Characterization of Mitral Valve Annular Dynamics in the Beating Heart

Tricuspid valve edge-to-edge repair simulations are highly sensitive to annular boundary conditions

Transcatheter edge-to-edge repair (TEER) simulations may provide insight into this novel therapeutic technology and help optimize its use. However, because of the relatively short history and technical complexity of TEER simulations, important questions remain unanswered. For example, there is no consensus on how to handle the annular boundary conditions in these simulations. In this short communication, we tested the sensitivity of such simulations to the choice of annular boundary conditions using a high-fidelity finite element model of a human tricuspid valve. Therein, we embedded the annulus among elastic springs to simulate the compliance of the perivalvular myocardium. Next, we varied the spring stiffness parametrically and explored the impact on two key measures of valve function: coaptation area and leaflet stress. Additionally, we compared our results to simulations with a pinned annulus. We found that a compliant annular boundary condition led to a TEER-induced "annuloplasty effect," i.e., annular remodeling, as observed clinically. Moreover, softer springs led to a larger coaptation area and smaller leaflet stresses. On the other hand, pinned annular boundary conditions led to unrealistically high stresses and no "annuloplasty effect." Furthermore, we found that the impact of the boundary conditions depended on the clip position. Our findings in this case study emphasize the importance of the annular boundary condition in tricuspid TEER simulations. Thus, we recommend that care be taken when choosing annular boundary conditions and that results from simulations using pinned boundaries should be interpreted with caution.

A method for non-invasive estimation of mitral valve annular regional strains

INTRODUCTION

In this study, a method to assess local deformations along the mitral annulus curve is proposed.

METHODS

It employs the known global geometry (from echocardiography) of the annulus during the cardiac cycle which is approximated with a closed cubic spline to generate a smooth mathematical representation of the annulus at each available time point. A point-wise mapping of the annular geometries between two consecutive times is established by minimizing the global displacements of the annulus. Hence, the displacements of the mitral annulus are determined and used to calculate the regional strains along the annulus line.

RESULTS

Data obtained from sonomicrometric markers are used to test the method detailed herein. The results show that our method can predict annular displacements throughout the cardiac cycle. Strain values computed with this approach are in line with previously experimentally measured strains reported in the literature. Finally, the method is applied and illustrated on an echocardiographic recording of a healthy individual.

CONCLUSION

The numerical method provided can be used to capture the regional annular strains by echocardiography and may help to predict regional dysfunctions in the mitral annulus, providing information on the pathological mechanisms.

Intrathoracic pressure deviations attenuate left ventricular filling and stroke volume without pronounced myocardial mechanical alterations in healthy adults

Intrathoracic pressure modulates cardiac loading conditions, which then influence left ventricular (LV) chamber function, and may occur with underlying myocardial mechanical alterations. We investigated the independent effects of inspiratory negative and expiratory positive intrathoracic pressure on septal geometry, LV chamber function, and rotation, twist, and strain indices. After baseline, 20 healthy adults (11M/9F, 23 ± 4 yr) performed resistive breathing to manipulate inspiratory (-30, -20, -10 cmH2O) or expiratory (+10, +20 cmH2O) intrathoracic pressure. Echocardiography was used to acquire LV-focused two-dimensional (2-D) images, and mitral Doppler inflow and annular tissue velocity spectra. Images were analyzed for LV chamber volumes, tissue velocities, transmitral filling velocities, and speckle tracking-derived LV longitudinal, radial, and circumferential strain and strain-rate, basal and apical rotation, and twist. Across negative pressure trials, most profoundly at -30 cmH2O, we observed progressive end-diastolic septal flattening (3.9 ± 0.4 vs. 3.2 ± 0.4 cm, P < 0.05) and decreases in LV end-diastolic volume (103 ± 23 vs. 115 ± 25 mL, P < 0.05) and stroke volume, whereas end-systolic volume was unchanged. However, LV apical and basal rotation, twist (13.3° ± 3.6° vs. 13.9° ± 3.7°, P = 0.890), and circumferential, radial, and longitudinal strain indices were largely unchanged. During positive pressure trials, we observed main effects for septal flattening (P = 0.014) confined to inspiration, and modestly reduced LV end-diastolic volume (P < 0.001), end-systolic volume (P = 0.033), and stroke volume. Again, myocardial mechanics parameters changed little. Collectively, our data suggest that both positive and negative intrathoracic pressures can exacerbate direct ventricular interaction through opposing mechanisms that attenuate LV end-diastolic volume and stroke volume, but without specific changes in myocardial mechanics or mitral inflow.NEW & NOTEWORTHY Incrementally more negative or positive intrathoracic pressures, relative to normal dynamic breathing, progressively attenuate left ventricular end-diastolic volume and stroke volume in healthy younger adults. Incrementally more negative or positive intrathoracic pressures were each associated with progressive septal flattening during inspiration, indicating direct ventricular interaction. However, left ventricular transmitral inflow velocities, and myocardial rotation, twist, and circumferential, longitudinal, and radial strain parameters changed little.

Ovine tricuspid annular dynamics and three-dimensional geometry during acute atrial fibrillation

OBJECTIVES

Long-standing atrial fibrillation (AF) may lead to tricuspid regurgitation (TR) and right ventricular dysfunction. However, the effect of acute AF on tricuspid annular (TA) dynamics and three-dimensional geometry is unknown.

METHODS

In eight adult sheep, sonomicrometry crystals were implanted around the tricuspid annulus and right ventricular free wall. Pressure transducers were placed in the right ventricle, left ventricle, and right atrium. After weaning from cardiopulmonary bypass and a period of hemodynamic stabilization, simultaneous sonomicrometry and hemodynamic data were collected in sinus rhythm (SR) and during experimental AF (400b/min right atrial pacing). Annular area, perimeter, dimensions, height, global and regional annular contraction, and strain were calculated based on cubic spline fits to crystal 3D locations.

RESULTS

Maximal TA area increased from 1084.9±273.9mm2 in SR to 1207.5±322.1mm2 during AF (p = 0.002). Anteroposterior diameter increased from 36.5±5.0mm to 38.4±5.5mm (p = 0.05). TA contraction decreased from 7±2% in SR to 2±1% in AF (p = 0.001). Anterior, posterior, and septal regional annular contraction decreased from 10±4%, 8±3% and 6±2% to 4±2%, 3±1% and 2±1% for SR and AF, respectively (p<0.05). AF perturbed systolic global annular strain (from -6.52±1.74% to -2.78±1.79%; p = 0.003) and caused annular stretch. Annular height marginally decreased with AF from 5.8±1.9mm to 5.7±2.0mm; p = 0.039.

