition that involves structural and functional changes of the heart and a progressive reduction in cardiac output. Heart failure is classified into two categories: diastolic heart failure, a thickening of the ventricular wall associated with impaired filling; and systolic heart failure, a dilation of the ventricles associated with reduced pump function. In theory, the pathophysiology of heart failure is well understood. In practice, however, heart failure is highly sensitive to cardiac microstructure, geometry, and loading. This makes it virtually impossible to predict the time line of heart failure for a diseased individual. Here we show that computational modeling allows us to integrate knowledge from different scales to create an individualized model for cardiac growth and remodeling during chronic heart failure. Our model naturally connects molecular events of parallel and serial sarcomere deposition with cellular phenomena of myofibrillogenesis and sarcomerogenesis to whole organ function. Our simulations predict chronic alterations in wall thickness, chamber size, and cardiac geometry, which agree favorably with the clinical observations in patients with diastolic and systolic heart failure. In contrast to existing single- or bi-ventricular models, our new four-chamber model can also predict characteristic secondary effects including papillary muscle dislocation, annular dilation, regurgitant flow, and outflow obstruction. Our prototype study suggests that computational modeling provides a patient-specific window into the progression of heart failure with a view towards personalized treatment planning.

A novel coaptation plate device for functional mitral regurgitation: an in vitro study

A novel mitral valve repair device, coaptation plate (CP), was proposed to treat functional mitral regurgitation. The objective of this study was to test efficacy of the CP in an in vitro model of functional mitral regurgitation. Ten fresh porcine mitral valves were mounted in a left heart simulator, Mitral regurgitation was emulated by means of annular dilatation, and the asymmetrical or symmetrical papillary muscles (PM) displacement. A rigid and an elastic CPs were fabricated and mounted in the orifice of regurgitant mitral valves. Steady flow leakage in a hydrostatic condition and regurgitant volume in a pulsatile flow were measured before and after implantation of the CPs. The rigid and elastic CPs reduced mitral valve regurgitant volume fraction from 60.5 ± 11.4 to 35 ± 11.6 and 36.5 ± 9.9%, respectively, in the asymmetric PM displacement. Mitral regurgitation was much lower in the symmetric PM displacement than in the asymmetric PM displacement, and was not significantly reduced after implantation of either CP. In conclusion, both the rigid and elastic CPs are effective and have no difference in reduction of functional mitral regurgitation. The CP does not aggravate mitral valve coaptation and may be used as a preventive way.

On the mechanics of growing thin biological membranes

Despite their seemingly delicate appearance, thin biological membranes fulfill various crucial roles in the human body and can sustain substantial mechanical loads. Unlike engineering structures, biological membranes are able to grow and adapt to changes in their mechanical environment. Finite element modeling of biological growth holds the potential to better understand the interplay of membrane form and function and to reliably predict the effects of disease or medical intervention. However, standard continuum elements typically fail to represent thin biological membranes efficiently, accurately, and robustly. Moreover, continuum models are typically cumbersome to generate from surface-based medical imaging data. Here we propose a computational model for finite membrane growth using a classical midsurface representation compatible with standard shell elements. By assuming elastic incompressibility and membrane-only growth, the model a priori satisfies the zero-normal stress condition. To demonstrate its modular nature, we implement the membrane growth model into the general-purpose non-linear finite element package Abaqus/Standard using the concept of user subroutines. To probe efficiently and robustness, we simulate selected benchmark examples of growing biological membranes under different loading conditions. To demonstrate the clinical potential, we simulate the functional adaptation of a heart valve leaflet in ischemic cardiomyopathy. We believe that our novel approach will be widely applicable to simulate the adaptive chronic growth of thin biological structures including skin membranes, mucous membranes, fetal membranes, tympanic membranes, corneoscleral membranes, and heart valve membranes. Ultimately, our model can be used to identify diseased states, predict disease evolution, and guide the design of interventional or pharmaceutic therapies to arrest or revert disease progression.

Growing matter: a review of growth in living systems

Living systems can grow, develop, adapt, and evolve. These phenomena are non-intuitive to traditional engineers and often difficult to understand. Yet, classical engineering tools can provide valuable insight into the mechanisms of growth in health and disease. Within the past decade, the concept of incompatible configurations has evolved as a powerful tool to model growing systems within the framework of nonlinear continuum mechanics. However, there is still a substantial disconnect between the individual disciplines, which explore the phenomenon of growth from different angles. Here we show that the nonlinear field theories of mechanics provide a unified concept to model finite growth by means of a single tensorial internal variable, the second order growth tensor. We review the literature and categorize existing growth models by means of two criteria: the microstructural appearance of growth, either isotropic or anisotropic; and the microenvironmental cues that drive the growth process, either chemical or mechanical. We demonstrate that this generic concept is applicable to a broad range of phenomena such as growing arteries, growing tumors, growing skin, growing airway walls, growing heart valve leaflets, growing skeletal muscle, growing plant stems, growing heart valve annuli, and growing cardiac muscle. The proposed approach has important biological and clinical applications in atherosclerosis, in-stent restenosis, tumor invasion, tissue expansion, chronic bronchitis, mitral regurgitation, limb lengthening, tendon tear, plant physiology, dilated and hypertrophic cardiomyopathy, and heart failure. Understanding the mechanisms of growth in these chronic conditions may open new avenues in medical device design and personalized medicine to surgically or pharmacologically manipulate development and alter, control, or revert disease progression.