Effect of adjustable passive constraint on the failing left ventricle: a finite-element model study

Optimizing Epicardial Restraint and Reinforcement Following Myocardial Infarction: Moving Towards Localized, Biomimetic, and Multitherapeutic Options

The mechanical reinforcement of the ventricular wall after a myocardial infarction has been shown to modulate and attenuate negative remodeling that can lead to heart failure. Strategies include wraps, meshes, cardiac patches, or fluid-filled bladders. Here, we review the literature describing these strategies in the two broad categories of global restraint and local reinforcement. We further subdivide the global restraint category into biventricular and univentricular support. We discuss efforts to optimize devices in each of these categories, particularly in the last five years. These include adding functionality, biomimicry, and adjustability. We also discuss computational models of these strategies, and how they can be used to predict the reduction of stresses in the heart muscle wall. We discuss the range of timing of intervention that has been reported. Finally, we give a perspective on how novel fabrication technologies, imaging techniques, and computational models could potentially enhance these therapeutic strategies.

Residual Stress Impairs Pump Function After Surgical Ventricular Remodeling: A Finite Element Analysis

BACKGROUND

Surgical ventricular restoration (Dor procedure) is generally thought to reduce left ventricular (LV) myofiber stress (FS) but to adversely affect pump function. However, the underlying mechanism is unclear. The goal of this study was to determine the effect of residual stress (RS) on LV FS and pump function after the Dor procedure.

METHODS

Previously described finite element models of the LV based on magnetic resonance imaging data obtained in 5 sheep 16 weeks after anteroapical myocardial infarction were used. Simulated polyethylene terephthalate fiber (Dacron) patches that were elliptical and 25% of the infarct opening area were implanted using a virtual suture technique (VIRTUAL-DOR). In each case, diastole and systole were simulated, and RS, FS, LV volumes, systolic and diastolic function, and pump (Starling) function were calculated.

RESULTS

VIRTUAL-DOR was associated with significant RS that was tensile (2.89 ± 1.31 kPa) in the remote myocardium and compressive (234.15 ± 65.53 kPa) in the border zone. VIRTUAL-DOR+RS (compared with VIRTUAL-DOR-NO-RS) was associated with further reduction in regional diastolic and systolic FS, with the greatest change in the border zone (43.5-fold and 7.1-fold, respectively; p < 0.0001). VIRTUAL-DOR+RS was also associated with further reduction in systolic and diastolic volumes (7.9%; p = 0.0606, and 10.6%; p = 0.0630, respectively). The resultant effect was a further reduction in pump function after VIRTUAL-DOR+RS.

CONCLUSIONS

Residual stress that occurs after the Dor procedure is positive (tensile) in the remote myocardium and negative (compressive) in the border zone and associated with reductions in FS and LV volumes. The resultant effect is a further reduction in LV pump (Starling) function.

Numerical evaluation of myofiber orientation and transmural contractile strength on left ventricular function

The left ventricle (LV) of the heart is composed of a complex organization of cardiac muscle fibers, which contract to generate force and pump blood into the body. It has been shown that both the orientation and contractile strength of these myofibers vary across the ventricular wall. The hypothesis of the current study is that the transmural distributions of myofiber orientation and contractile strength interdependently impact LV pump function. In order to quantify these interactions a finite element (FE) model of the LV was generated, which incorporated transmural variations. The influences of myofiber orientation and contractile strength on the Starling relationship and the end-systolic (ES) apex twist of the LV were assessed. The results suggest that reductions in contractile strength within a specific transmural layer amplified the effects of altered myofiber orientation in the same layer, causing greater changes in stroke volume (SV). Furthermore, when the epicardial myofibers contracted the strongest, the twist of the LV apex was greatest, regardless of myofiber orientation. These results demonstrate the important role of transmural distribution of myocardial contractile strength and its interplay with myofiber orientation. The coupling between these two physiologic parameters could play a critical role in the progression of heart failure.