CONCLUSIONS

Acute experimental AF in healthy sheep was associated with TA dilation, flattening, and decreased total and regional annular contractility. These data may help elucidate the pathophysiology of functional TR associated with AF.

Characterisation of global and regional mitral annular strains in an acute porcine model

OBJECTIVES

This study aimed to explore regional mitral annular strain using a novel computational method.

METHODS

Eight pigs underwent implantation with piezoelectric transducers around the mitral annulus. Interventions of pre- and afterload were performed by inferior vena cava constriction and endovascular balloon occlusion of the descending aorta. The mitral annulus was reconstructed in a mathematical model and divided into 6 segments. Global and segmental annular strain were calculated from a discrete mathematical representation.

RESULTS

Global annular strain gradually decreased after isovolumetric contraction until late systole. Mitral annular end-systolic strain demonstrated shortening in all segments except the anterior segment, which showed the least deformation. The P2 annular segment demonstrated the most end-systolic shortening (-7.6 ± 1.1% at baseline, P < 0.001 compared to anterior segment). Systolic global annular strain showed no significant change in response to load interventions but correlated positively with left ventricular contractility at baseline and after preload reduction.

CONCLUSIONS

Mitral annular systolic strain demonstrates cyclical variations with considerable regional heterogeneity, with the most pronounced deformation in posterior annular segments. Measurements appear independent of changes to pre- and afterload.

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.

Three-dimensional echocardiography reveals early mitral valve alterations in hypertrophic cardiomyopathy genetic mutation carriers

BACKGROUND

The mitral valve undergoes structural modifications in response to cardiac functional changes, often predating cardiac decompensation and overt clinical signs. Our study assessed the potential of mitral valve morphological changes as early indicators for detecting carriers of hypertrophic cardiomyopathy (HCM)-associated gene mutations.

METHODS

We studied 505 participants: 189 without the pathogenic gene mutations and left ventricular hypertrophy (G-/LVH-), 149 carriers without LV hypertrophy (G+/LVH-), and 167 manifest HCM patients (G+/LVH+). We juxtaposed the mitral valve morphology and associated metrics across these groups, emphasizing those carrying MYH7 and MYBPC3 mutations.

RESULTS

We discerned pronounced disparities in the mitral annulus and leaflet structures across the groups. The mitral valve apparatus in mutation carriers exhibited a tendency towards a flattened profile. Detailed analysis spotlighted MYBPC3 mutation carriers, whose mitral valves were notably flatter (with notably lower AHCWR values than non-carriers); this contrast was not evident in MYH7 mutation carriers. This mitral valve flattening, manifest in the mutation carriers, suggests it might be an adaptive response to incipient cardiac dysfunction in HCM's nascent stages.

CONCLUSIONS

Three-dimensional echocardiography illuminates the initial mitral valve structural changes in HCM patients bearing pathogenic gene mutations. These morphological signatures hold promise as sensitive imaging markers, especially for asymptomatic carriers of the MYBPC3 mutation.

A Computational Pipeline for Patient-Specific Prediction of the Postoperative Mitral Valve Functional State

While mitral valve (MV) repair remains the preferred clinical option for mitral regurgitation (MR) treatment, long-term outcomes remain suboptimal and difficult to predict. Furthermore, pre-operative optimization is complicated by the heterogeneity of MR presentations and the multiplicity of potential repair configurations. In the present work, we established a patient-specific MV computational pipeline based strictly on standard-of-care pre-operative imaging data to quantitatively predict the post-repair MV functional state. First, we established human mitral valve chordae tendinae (MVCT) geometric characteristics obtained from five CT-imaged excised human hearts. From these data, we developed a finite-element model of the full patient-specific MV apparatus that included MVCT papillary muscle origins obtained from both the in vitro study and the pre-operative three-dimensional echocardiography images. To functionally tune the patient-specific MV mechanical behavior, we simulated pre-operative MV closure and iteratively updated the leaflet and MVCT prestrains to minimize the mismatch between the simulated and target end-systolic geometries. Using the resultant fully calibrated MV model, we simulated undersized ring annuloplasty (URA) by defining the annular geometry directly from the ring geometry. In three human cases, the postoperative geometries were predicted to 1 mm of the target, and the MV leaflet strain fields demonstrated close agreement with noninvasive strain estimation technique targets. Interestingly, our model predicted increased posterior leaflet tethering after URA in two recurrent patients, which is the likely driver of long-term MV repair failure. In summary, the present pipeline was able to predict postoperative outcomes from pre-operative clinical data alone. This approach can thus lay the foundation for optimal tailored surgical planning for more durable repair, as well as development of mitral valve digital twins.

Mitral annular dynamics are influenced by left ventricular load and contractility in an acute animal model

The purpose of this study was to investigate the effects of loading conditions and left ventricular (LV) contractility on mitral annular dynamics. In 10 anesthetized pigs, eight piezoelectric transducers were implanted equidistantly around the mitral annulus. High-fidelity catheters measured left ventricular pressures and the slope of the end-systolic pressure-volume relationship (Ees ) determined LV contractility. Adjustments of pre- and afterload were done by constriction of the inferior caval vein and occlusion of the descending aorta. Mitral annulus area indexed to body surface area (MAAi ), annular circularity index (ACI), and non-planarity angle (NPA) were calculated by computational analysis. MAAi was more dynamic in response to loading interventions than ACI and NPA. However, MAAi maximal cyclical reduction (-Δr) and average deformational velocity (-v ¯ $$ \overline{v} $$ ) did not change accordingly (p = 0.31 and p = 0.22). Reduced Ees was associated to attenuation in MAAi -Δr and MAAi -v ¯ $$ \overline{v} $$ (r2  = 0.744; p = 0.001 and r2  = 0.467; p = 0.029). In conclusion, increased cardiac load and reduced LV contractility may cause deterioration of mitral annular dynamics, likely impairing coaptation and increasing susceptibility to valvular incompetence.

