Scar remodeling and transmural deformation after infarction in the pig

Combined In-silico and Machine Learning Approaches Toward Predicting Arrhythmic Risk in Post-infarction Patients

Background: Remodeling due to myocardial infarction (MI) significantly increases patient arrhythmic risk. Simulations using patient-specific models have shown promise in predicting personalized risk for arrhythmia. However, these are computationally- and time- intensive, hindering translation to clinical practice. Classical machine learning (ML) algorithms (such as K-nearest neighbors, Gaussian support vector machines, and decision trees) as well as neural network techniques, shown to increase prediction accuracy, can be used to predict occurrence of arrhythmia as predicted by simulations based solely on infarct and ventricular geometry. We present an initial combined image-based patient-specific in silico and machine learning methodology to assess risk for dangerous arrhythmia in post-infarct patients. Furthermore, we aim to demonstrate that simulation-supported data augmentation improves prediction models, combining patient data, computational simulation, and advanced statistical modeling, improving overall accuracy for arrhythmia risk assessment.

Methods: MRI-based computational models were constructed from 30 patients 5 days post-MI (the “baseline” population). In order to assess the utility biophysical model-supported data augmentation for improving arrhythmia prediction, we augmented the virtual baseline patient population. Each patient ventricular and ischemic geometry in the baseline population was used to create a subfamily of geometric models, resulting in an expanded set of patient models (the “augmented” population). Arrhythmia induction was attempted via programmed stimulation at 17 sites for each virtual patient corresponding to AHA LV segments and simulation outcome, “arrhythmia,” or “no-arrhythmia,” were used as ground truth for subsequent statistical prediction (machine learning, ML) models. For each patient geometric model, we measured and used choice data features: the myocardial volume and ischemic volume, as well as the segment-specific myocardial volume and ischemia percentage, as input to ML algorithms. For classical ML techniques (ML), we trained k-nearest neighbors, support vector machine, logistic regression, xgboost, and decision tree models to predict the simulation outcome from these geometric features alone. To explore neural network ML techniques, we trained both a three - and a four-hidden layer multilayer perceptron feed forward neural networks (NN), again predicting simulation outcomes from these geometric features alone. ML and NN models were trained on 70% of randomly selected segments and the remaining 30% was used for validation for both baseline and augmented populations.

Results: Stimulation in the baseline population (30 patient models) resulted in reentry in 21.8% of sites tested; in the augmented population (129 total patient models) reentry occurred in 13.0% of sites tested. ML and NN models ranged in mean accuracy from 0.83 to 0.86 for the baseline population, improving to 0.88 to 0.89 in all cases.

Conclusion: Machine learning techniques, combined with patient-specific, image-based computational simulations, can provide key clinical insights with high accuracy rapidly and efficiently. In the case of sparse or missing patient data, simulation-supported data augmentation can be employed to further improve predictive results for patient benefit. This work paves the way for using data-driven simulations for prediction of dangerous arrhythmia in MI patients.

Mechano-chemo signaling interactions modulate matrix production by cardiac fibroblasts

Extracellular matrix remodeling after myocardial infarction occurs in a dynamic environment in which local mechanical stresses and biochemical signaling species stimulate the accumulation of collagen-rich scar tissue. It is well-known that cardiac fibroblasts regulate post-infarction matrix turnover by secreting matrix proteins, proteases, and protease inhibitors in response to both biochemical stimuli and mechanical stretch, but how these stimuli act together to dictate cellular responses is still unclear. We developed a screen of cardiac fibroblast-secreted proteins in response to combinations of biochemical agonists and cyclic uniaxial stretch in order to elucidate the relationships between stretch, biochemical signaling, and cardiac matrix turnover. We found that stretch significantly synergized with biochemical agonists to inhibit the secretion of matrix metalloproteinases, with stretch either amplifying protease suppression by individual agonists or antagonizing agonist-driven upregulation of protease expression. Stretch also modulated fibroblast sensitivity towards biochemical agonists by either sensitizing cells towards agonists that suppress protease secretion or de-sensitizing cells towards agonists that upregulate protease secretion. These findings suggest that the mechanical environment can significantly alter fibrosis-related signaling in cardiac fibroblasts, suggesting caution when extrapolating in vitro data to predict effects of fibrosis-related cytokines in situations like myocardial infarction where mechanical stretch occurs.