Continuous flow left ventricular pump support and its effect on regional left ventricular wall stress: finite element analysis study

Left ventricular assist device (LVAD) support unloads left ventricular (LV) pressure and volume and decreases wall stress. This study investigated the effect of systematic LVAD unloading on the 3-dimensional myocardial wall stress by employing finite element models containing layered fiber structure, active contractility, and passive stiffness. The HeartMate II(®) (Thoratec, Inc., Pleasanton, CA) was used for LV unloading. The model geometries and hemodynamic conditions for baseline (BL) and LVAD support (LVsupport) were acquired from the Penn State mock circulatory cardiac simulator. Myocardial wall stress of BL was compared with that of LVsupport at 8,000, 9,000, 10,000 RPM, providing mean pump flow (Q(mean)) of 2.6, 3.2, and 3.7 l/min, respectively. LVAD support was more effective at unloading during diastole as compared to systole. Approximately 40, 50, and 60% of end-diastolic wall stress reduction were achieved at Q(mean) of 2.6, 3.2, and 3.7 l/min, respectively, as compared to only a 10% reduction of end-systolic wall stress at Q(mean) of 3.7 l/min. In addition, there was a stress concentration during systole at the apex due to the cannulation and reduced boundary motion. This modeling study can be used to further understand optimal unloading, pump control, patient management, and cannula design.

Distribution of normal human left ventricular myofiber stress at end diastole and end systole: a target for in silico design of heart failure treatments

Ventricular wall stress is believed to be responsible for many physical mechanisms taking place in the human heart, including ventricular remodeling, which is frequently associated with heart failure. Therefore, normalization of ventricular wall stress is the cornerstone of many existing and new treatments for heart failure. In this paper, we sought to construct reference maps of normal ventricular wall stress in humans that could be used as a target for in silico optimization studies of existing and potential new treatments for heart failure. To do so, we constructed personalized computational models of the left ventricles of five normal human subjects using magnetic resonance images and the finite-element method. These models were calibrated using left ventricular volume data extracted from magnetic resonance imaging (MRI) and validated through comparison with strain measurements from tagged MRI (950 ± 170 strain comparisons/subject). The calibrated passive material parameter values were C0 = 0.115 ± 0.008 kPa and B0 = 14.4 ± 3.18; the active material parameter value was Tmax = 143 ± 11.1 kPa. These values could serve as a reference for future construction of normal human left ventricular computational models. The differences between the predicted and the measured circumferential and longitudinal strains in each subject were 3.4 ± 6.3 and 0.5 ± 5.9%, respectively. The predicted end-diastolic and end-systolic myofiber stress fields for the five subjects were 2.21 ± 0.58 and 16.54 ± 4.73 kPa, respectively. Thus these stresses could serve as targets for in silico design of heart failure treatments.

Applications of computational modeling in cardiac surgery

Although computational modeling is common in many areas of science and engineering, only recently have advances in experimental techniques and medical imaging allowed this tool to be applied in cardiac surgery. Despite its infancy in cardiac surgery, computational modeling has been useful in calculating the effects of clinical devices and surgical procedures. In this review, we present several examples that demonstrate the capabilities of computational cardiac modeling in cardiac surgery. Specifically, we demonstrate its ability to simulate surgery, predict myofiber stress and pump function, and quantify changes to regional myocardial material properties. In addition, issues that would need to be resolved in order for computational modeling to play a greater role in cardiac surgery are discussed.

Modulation of Diastolic Filling Using an Epicardial Diastolic Recoil Device

Diastolic dysfunction likely contributes to all cases of congestive heart failure and is solely responsible for many. Existing cardiac support devices largely ignore diastolic dysfunction and may exacerbate it. Current diastolic devices in development rely on either extensive extraventricular fixation or intraventricular implantation with complications associated with blood contact. A diastolic recoil device is proposed that pneumatically locks to the outside of the heart wall. The end-diastolic total biventricular pressure-volume relationship (EDTBPVR) was used to evaluate, in vitro, the ability of a recoil device to modulate filling mechanics through pneumatic locking as the method of fixation. The pressure in a model heart was incremented and the corresponding volume changes were measured. The heart model and device were pneumatically locked together using a vacuum sac to model the pericardium. The diastolic recoil component shifted the EDTBPVR towards lower pressures at low volumes, providing up to 0.9 kPa (9 cm H2O) of suction, demonstrating enhanced diastolic recoil at beginning diastole. We conclude that pneumatic locking appears to be a viable method for a recoil device to engage the heart.

Biventricular finite element modeling of the Acorn CorCap Cardiac Support Device on a failing heart

BACKGROUND

The Acorn CorCap Cardiac Support Device (CSD; Acorn Cardiovascular Inc, St. Paul, MN) is a woven polyester jacket that is placed around the heart and designed to reverse the progressive remodeling associated with dilated cardiomyopathy. However, the effects of the Acorn CSD on myofiber stress and ventricular function remain unknown. We tested the hypothesis that the Acorn CSD reduces end-diastolic (ED) myofiber stress.