Traction mechanical characterization of porcine mitral valve annulus

The Mitral Annulus (MA) is an anisotropic, fibrous, flexible and dynamical structure. While MA dynamics are well documented, its passive mechanical properties remain poorly investigated to complete the design of adequate prostheses. Mechanical properties in traction on four sections of the MA (aortic, left, posterior and right segments) were assessed using a traction test system with a 30 N load cell and pulling jaws for sample fixation. Samples were submitted to a 1.5 N pre-load, 10 pre-conditioning cycles. Three strain rates were tested (5 %/min, 7 %/min and 13 %/min), the first two up to 10 % strain and the last until rupture. High-resolution diffusion-MRI provided microstructural mapping of fractional anisotropy and mean diffusion within muscle and collagen fibres. Ten MA from porcine hearts were excised resulting in 40 tested samples, out of which 28 were frozen prior to testing. Freezing samples significantly increased Young Moduli for all strain rates. No significant differences were found between Young Moduli at different strain rates (fresh samples 2.4 ± 1.1 MPa, 3.8 ± 2.2 MPa and 3.1 ± 1.8 MPa for increasing strain rates in fresh samples), while significant differences were found when comparing aortic with posterior and posterior with lateral (p < 0.012). Aortic segments deformed the most (24.1 ± 9.4 %) while lateral segments endured the highest stress (>0.3 MPa), corresponding to higher collagen fraction (0.46) and fractional anisotropy. Passive machinal properties differed between aortic and lateral segments of the MA. The process of freezing samples altered their mechanical properties. Underlying microstructural differences could be linked to changes in strain response.

The Geometry of Incompatibility in Growing Soft Tissues: Theory and Numerical Characterization

Tissues in vivo are not stress-free. As we grow, our tissues adapt to different physiological and disease conditions through growth and remodeling. This adaptation occurs at the microscopic scale, where cells control the microstructure of their immediate extracellular environment to achieve homeostasis. The local and heterogeneous nature of this process is the source of residual stresses. At the macroscopic scale, growth and remodeling can be accurately captured with the finite volume growth framework within continuum mechanics, which is akin to plasticity. The multiplicative split of the deformation gradient into growth and elastic contributions brings about the notion of incompatibility as a plausible description for the origin of residual stress. Here we define the geometric features that characterize incompatibility in biological materials. We introduce the geometric incompatibility tensor for different growth types, showing that the constraints associated with growth lead to specific patterns of the incompatibility metrics. To numerically investigate the distribution of incompatibility measures, we implement the analysis within a finite element framework. Simple, illustrative examples are shown first to explain the main concepts. Then, numerical characterization of incompatibility and residual stress is performed on three biomedical applications: brain atrophy, skin expansion, and cortical folding. Our analysis provides new insights into the role of growth in the development of tissue defects and residual stresses. Thus, we anticipate that our work will further motivate additional research to characterize residual stresses in living tissue and their role in development, disease, and clinical intervention.

Annular Dynamics and Leaflet Geometry in Patch Reconstruction of the Posterior Mitral Leaflet After Adding a Flexible Annuloplasty Ring

PURPOSE

Patch reconstruction of the posterior mitral leaflet using small intestinal submucosa extracellular matrix has been successfully performed in a porcine study. The patch reconstruction, however, resulted in non-physiological systolic widening of the mitral annulus, suggesting the need for an annuloplasty ring. The objective was to characterize the impact on annular dynamics and leaflet geometry of adding a flexible annuloplasty ring to the posterior mitral leaflet patch reconstruction.

METHODS

Measurements were performed in an acute 80-kg porcine model, with seven pigs acting as their own controls. The posterior mitral leaflet was reconstructed with a 2-ply small intestinal submucosa extracellular matrix patch (CorMatrix®). Additionally, a Simulus® Flexible Annuloplasty Ring (Medtronic Inc., Minneapolis, MN, USA) was inserted. Mitral annular dynamics were evaluated using sonomicrometry, and leaflet geometry was described using echocardiography.

RESULTS

The annuloplasty ring reduced mitral annular dimensions and restricted cyclic changes in mitral annular area (126 ± 19 vs. 30 ± 13 mm², p < 0.001), septal-lateral and commisure-commisure distances. Ring annuloplasty prevented systolic widening in the mitral annulus after posterior mitral leaflet reconstruction. The annular saddle shape and leaflet coaptation length (8.7 ± 2.3 vs. 9.7 ± 1.3 mm, p = 0.221) were comparable before and after ring insertion.

CONCLUSIONS

The flexible annuloplasty ring resulted in a downsized annulus with restriction of cyclic annular changes in the reconstructed mitral valve. Ring insertion preserved the annular saddle shape and coaptation length. The ring annuloplasty counteracted the non-physiological annular dynamics, and this may improve durability of the posterior mitral leaflet patch reconstruction.

Tricuspid Annuloplasty Rings: A Quantitative Comparison of Size, Nonplanar Shape, and Stiffness

BACKGROUND

Functional tricuspid regurgitation due to annular and ventricular dilatation is increasingly recognized as a significant source of morbidity and mortality. To repair the annulus, surgeons implant one of many annuloplasty devices that differ in size, 3-dimensional (3D) shape, and stiffness. However, there have been no quantitative comparisons between various available devices.

METHODS

Three-dimensional scanning, micro-computed tomography imaging, analytical methods, and mechanical tests were used to compare 3 Edwards Lifesciences (Irvine, CA) and 3 Medtronic (Minneapolis, MN) annuloplasty devices of all available sizes. We measured in-plane metrics of maximum diameter, perimeter, area, height, as well as elevation and curvature profiles. Furthermore, we computed bending stiffness as well as the maximum and minimum axes of the bending stiffness.

RESULTS

Most annular prostheses differed little in their in-plane geometries but varied significantly in height. In-plane properties deviated significantly from measurements of healthy human tricuspid annuli. Height of the Edwards' MC3 and Medtronic's Contour 3D resembled healthy human tricuspid valve annuli, whereas the Edwards' Physio and Classic, and Medtronic's TriAd, did not. Additionally, the elevation profiles of the MC3 and Contour 3D and curvature profiles between all devices were consistent and matched those of healthy human annuli. The tested devices also differed in their bending stiffness, both in terms of absolute values and their maximum and minimum axes.

CONCLUSIONS

Contoured devices, such as Edwards' MC3 and Medtronic's Contour 3D, most accurately resembled the healthy human tricuspid annulus but differed significantly in bending stiffness. To what extent prosthesis properties and shape affect tricuspid valve function remains to be determined.

Geometric description for the anatomy of the mitral valve: A review

The mitral valve is a complex anatomical structure whose physiological functioning relies on the biomechanical properties and structural integrity of its components. Their compromise can lead to mitral valve dysfunction, associated with morbidity and mortality. Therefore, a review on the morphometry of the mitral valve is crucial, more specifically on the importance of valve dimensions and shape for its function. This review initially provides a brief background on the anatomy and physiology of the mitral valve, followed by an analysis of the morphological information available. A characterisation of mathematical descriptions of several parts of the valve is performed and the impact of different dimensions and shape changes in disease is then outlined. Finally, a section regarding future directions and recommendations for the use of morphometric information in clinical analysis of the mitral valve is presented.