Biomechanical properties of acellular scar ECM during the acute to chronic stages of myocardial infarction

After myocardial infarction (MI), the infarcted tissue undergoes dynamic and time-dependent changes. Previous knowledge on MI biomechanical alterations has been obtained by studying the explanted scar tissues. In this study, we decellularized MI scar tissue and characterized the biomechanics of the obtained pure scar ECM. By thoroughly removing the cellular content in the MI scar tissue, we were able to avoid its confounding effects. Rat MI hearts were obtained from a reliable and reproducible model based on permanent left coronary artery ligation (PLCAL). MI heart explants at various time points (15 min, 1 week, 2 weeks, 4 weeks, and 12 weeks) were subjected to decellularization with 0.1% sodium dodecyl sulfate solution for ~1-2 weeks to obtain acellular scar ECM. A biaxial mechanical testing system was used to characterize the acellular scar ECM under physiologically relevant loading conditions. After decellularization, large decrease in wall thickness was observed in the native heart ECM and 15 min scar ECM, implying the collapse of cardiomyocyte lacunae after removal of heart muscle fibers. For scar ECM 1 week, 2 weeks, and 4 weeks post infarction, the decrease in wall thickness after decellularization was small. For scar ECM 12 weeks post infarction, the reduction amount of wall thickness due to decellularization was minimal. We found that the scar ECM preserved the overall mechanical anisotropy of the native ventricle wall and MI scar tissue, in which the longitudinal direction is more extensible. Acellular scar ECM from 15 min to 12 weeks post infarction showed an overall stiffening trend in biaxial behavior, in which longitudinal direction was mostly affected and manifested with a decreased extensibility and increased modulus. This reduction trend of longitudinal extensibility also led to a decreased anisotropy index in the scar ECM from the acute to chronic stages of MI. The post-MI change in biomechanical properties of the scar ECM reflected the alterations of collagen fiber network, confirmed by the histology of scar ECM. In short, the reported structure-property relationship reveals how scar ECM biophysical properties evolve from the acute to chronic stages of MI. The obtained information will help establish a knowledge basis about the dynamics of scar ECM to better understand post-MI cardiac remodeling.

Apparent growth tensor of left ventricular post myocardial infarction - In human first natural history study

An outstanding challenge in modelling biomechanics after myocardial infarction (MI) is to estimate the so-called growth tensor. Since it is impossible to track pure growth induced geometry change from in vivo magnetic resonance images alone, in this work, we propose a way of estimating a surrogate or apparent growth tensor of the human left ventricle using cine magnetic resonance (CMR) and late gadolinium enhanced (LGE) images of 16 patients following acute MI. The apparent growth tensor is evaluated at four time-points following myocardial reperfusion: 4-12 h (baseline), 3 days, 10 days and 7 months. We have identified three different growth patterns classified as the Dilation, No-Change and Shrinkage groups defined by the left ventricle end-diastole cavity volume change from baseline. We study the- trends in both the infarct and remote regions. Importantly, although the No-Change group has little change in the ventricular cavity volume, significant remodelling changes are seen within the myocardial wall, both in the infarct and remote regions. Through statistical analysis, we show that the growth tensor invariants can be used as effective biomarkers for adverse and favourable remodelling of the heart from 10 days onwards post-MI with statistically significant changes over time, in contrast to most of the routine clinical indices. We believe this is the first time that the apparent growth tensor has been estimated from in vivo CMR images post-MI. Our study not only provides much-needed information for understanding growth and remodelling in the human heart following acute MI, but also identifies novel biomarker for assessing heart disease progression.