METHODS

A previously described weakly coupled biventricular finite element (FE) model and circulatory model based on magnetic resonance images of a dog with dilated cardiomyopathy was used. Virtual applications of the CSD alone (Acorn), CSD with rotated fabric fiber orientation (rotated), CSD with 5% prestretch (tight), and CSD wrapped only around the left ventricle (LV; LV-only) were performed, and the effect on myofiber stress at ED and pump function was calculated.

RESULTS

The Acorn CSD has a large effect on ED myofiber stress in the LV free wall, with reductions of 55%, 79%, 92%, and 40% in the Acorn, rotated, tight, and LV-only cases, respectively. However, there is a tradeoff in which the Acorn CSD reduces stroke volume at LV end-diastolic pressure of 8 mm Hg by 23%, 25%, 30%, and 7%, respectively, in the Acorn, rotated, tight, and LV-only cases.

CONCLUSIONS

The Acorn CSD significantly reduces ED myofiber stress. However, CSD wrapped only around the LV was the only case with minimal negative effect on pump function. Findings suggest that LV-only CSD and Acorn fabric orientation should be optimized to allow maximal myofiber stress reduction with minimal reduction in pump function.

Algisyl-LVR™ with coronary artery bypass grafting reduces left ventricular wall stress and improves function in the failing human heart

BACKGROUND

Left ventricular (LV) wall stress reduction is a cornerstone in treating heart failure. Large animal models and computer simulations indicate that adding non-contractile material to the damaged LV wall can potentially reduce myofiber stress. We sought to quantify the effects of a novel implantable hydrogel (Algisyl-LVR™) treatment in combination with coronary artery bypass grafting (i.e. Algisyl-LVR™+CABG) on both LV function and wall stress in heart failure patients.

METHODS AND RESULTS

Magnetic resonance images obtained before treatment (n=3), and at 3 months (n=3) and 6 months (n=2) afterwards were used to reconstruct the LV geometry. Cardiac function was quantified using end-diastolic volume (EDV), end-systolic volume (ESV), regional wall thickness, sphericity index and regional myofiber stress computed using validated mathematical modeling. The LV became more ellipsoidal after treatment, and both EDV and ESV decreased substantially 3 months after treatment in all patients; EDV decreased from 264 ± 91 ml to 146 ± 86 ml and ESV decreased from 184 ± 85 ml to 86 ± 76 ml. Ejection fraction increased from 32 ± 8% to 47 ± 18% during that period. Volumetric-averaged wall thickness increased in all patients, from 1.06 ± 0.21 cm (baseline) to 1.3 ± 0.26 cm (3 months). These changes were accompanied by about a 35% decrease in myofiber stress at end-of-diastole and at end-of-systole. Post-treatment myofiber stress became more uniform in the LV.

CONCLUSIONS

These results support the novel concept that Algisyl-LVR™+CABG treatment leads to decreased myofiber stress, restored LV geometry and improved function.

Optimized local infarct restraint improves left ventricular function and limits remodeling

BACKGROUND

Preventing expansion and dyskinetic movement of a myocardial infarction (MI) can limit left ventricular (LV) remodeling. Using a device designed to produce variable alteration of infarct stiffness and geometry, we sought to understand how these parameters affect LV function and remodeling early after MI.

METHODS

Ten pigs had posterolateral infarctions. An unexpanded device was placed in 5 animals at the time of infarction and 5 animals served as untreated controls. One week after MI animals underwent magnetic resonance imaging to assess LV size and regional function. In the treatment group, after initial imaging, the device was expanded with 2, 4, 6, 8, and 10 mL of saline. The optimal degree of inflation was defined as that which maximized stroke volume (SV). The device was left optimally inflated in the treatment animals for 3 additional weeks.

RESULTS

One week after MI, device inflation to 6 mL or greater significantly (p < 0.05) decreased end-systolic volume (0 mL, 59.9 mL ± 3.8; 6 mL, 54.0 mL ± 3.1; 8 mL, 50.5 mL ± 4.8; and 10 mL, 46.1 mL ± 2.2) and increased ejection fraction (EF) (0 mL, 0.346 ± 0.016; 6 mL, 0.0397 ± 0.009; 8 mL, 0.431 ± 0.027; and 10 mL, 0.441 ± 0.009). Systolic volume significantly (p < 0.05) improved for the 6 mL and 8 mL volumes (0 mL, 31.2 mL ± 2.6; 6 mL, 35.7 mL ± 2.0; and 8 mL, 37.5 mL ± 1.9) but trended downward for 10 mL (36.6 mL ± 2.8). At 4 weeks after MI, end-diastolic volume and end-systolic volume were unchanged from 1-week values in the treatment group while the control group continued to dilate. Systolic volume (38.2 ± 4.4 mL vs 34.0.1 ± 4.8 mL, p = 0.08) and EF (0.360 ± 0.026 vs 0.276 ± 0.014, p = 0.04) were also better in the treatment animals.