Dynamic computed tomographic assessment of the mitral annulus in patients with and without mitral prolapse

OBJECTIVES

To obtain 3D CT measurements of mitral annulus throughout cardiac cycle using prototype mitral modeling software, assess interobserver agreement, and compare among patients with mitral prolapse (MP) and control group.

BACKGROUND

Pre-procedural imaging is critical for planning of transcatheter mitral valve (MV) replacement. However, there is limited data regarding reliable CT-based measurements to accurately characterize the dynamic geometry of the mitral annulus in patients with MV disease.

METHODS

Patients with MP and control subjects without any MV disease who underwent ECG-gated cardiac CT were retrospectively identified. Multiphasic CT data was loaded into a prototype mitral modeling software. Multiple anatomical parameters in 3D space were recorded throughout the cardiac cycle (0-95%): annular circumference, planar-surface-area (PSA), anterior-posterior (A-P) distance, and anterolateral-posteromedial (AL-PM) distance. Comparisons were made among the two groups, with p < 0.05 considered statistically significant. Interobserver agreement was assessed on ten patients using intraclass correlation coefficient (ICC) among 4 experienced readers.

RESULTS

A total of 100 subjects were included: 50 with MP and 50 control. Annular dimensions were significantly higher in the MP group than control group, with circumference (144 ± 11 vs. 117±8 mm), PSA (1533 ± 247 vs. 1005 ± 142 mm²), A-P distance (38 ± 4 vs. 32±2 mm), and AL-PM distance (47 ± 4 vs. 39±3 mm) (all p < 0.001). Substantial size changes were observed throughout the cardiac cycle, but with maximal and minimal sizes at different cardiac phases for the two groups. The interobserver agreement was excellent (ICC≥0.75) for annular circumference, PSA, A-P- and AL-PM distance.

CONCLUSION

A significant variation in the mitral annular measures between different cardiac phases and two groups was observed with excellent interobserver agreement.

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.

A computational model to predict cell traction-mediated prestretch in the mitral valve

Prestretch is observed in many soft biological tissues, directly influencing the mechanical behavior of the tissue in question. The development of this prestretch occurs through complex growth and remodeling phenomena, which yet remain to be elucidated. In the present study it was investigated whether local cell-mediated traction forces can explain the development of global anisotropic tissue prestretch in the mitral valve. Towards this end, a model predicting actin stress fiber-generated traction forces was implemented in a finite element framework of the mitral valve. The overall predicted magnitude of prestretch induced valvular contraction after release of in vivo boundary constraints was in good agreement with data reported on valvular retraction after excision from the heart. Next, by using a systematic variation of model parameters and structural properties, a more anisotropic prestretch development in the valve could be obtained, which was also similar to physiological values. In conclusion, this study shows that cell-generated traction forces could explain prestretch magnitude and anisotropy in the mitral valve.

Novel In Vitro Test Systems and Insights for Transcatheter Mitral Valve Design, Part II: Radial Expansion Forces

Transcatheter mitral valve (TMV) replacement technology has great clinical potential for surgically inoperable patients suffering from mitral regurgitation. An important goal for robust TMV design is maximizing the likelihood of achieving a geometry post-implant that facilitates optimal performance. To support this goal, improved understanding of the annular forces that oppose TMV radial expansion is necessary. In Part II of this study, novel circular and D-shaped Radial Expansion Force Transducers (C-REFT and D-REFT) were developed and employed in porcine hearts (N = 12), to detect the forces required to radially expand the mitral annulus to discrete oversizing levels. Forces on both the septal-lateral and inter-commissural axes (FSL and FIC) scaled with device size. The D-REFT experienced lower FSL than the C-REFT (19.8 ± 7.4 vs. 17.4 ± 10.8 N, p = 0.002) and greater FIC (31.5 ± 14.0 vs. 36.9 ± 16.2 N; p = 0.002), and was more sensitive to degree of oversizing. Across all tests, FIC/FSL was 2.21 ± 1.33, likely reflecting low resistance to radial expansion at the aorto-mitral curtain. In conclusion, the annular forces opposing TMV radial expansion are non-uniform, and depend on final TMV shape and size. Based on this two-part study, we propose that radial force applied at the commissural aspect of the annulus has the most potent effect on paravalvular sealing.

Large animal model of acute right ventricular failure with functional tricuspid regurgitation

BACKGROUND

Functional tricuspid regurgitation (FTR) commonly arises secondary to conditions affecting the left heart and is associated with right ventricular dysfunction and tricuspid annular dilatation. We set out to establish an animal model of acute RV failure (RVF) with FTR resembling the clinical features.

METHODS

Ten adult sheep had pressure sensors placed in the LV, RV, and right atrium while sonomicrometry crystals were implanted around tricuspid annulus and on the RV. Animals were studied open-chest to assess for RV function and FTR after: (1) volume infusion, (2) pulmonary artery constriction, (3) 5 min posterior descending artery occlusion, and (4) combination of all interventions. Hemodynamic, echocardiographic, and sonomicrometry data were collected at baseline and after every intervention. RV dimensions, RV strain, and annular area, perimeter, and size were calculated from crystal coordinates. The model was validated in six additional sheep studied only before and after combined interventions.

RESULTS

Neither volume infusion, pulmonary hypertension, nor ischemia were associated with RVF or clinically significant TR when applied separately but combined resulted in RVF and greater than moderate FTR. In the validation group, maximal RV volume increased (62 ± 14 vs 70 ± 16 ml, p = 0.006), contractility decreased (20 ± 6 vs 12 ± 2%, p = 0.02), and strain increased. FTR increased from 0.4 ± 0.5 to 2.5 ± 0.8 (p < 0.001) and annular area from 652 ± 87 mm² to 739 ± 87 mm² (p = 0.005).

CONCLUSIONS

The developed ovine model of acute RVF was associated with significant annular and RV enlargement and FTR. This novel and clinically pertinent research platform offers insight into the acute RVF pathophysiology and can be utilized to evaluate treatment interventions.

A noninvasive method for the determination of in vivo mitral valve leaflet strains

Assessment of mitral valve (MV) function is important in many diagnostic, prognostic, and surgical planning applications for treatment of MV disease. Yet, to date, there are no accepted noninvasive methods for determination of MV leaflet deformation, which is a critical metric of MV function. In this study, we present a novel, completely noninvasive computational method to estimate MV leaflet in-plane strains from clinical-quality real-time three-dimensional echocardiography (rt-3DE) images. The images were first segmented to produce meshed medial-surface leaflet geometries of the open and closed states. To establish material point correspondence between the two states, an image-based morphing pipeline was implemented within a finite element (FE) modeling framework in which MV closure was simulated by pressurizing the open-state geometry, and local corrective loads were applied to enforce the actual MV closed shape. This resulted in a complete map of local systolic leaflet membrane strains, obtained from the final FE mesh configuration. To validate the method, we utilized an extant in vitro database of fiducially labeled MVs, imaged in conditions mimicking both the healthy and diseased states. Our method estimated local anisotropic in vivo strains with less than 10% error and proved to be robust to changes in boundary conditions similar to those observed in ischemic MV disease. Next, we applied our methodology to ovine MVs imaged in vivo with rt-3DE and compared our results to previously published findings of in vivo MV strains in the same type of animal as measured using surgically sutured fiducial marker arrays. In regions encompassed by fiducial markers, we found no significant differences in circumferential(P = 0.240) or radial (P = 0.808) strain estimates between the marker-based measurements and our novel noninvasive method. This method can thus be used for model validation as well as for studies of MV disease and repair.

Mechanical properties of a new thermally deformable mitral valve annuloplasty ring and its effects on the mitral valve

Ideally, an annuloplasty ring's shape should be changed intraoperatively if mitral valve repair is unsuccessful because of a short coaptation length or systolic anterior motion. Several post-implantation adjustable rings have been developed, but they are not freely deformable and are unsuitable for asymmetric repair of the valvular annulus. We developed a novel thermally deformable mitral annuloplasty ring to address these problems and assessed the ring's mechanical properties and its effect on the mitral valve anatomy. This ring was made of polycaprolactone. Tensile and bending tests were performed to evaluate the ring's mechanical properties. The ratio of the transverse and septal-lateral length was determined as 4:3. Using 10 pig hearts, we measured the post-deformation coaptation length and minimum distance from the coaptation to the ventricular septum, which is a factor of abnormal systolic anterior motion of the mitral valve. In the mechanical tests, the ring's yield point was greater than the deformation force of the annulus in humans. In pigs with deformation from "4:3" to "4:2", the coaptation length was significantly increased in each mitral valve part. In pigs with deformation from "4:3" to "4:4", the minimum distance from the coaptation to the ventricular septum was significantly increased. Asymmetrical ring deformation increased the coaptation length only at the deformed area. In conclusion, this new thermally deformable mitral annuloplasty ring could be "order-made" to effectively change the coaptation length in all parts of the mitral valve and the distance from the coaptation to septum post-deformation via intraoperative heating.

Validation of a Holographic Display for Quantification of Mitral Annular Dynamics by Three-Dimensional Echocardiography

BACKGROUND

Three-dimensional (3D) echocardiography with multiplanar reconstruction (MPR) is used clinically to quantify the mitral annulus. MPR images are, however, presented on a two-dimensional screen, calling into question their accuracy. An alternative to MPR is an autostereoscopic holographic display that enables in-depth visualization of 3D echocardiographic data without the need for special glasses. The aim of this study was to validate an autostereoscopic display using sonomicrometry as a gold standard.

METHODS

In 11 anesthetized open-chest pigs, sonomicrometric crystals were placed along the mitral annulus and near the left ventricular apex. High-fidelity catheters measured left atrial and ventricular pressures. Adjustments of pre- and afterload were done by constriction of the inferior vena cava and the ascending aorta, respectively. Three-dimensional epicardial echocardiography was obtained from an apical view and converted to the autostereoscopic display. A 3D virtual semitransparent annular surface (VSAS) was generated to measure commissure width (CW), septal-lateral length, area of the mitral annular surface, nonplanarity angle, and the annular height-to-commissure width ratio in mid-systole and late diastole.

RESULTS

Mitral annular measurements from the 3D VSAS derived from the 3D echocardiographic images and autostereoscopic display correlated well with sonomicrometry over a range of loading conditions: CW length (r = 0.98, P < .00001), septal-lateral length (r = 0.98, P < .00001), annular surface area (r = 0.93, P < .001), nonplanarity angle (r = 0.87, P < .001), and annular height-to-commissure width ratio (r = 0.85, P < .01). The 3D VSAS showed better agreement with the sonomicrometric measurements compared with MPR.

CONCLUSIONS

Mitral annular measurements using 3D VSAS correlate well with sonomicrometry over a range of loading conditions and may represent a powerful tool for noninvasive quantification of mitral annular dynamics.

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.

Effects of nonvalvular atrial fibrillation on the structure and function of mitral valves (a STROBE-compliant article)

The aim of this study was to explore the effects of nonvalvular atrial fibrillation (NVAF) on the structure and function of mitral valve and analyze independent risk factors of moderate to severe mitral regurgitation (MR) by quantitative measurement of mitral parameters using real-time 3-dimensional transesophageal echocardiography.This study included 30 subjects with sinus rhythm group, and 65 patients with NVAF. The 65 patients with NVAF were divided into 35 with paroxysmal atrial fibrillation group and 30 with persistent atrial fibrillation. According to MR degree, the patients with NVAF were again divided into no or mild MR group (n = 44) and moderate to severe MR group (n = 21).There were significant differences in anterolateral-to-posteromedial diameter (DAlPm), anterior-to-posterior diameter, 3-dimensional circumference (C3D), 2-dimensional area (A2D), mitral leaflet surface area in late systolic phase, the index of mitral valve coaptation and left atrial internal diameter (LAID) between different cardiac rhythm groups (all P < .05). The DAlPm, C3D, A2D, nonplanar angle (θNPA), and LAID were greater but the mitral valve coaptation index was smaller in the moderate to severe MR group than in the no or mild MR group (all P < .05). Logistic regression analysis indicated that DAlPm and LAID were independent risk factors of moderate to severe MR in the patients with NVAF (OR > 1, P < .05).DAlPm and LAID are independent risk factors of moderate to severe MR in the patients with NVAF. NVAF can change the structure and function of mitral valve, which leads to MR.

Mitral annuloplasty ring flexibility preferentially reduces posterior suture forces

Annuloplasty ring repair is a common procedure for the correction of mitral valve regurgitation. Commercially available rings vary in dimensions and material properties. Annuloplasty ring suture dehiscence from the native annulus is a catastrophic yet poorly understood phenomenon that has been reported across ring types. Recognizing that sutures typically dehisce from the structurally weaker posterior annulus, our group is conducting a multi-part study in search of ring design parameters that influence forces acting on posterior annular sutures in the beating heart. Herein, we report the effect of ring rigidity on suture forces. Measurements utilized custom force sensors, attached to annuloplasty rings and implanted in normal ovine subjects via standard surgical procedure. Tested rings included the semi-rigid Physio (Edwards Lifesciences) and rigid and flexible prototypes of matching geometry. While no significant differences due to ring stiffness existed for sutures in the anterior region, posterior forces were significantly reduced with use of the flexible ring (rigid: 1.95 ± 0.96 N, semi-rigid: 1.76 ± 1.19 N, flexible: 1.04 ± 0.63 N; p < 0.001). The ratio of anterior to posterior F_C scaled positively with increasing flexibility (p < 0.001), and posterior forces took more time to reach their peak load when a flexible ring was used (p < 0.001). This suggests a more rigid ring enables more rapid/complete force equilibration around the suture network, transferring higher anterior forces to the weaker posterior tissue. For mitral annuloplasties requiring ring rigidity, we propose a ring design concept to potentially disrupt this force transfer and improve suture retention.