CONCLUSIONS

Optimized isolated infarct restraint can limit adverse LV remodeling after MI. The tested device affords the potential for a patient-specific therapy to preserve cardiac function after MI.

Growth and remodeling of the left ventricle: A case study of myocardial infarction and surgical ventricular restoration

Cardiac growth and remodeling in the form of chamber dilation and wall thinning are typical hallmarks of infarct-induced heart failure. Over time, the infarct region stiffens, the remaining muscle takes over function, and the chamber weakens and dilates. Current therapies seek to attenuate these effects by removing the infarct region or by providing structural support to the ventricular wall. However, the underlying mechanisms of these therapies are unclear, and the results remain suboptimal. Here we show that myocardial infarction induces pronounced regional and transmural variations in cardiac form. We introduce a mechanistic growth model capable of predicting structural alterations in response to mechanical overload. Under a uniform loading, this model predicts non-uniform growth. Using this model, we simulate growth in a patient-specific left ventricle. We compare two cases, growth in an infarcted heart, pre-operative, and growth in the same heart, after the infarct was surgically excluded, post-operative. Our results suggest that removing the infarct and creating a left ventricle with homogeneous mechanical properties does not necessarily reduce the driving forces for growth and remodeling. These preliminary findings agree conceptually with clinical observations.

Patient-specific finite element-based analysis of ventricular myofiber stress after Coapsys: importance of residual stress

BACKGROUND

We sought to determine regional myofiber stress after Coapsys device (Myocor, Inc, Maple Grove, MN) implantation using a finite element model of the left ventricle (LV). Chronic ischemic mitral regurgitation is caused by LV remodeling after posterolateral myocardial infarction. The Coapsys device consists of a single trans-LV chord placed below the mitral valve such that when tensioned it alters LV shape and decreases chronic ischemic mitral regurgitation.

METHODS

Finite element models of the LV were based on magnetic resonance images obtained before (preoperatively) and after (postoperatively) coronary artery bypass grafting with Coapsys implantation in a single patient. To determine the effect of Coapsys and LV before stress, virtual Coapsys was performed on the preoperative model. Diastolic and systolic material variables in the preoperative, postoperative, and virtual Coapsys models were adjusted so that model LV volume agreed with magnetic resonance imaging data. Chronic ischemic mitral regurgitation was abolished in the postoperative models. In each case, myofiber stress and pump function were calculated.

RESULTS

Both postoperative and virtual Coapsys models shifted end-systolic and end-diastolic pressure-volume relationships to the left. As a consequence and because chronic ischemic mitral regurgitation was reduced after Coapsys, pump function was unchanged. Coapsys decreased myofiber stress at end-diastole and end-systole in both the remote and infarct regions of the myocardium. However, knowledge of Coapsys and LV prestress was necessary for accurate calculation of LV myofiber stress, especially in the remote zone.

CONCLUSIONS

Coapsys decreases myofiber stress at end-diastole and end-systole. The improvement in myofiber stress may contribute to the long-term effect of Coapsys on LV remodeling.

A coupled biventricular finite element and lumped-parameter circulatory system model of heart failure

Numerical modelling of the cardiovascular system is becoming an important tool for assessing the influence of heart disease and treatment therapies. In the current study, we present an approach for modelling the interaction between the heart and the circulatory system. This was accomplished by creating animal-specific biventricular finite element (FE) models, which characterise the mechanical response of the heart, and by coupling them to a lumped-parameter model that represents the systemic and pulmonic circulatory system. In order to minimise computation time, the coupling was enforced in a weak (one-way) manner, where the ventricular pressure-volume relationships were generated by the FE models and then passed into the circulatory system model to ensure volume conservation and physiological pressure changes. The models were first validated by tuning the parameters, such that the output of the models matched experimentally measured pressures and volumes. Then the models were used to examine cardiac function and the myofibre stress in a healthy canine heart and a canine heart with dilated cardiomyopathy. The results showed good agreement with experimental measurements. The stress in the case of cardiomyopathy was found to increase significantly, while the pump function was decreased, compared to the healthy case. The total runtime of the simulations is lesser than that of many fully coupled models presented in the literature. This will allow for a much quicker evaluation of possible treatment strategies for combating the effects of heart failure, especially in optimisation schemes that require numerous FE simulations.