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.

Assessment of mitral valve geometric deformity in patients with ischemic heart disease using three-dimensional echocardiography

BACKGROUND

A full understanding of the geometry of the nonplanar saddle-shaped mitral annulus can provide valuable information regarding the pathophysiology of mitral regurgitation (MR).

AIM OF THE WORK

To investigate mitral annular geometric deformities using three-dimensional echocardiography among patients with ischemic coronary illness with and without mitral regurgitation.

METHODS

Three-dimensional transesophageal echocardiographic data were acquired intraoperatively from patients with ischemic heart disease with or without associated mitral regurgitation who experienced coronary artery bypass grafting and normal control subjects. The mitral annulus was analyzed for differences in geometry using QLAB software.

RESULTS

Left ventricular ejection fraction was reduced in patients with ischemic heart disease and MR (n = 21; Group 1) and without MR (n = 7; Group 2) compared with that in normal subjects (n = 14; Group 3) (43.4% ± 11.8% and 35.9% ± 13.6% vs. 52.6% ± 9.3%, respectively; p = 0.015). Mitral annular height and mitral annular saddle-shaped nonplanarity were significantly lower in Group 1 compared to Group 2 and Group 3 (6.00 ± 1.07 mm, 7.96 ± 0.93 mm and 8.31 ± 1.12 mm; p < 0.0001) and (0.19 ± 0.04, 0.26 ± 0.04 and 0.26 ± 0.03; p < 0.0001) respectively while mitral annular ellipsicity and Mitral valve tenting volume were significantly higher in the same group (1) (114.82% ± 22.47%, 100.21% ± 9.87% and 97.29% ± 14.37%; p = 0.0421) and (2.73 ± 1.11, 2.20 ± 1.39 and 0.87 ± 0.67) respectively. Vena contracta diameter was inversely correlated with the mitral annular height (r = -0.82; p < 0.0001) and saddle-shaped nonplanarity of the annulus (r = -0.68; p < 0.0001).

CONCLUSION

Among patients with ischemic heart disease, there are significant increases in mitral valve tenting volume and height, and those with mitral regurgitation exhibited a reduced mitral annular height, a shallower saddle shape annulus and losses of ellipsicity of the annulus.

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.

Role of Mitral Annulus Diastolic Geometry on Intraventricular Filling Dynamics

The mitral valve (MV) is a bileaflet valve positioned between the left atrium and ventricle of the heart. The annulus of the MV has been observed to undergo geometric changes during the cardiac cycle, transforming from a saddle D-shape during systole to a flat (and less eccentric) D-shape during diastole. Prosthetic MV devices, including heart valves and annuloplasty rings, are designed based on these two configurations, with the circular design of some prosthetic heart valves (PHVs) being an approximation of the less eccentric, flat D-shape. Characterizing the effects of these geometrical variations on the filling efficiency of the left ventricle (LV) is required to understand why the flat D-shaped annulus is observed in the native MV during diastole in addition to optimizing the design of prosthetic devices. We hypothesize that the D-shaped annulus reduces energy loss during ventricular filling. An experimental left heart simulator (LHS) consisting of a flexible-walled LV physical model was used to characterize the filling efficiency of the two mitral annular geometries. The strength of the dominant vortical structure formed and the energy dissipation rate (EDR) of the measured fields, during the diastolic period of the cardiac cycle, were used as metrics to quantify the filling efficiency. Our results indicated that the O-shaped annulus generates a stronger (25% relative to the D-shaped annulus) vortical structure than that of the D-shaped annulus. It was also found that the O-shaped annulus resulted in higher EDR values throughout the diastolic period of the cardiac cycle. The results support the hypothesis that a D-shaped mitral annulus reduces dissipative energy losses in ventricular filling during diastole and in turn suggests that a symmetric stent design does not provide lower filling efficiency than an equivalent asymmetric design.

Multi-scale Modeling of the Cardiovascular System: Disease Development, Progression, and Clinical Intervention

Cardiovascular diseases (CVDs) are the leading cause of death in the western world. With the current development of clinical diagnostics to more accurately measure the extent and specifics of CVDs, a laudable goal is a better understanding of the structure-function relation in the cardiovascular system. Much of this fundamental understanding comes from the development and study of models that integrate biology, medicine, imaging, and biomechanics. Information from these models provides guidance for developing diagnostics, and implementation of these diagnostics to the clinical setting, in turn, provides data for refining the models. In this review, we introduce multi-scale and multi-physical models for understanding disease development, progression, and designing clinical interventions. We begin with multi-scale models of cardiac electrophysiology and mechanics for diagnosis, clinical decision support, personalized and precision medicine in cardiology with examples in arrhythmia and heart failure. We then introduce computational models of vasculature mechanics and associated mechanical forces for understanding vascular disease progression, designing clinical interventions, and elucidating mechanisms that underlie diverse vascular conditions. We conclude with a discussion of barriers that must be overcome to provide enhanced insights, predictions, and decisions in pre-clinical and clinical applications.

How Local Annular Force and Collagen Density Govern Mitral Annuloplasty Ring Dehiscence Risk

BACKGROUND

Annuloplasty ring dehiscence is a well described mode of mitral valve repair failure. Defining the mechanisms underlying dehiscence may facilitate its prevention.

METHODS

Factors that govern suture dehiscence were examined with an ovine model. After undersized ring annuloplasty in live animals (n = 5), cyclic force (FC) that acts on sutures during cardiac contraction was measured with custom transducers. FC was measured at ten suture positions, throughout cardiac cycles with peak left ventricular pressure (LVPmax) of 100, 125, and 150 mm Hg. Suture pullout testing was conducted on explanted mitral annuli (n = 12) to determine suture holding strength at each position. Finally, relative collagen density differences at suture sites around the annulus were assessed by two-photon excitation fluoroscopy.

RESULTS

Anterior FC exceeded posterior FC at each LVPmax (eg, 2.8 ± 1.3 N versus 1.8 ± 1.2 N at LVPmax = 125 mm Hg, p < 0.01). Anterior holding strength exceeded posterior holding strength (6.4 ± 3.6 N versus 3.9 ± 1.6 N, p < 0.0001). On the basis of FC at LVPmax of 150 mm Hg, margin of safety before suture pullout was vastly higher between the trigones (exclusive) versus elsewhere (4.8 ± 0.9 N versus 1.9 ± 0.5 N, p < 0.001). Margin of safety exhibited strong correlation to collagen density (R(2) = 0.947).

CONCLUSIONS

Despite lower cyclic loading on posterior sutures, the weaker posterior mitral annular tissue creates higher risk of dehiscence, apparently because of reduced collagen content. Sutures placed atop the trigones are less secure than predicted, because of a combination of reduced collagen and higher overall rigidity in this region. These findings highlight the inter-trigonal tissue as the superior anchor and have implications on the design and implantation techniques for next-generation mitral prostheses.

Comparison of saddle-shape flexibility and elliptical-shape stability between Cosgrove-Edwards and Memo-3D annuloplasty rings using three-dimensional analysis software

OBJECTIVE

To compare three-dimensional dynamics between implanted Cosgrove-Edwards and Sorin Memo-3D annuloplasty rings during the cardiac cycle.

METHODS

We examined 11 Cosgrove-Edwards rings and 20 Sorin Memo-3D rings after mitral plasty using real-time three-dimensional transesophageal echocardiography. We evaluated ring height, ellipticity, and geometry during one cardiac cycle. Four evenly spaced phases each selected during systole and diastole were assessed using REAL VIEW software.

RESULTS

The height of the Cosgrove-Edwards and Sorin Memo-3D rings was similar (2.3 ± 0.8 vs. 1.9 ± 0.9 mm, p = 0.44). The maximum difference in ring height during one cardiac cycle (change in height) was larger for the Cosgrove-Edwards than the Sorin Memo-3D rings (2.3 ± 0.8 vs. 1.5 ± 0.6 mm, p = 0.014). Ellipticity and the maximum difference in ellipticity during one cardiac cycle (change in ellipticity) were larger for Cosgrove-Edwards than Sorin Memo-3D rings (80.0 ± 9.1 vs. 72.0 ± 4.8 %, p = 0.014, respectively, and 12.0 ± 3.1 vs. 6.0 ± 1.8 %, p < 0.001).

CONCLUSIONS

Cosgrove-Edwards rings were more flexible, whereas Sorin Memo-3D rings maintained the elliptical shape more effectively.

Mitral Annular Kinetics, Left Atrial, and Left Ventricular Diastolic Function Post Mitral Valve Repair in Degenerative Mitral Regurgitation

Objective

The relationship of mitral annular (MA) kinetics to left ventricular (LV) and left atrial (LA) function before and after mitral valve (MV) repair has not been well studied. Here we sought to provide comprehensive analysis that relates to MA motions, and LA and LV diastolic function post MV repair.

Methods

Three-dimensional analyses of mitral annular motion, LA function, and LV volumetric and diastolic strain rates were performed on 35 degenerative mitral regurgitation (MR) patients at baseline and 1-year post MV repair, and 51 normal controls, utilizing cardiac magnetic resonance imaging with tissue tagging.

Results

All had normal LV ejection fraction (EF) at baseline. LV and LA EFs decreased 1-year post-surgery vs. controls. LV early diastolic myocardial strain rates decreased post-surgery along with decreases in normalized early diastolic filling rate, E/A ratio, and early diastolic MA relaxation rates. Post-surgical LA late active kick remained higher in MR patients vs. control. LV and LA EFs were significantly associated with peak MA centroid to apex shortening. Furthermore, during LV systolic phase, peak LV ejection and LA filling rates were significantly correlated with peak MA centroid to apex shortening rate, respectively. While during LV diastolic phase, both peak early diastolic MA centroid to apex relaxation rate and LA ejection rate were positively significantly associated with LV peak early diastolic filling rate.

Conclusion

MA motion is significantly associated with LA and LV function. Mitral annular motion, left atrial function, and LV diastolic strain rates are still impaired 1 year post MV repair. Long-term effects of these impairments should be prospectively evaluated.

Mitral valve analysis adding a virtual semi-transparent annulus plane for detection of prolapsing segments

Background

We hypothesized that a novel three-dimensional virtual semi-transparent annulus plane (3D VSAP) presented on a holographic screen can be used to visualize the prolapsing tissue in degenerative mitral valve disease and furthermore, provide us with geometrical data of the mitral valve apparatus. Phantom and patient studies were designed to demonstrate the feasibility of creating a semi-automatic, semi-transparent mitral annulus plane visualized on a holographic display.

Methods

Ten pipe cleaners mimicking the mitral annulus with different shapes and three types of annuloplasty rings served as phantoms. We obtained 3D transoesophageal examination of the phantoms in a special designed box filled with water. Recordings were converted to the holographic display and a 3D VSAP was created. The ratio of the major and minor axes as well as the non-planar angles were calculated and compared with direct measures of the phantoms. Forty patients with degenerative mitral valve disease were then analyzed with 3D transthoracic echocardiography (TTE) and a 3D VSAP was created on the holographic display. A total of 240 segments were analyzed by two independent observers, one echo expert (observer I), and the other novice with limited echo experience (observer II). The two observers created the 3D VSAP in each patient before suggesting the valve pathology.

Results

The major/minor axes ratio and non-planar angles by 3D VSAP correlated with direct measurements by r = 0.65, p < 0.02 and r = 0.99, p < 0.0001, respectively. The sensitivity and specificity of the 3D VSAP method in patients was 81 and 97 %, respectively (observer I) and for observer II 77 and 96 %, respectively. The accuracy and precisions were 93.9 and 89.4 %, respectively (observer I), 92.3 and 85.1 % (observer II). Mitral valve analysis adding a 3D VSAP was feasible with high accuracy and precision, providing a quick and less subjective method for diagnosing mitral valve prolapse. This novel method may improve preoperative diagnostics and may relieve a better understanding of the pathophysiology of mitral valve disease. Thus, based on the specific findings in each patient, a tailored surgical repair can be planned and hopefully enhance long-term repair patency in the future.