Electromechanical feedback with reduced cellular connectivity alters electrical activity in an infarct injured left ventricle: a finite element model study

Myocardial infarction (MI) significantly alters the structure and function of the heart. As abnormal strain may drive heart failure and the generation of arrhythmias, we used computational methods to simulate a left ventricle with an MI over the course of a heartbeat to investigate strains and their potential implications to electrophysiology. We created a fully coupled finite element model of myocardial electromechanics consisting of a cellular physiological model, a bidomain electrical diffusion solver, and a nonlinear mechanics solver. A geometric mesh built from magnetic resonance imaging (MRI) measurements of an ovine left ventricle suffering from a surgically induced anteroapical infarct was used in the model, cycled through the cardiac loop of inflation, isovolumic contraction, ejection, and isovolumic relaxation. Stretch-activated currents were added as a mechanism of mechanoelectric feedback. Elevated fiber and cross fiber strains were observed in the area immediately adjacent to the aneurysm throughout the cardiac cycle, with a more dramatic increase in cross fiber strain than fiber strain. Stretch-activated channels decreased action potential (AP) dispersion in the remote myocardium while increasing it in the border zone. Decreases in electrical connectivity dramatically increased the changes in AP dispersion. The role of cross fiber strain in MI-injured hearts should be investigated more closely, since results indicate that these are more highly elevated than fiber strain in the border of the infarct. Decreases in connectivity may play an important role in the development of altered electrophysiology in the high-stretch regions of the heart.

Hemodynamics and myocardial blood flow patterns after placement of a cardiac passive restraint device in a model of dilated cardiomyopathy

BACKGROUND

The present study examined a cardiac passive restraint device which applies epicardial pressure (HeartNet Implant; Paracor Medical, Inc, Sunnyvale, Calif) in a clinically relevant model of dilated cardiomyopathy to determine effects on hemodynamic and myocardial blood flow patterns.

METHODS

Dilated cardiomyopatht was established in 10 pigs (3 weeks of atrial pacing, 240 beats/min). Hemodynamic parameters and regional left ventricular blood flow were measured under baseline conditions and after acute placement of the HeartNet Implant. Measurements were repeated after adenosine infusion, allowing maximal coronary vasodilation and coronary flow reserve to be determined.

RESULTS

Left ventricular dilation and systolic dysfunction occurred relative to baseline as measured by echocardiography. Left ventricular end-diastolic dimension increased and left ventricular fractional shortening decreased (3.8 ± 0.1 vs 6.1 ± 0.2 cm and 31.6% ± 0.5% vs 16.2% ± 2.1%, both P < .05, respectively), consistent with the dilated cardiomyopathy phenotype. The HeartNet Implant was successfully deployed without arrhythmias and a computed median mid-left ventricular epicardial pressure of 1.4 mm Hg was applied by the HeartNet Implant throughout the cardiac cycle. Acute HeartNet placement did not adversely affect steady state hemodynamics. With the HeartNet Implant in place, coronary reserve was significantly blunted.

CONCLUSIONS

In a large animal model of dilated cardiomyopathy, the cardiac passive restraint device did not appear to adversely affect basal resting myocardial blood flow. However, after acute HeartNet Implant placement, left ventricular maximal coronary reserve was blunted. These unique results suggest that cardiac passive restraint devices that apply epicardial transmural pressure can alter myocardial blood flow patterns in a model of dilated cardiomyopathy. Whether this blunting of coronary reserve holds clinical relevance with chronic passive restraint device placement remains unestablished.

Model-based design of mechanical therapies for myocardial infarction

The mechanical properties of healing myocardial infarcts are a critical determinant of pump function and the transition to heart failure. Recent reports suggest that modifying infarct mechanical properties can improve function and limit ventricular remodeling. However, little attempt has been made to identify the specific infarct material properties that would optimize left ventricular (LV) function. We utilized a finite-element model of a large anteroapical infarct in a dog heart to explore a wide range of infarct mechanical properties. Isotropic stiffening of the infarct reduced end-diastolic (EDV) and end-systolic (ESV) volumes, improved LV contractility, but had little effect on stroke volume. A highly anisotropic infarct, with high longitudinal stiffness but low circumferential stiffness coefficients, produced the best stroke volume by increasing diastolic filling, without affecting contractility or ESV. Simulated infarcts in two different locations displayed different transmural strain patterns. Our results suggest that there is a general trade-off between acutely reducing LV size and acutely improving LV pump function, that isotropically stiffening the infarct is not the only option of potential therapeutic interest, and that customizing therapies for different infarct locations may be important. Our model results should provide guidance for design and development of therapies to improve LV function by modifying infarct mechanical properties.