In vitro comparison of Doppler and catheter-measured pressure gradients in 3D models of mitral valve calcification

Mitral annular calcification (MAC) involves calcium deposition in the fibrous annulus supporting the mitral valve (MV). When calcification extends onto the leaflets, valve opening can be restricted. The influence of MAC MV geometry on Doppler gradients is unknown. This study describes a novel methodology to rapid-prototype subject-specific MAC MVs. Replicated valves were used to assess the effects of distorted annular-leaflet geometry on Doppler-derived, transmitral gradients in comparison to direct pressure measurements and to determine if transmitral gradients vary according to measurement location. Three-dimensional echocardiography data sets were selected for two MAC MVs and one healthy MV. These MVs were segmented and rapid prototyped in their middiastolic configuration for in vitro testing. The effects of MV geometry, measurement modality, and measurement location on transmitral pressure gradient were assessed by Doppler and catheter at three locations along the MV's intercommissural axis. When comparing dimensions of the rapid-prototyped valves to the subject echocardiography data sets, mean relative errors ranged from 6.2% to 35%. For the evaluated MVs, Doppler pressure gradients exhibited good agreement with catheter-measured gradients at a variety of flow rates, though with slight systematic overestimation in the recreated MAC valves. For all of the tested MVs, measuring the transmitral pressure gradient at differing valve orifice positions had minimal impact on observed gradients. Upon the testing of additional normal and calcific MVs, these data may contribute to an improved clinical understanding of MAC-related mitral stenosis. Moreover, they provide the ability to statistically evaluate between measurement locations, flow rates, and valve geometries for Doppler-derived pressure gradients. Determining these end points will contribute to greater clinical understanding for the diagnosis MAC patients and understanding the use and application of Doppler echocardiography to estimate transmitral pressure gradients.

Isolated effect of geometry on mitral valve function for in silico model development

Computational models for the heart's mitral valve (MV) exhibit several uncertainties that may be reduced by further developing these models using ground-truth data-sets. This study generated a ground-truth data-set by quantifying the effects of isolated mitral annular flattening, symmetric annular dilatation, symmetric papillary muscle (PM) displacement and asymmetric PM displacement on leaflet coaptation, mitral regurgitation (MR) and anterior leaflet strain. MVs were mounted in an in vitro left heart simulator and tested under pulsatile haemodynamics. Mitral leaflet coaptation length, coaptation depth, tenting area, MR volume, MR jet direction and anterior leaflet strain in the radial and circumferential directions were successfully quantified at increasing levels of geometric distortion. From these data, increase in the levels of isolated PM displacement resulted in the greatest mean change in coaptation depth (70% increase), tenting area (150% increase) and radial leaflet strain (37% increase) while annular dilatation resulted in the largest mean change in coaptation length (50% decrease) and regurgitation volume (134% increase). Regurgitant jets were centrally located for symmetric annular dilatation and symmetric PM displacement. Asymmetric PM displacement resulted in asymmetrically directed jets. Peak changes in anterior leaflet strain in the circumferential direction were smaller and exhibited non-significant differences across the tested conditions. When used together, this ground-truth data-set may be used to parametrically evaluate and develop modelling assumptions for both the MV leaflets and subvalvular apparatus. This novel data may improve MV computational models and provide a platform for the development of future surgical planning tools.

On the effect of prestrain and residual stress in thin biological membranes

Understanding the difference between ex vivo and in vivo measurements is critical to interpret the load carrying mechanisms of living biological systems. For the past four decades, the ex vivo stiffness of thin biological membranes has been characterized using uniaxial and biaxial tests with remarkably consistent stiffness parameters, even across different species. Recently, the in vivo stiffness was characterized using combined imaging techniques and inverse finite element analyses. Surprisingly, ex vivo and in vivo stiffness values differed by up to three orders of magnitude. Here, for the first time, we explain this tremendous discrepancy using the concept of prestrain. We illustrate the mathematical modeling of prestrain in nonlinear continuum mechanics through the multiplicative decomposition of the total elastic deformation into prestrain-induced and load-induced parts. Using in vivo measured membrane kinematics and associated pressure recordings, we perform an inverse finite element analysis for different prestrain levels and show that the resulting membrane stiffness may indeed differ by four orders of magnitude depending on the prestrain level. Our study motivates the hypothesis that prestrain is important to position thin biological membranes in vivo into their optimal operating range, right at the transition point of the stiffening regime. Understanding the effect of prestrain has direct clinical implications in regenerative medicine, medical device design, and and tissue engineering of replacement constructs for thin biological membranes.

Mechanics of healthy and functionally diseased mitral valves: a critical review

The mitral valve is a complex apparatus with multiple constituents that work cohesively to ensure unidirectional flow between the left atrium and ventricle. Disruption to any or all of the components-the annulus, leaflets, chordae, and papillary muscles-can lead to backflow of blood, or regurgitation, into the left atrium, which deleteriously effects patient health. Through the years, a myriad of surgical repairs have been proposed; however, a careful appreciation for the underlying structural mechanics can help optimize long-term repair durability and inform medical device design. In this review, we aim to present the experimental methods and significant results that have shaped the current understanding of mitral valve mechanics. Data will be presented for all components of the mitral valve apparatus in control, pathological, and repaired conditions from human, animal, and in vitro studies. Finally, current strategies of patient specific and noninvasive surgical planning will be critically outlined.

Mechanics of the mitral annulus in chronic ischemic cardiomyopathy

Approximately one third of all patients undergoing open-heart surgery for repair of ischemic mitral regurgitation present with residual and recurrent mitral valve leakage upon follow up. A fundamental quantitative understanding of mitral valve remodeling following myocardial infarction may hold the key to improved medical devices and better treatment outcomes. Here we quantify mitral annular strains and curvature in nine sheep 5 ± 1 weeks after controlled inferior myocardial infarction of the left ventricle. We complement our marker-based mechanical analysis of the remodeling mitral valve by common clinical measures of annular geometry before and after the infarct. After 5 ± 1 weeks, the mitral annulus dilated in septal-lateral direction by 15.2% (p = 0.003) and in commissure-commissure direction by 14.2% (p < 0.001). The septal annulus dilated by 10.4% (p = 0.013) and the lateral annulus dilated by 18.4% (p < 0.001). Remarkably, in animals with large degree of mitral regurgitation and annular remodeling, the annulus dilated asymmetrically with larger distortions toward the lateral-posterior segment. Strain analysis revealed average tensile strains of 25% over most of the annulus with exception for the lateral-posterior segment, where tensile strains were 50% and higher. Annular dilation and peak strains were closely correlated to the degree of mitral regurgitation. A complementary relative curvature analysis revealed a homogenous curvature decrease associated with significant annular circularization. All curvature profiles displayed distinct points of peak curvature disturbing the overall homogenous pattern. These hinge points may be the mechanistic origin for the asymmetric annular deformation following inferior myocardial infarction. In the future, this new insight into the mechanism of asymmetric annular dilation may support improved device designs and possibly aid surgeons in reconstructing healthy annular geometry during mitral valve repair.