E3 Ubiquitin ligase NEDD4 family‑regulatory network in cardiovascular disease

Protein ubiquitination represents a critical modification occurring after translation. E3 ligase catalyzes the covalent binding of ubiquitin to the protein substrate, which could be degraded. Ubiquitination as an important protein post-translational modification is closely related to cardiovascular disease. The NEDD4 family, belonging to HECT class of E3 ubiquitin ligases can recognize different substrate proteins, including PTEN, ENaC, Nav1.5, SMAD2, PARP1, Septin4, ALK1, SERCA2a, TGFβR3 and so on, via the WW domain to catalyze ubiquitination, thus participating in multiple cardiovascular-related disease such as hypertension, arrhythmia, myocardial infarction, heart failure, cardiotoxicity, cardiac hypertrophy, myocardial fibrosis, cardiac remodeling, atherosclerosis, pulmonary hypertension and heart valve disease. However, there is currently no review comprehensively clarifying the important role of NEDD4 family proteins in the cardiovascular system. Therefore, the present review summarized recent studies about NEDD4 family members in cardiovascular disease, providing novel insights into the prevention and treatment of cardiovascular disease. In addition, assessing transgenic animals and performing gene silencing would further identify the ubiquitination targets of NEDD4. NEDD4 quantification in clinical samples would also constitute an important method for determining NEDD4 significance in cardiovascular disease.

Coagulation factor XIII activity predicts left ventricular remodelling after acute myocardial infarction

AIMS

Acute myocardial infarction (MI) is the major cause of chronic heart failure. The activity of blood coagulation factor XIII (FXIIIa) plays an important role in rodents as a healing factor after MI, whereas its role in healing and remodelling processes in humans remains unclear. We prospectively evaluated the relevance of FXIIIa after acute MI as a potential early prognostic marker for adequate healing.

METHODS AND RESULTS

This monocentric prospective cohort study investigated cardiac remodelling in patients with ST-elevation MI and followed them up for 1 year. Serum FXIIIa was serially assessed during the first 9 days after MI and after 2, 6, and 12 months. Cardiac magnetic resonance imaging was performed within 4 days after MI (Scan 1), after 7 to 9 days (Scan 2), and after 12 months (Scan 3). The FXIII valine-to-leucine (V34L) single-nucleotide polymorphism rs5985 was genotyped. One hundred forty-six patients were investigated (mean age 58 ± 11 years, 13% women). Median FXIIIa was 118% (quartiles, 102-132%) and dropped to a trough on the second day after MI: 109% (98-109%; P < 0.001). FXIIIa recovered slowly over time, reaching the baseline level after 2 to 6 months and surpassed baseline levels only after 12 months: 124% (110-142%). The development of FXIIIa after MI was independent of the genotype. FXIIIa on Day 2 was strongly and inversely associated with the relative size of MI in Scan 1 (Spearman's ρ = -0.31; P = 0.01) and Scan 3 (ρ = -0.39; P < 0.01) and positively associated with left ventricular ejection fraction: ρ = 0.32 (P < 0.01) and ρ = 0.24 (P = 0.04), respectively.

CONCLUSIONS

FXIII activity after MI is highly dynamic, exhibiting a significant decline in the early healing period, with reconstitution 6 months later. Depressed FXIIIa early after MI predicted a greater size of MI and lower left ventricular ejection fraction after 1 year. The clinical relevance of these findings awaits to be tested in a randomized trial.

Biomechanics of infarcted left Ventricle-A review of experiments

Myocardial infarction (MI) is one of leading diseases to contribute to annual death rate of 5% in the world. In the past decades, significant work has been devoted to this subject. Biomechanics of infarcted left ventricle (LV) is associated with MI diagnosis, understanding of remodelling, MI micro-structure and biomechanical property characterizations as well as MI therapy design and optimization, but the subject has not been reviewed presently. In the article, biomechanics of infarcted LV was reviewed in terms of experiments achieved in the subject so far. The concerned content includes experimental remodelling, kinematics and kinetics of infarcted LVs. A few important issues were discussed and several essential topics that need to be investigated further were summarized. Microstructure of MI tissue should be observed even carefully and compared between different methods for producing MI scar in the same animal model, and eventually correlated to passive biomechanical property by establishing innovative constitutive laws. More uniaxial or biaxial tensile tests are desirable on MI, border and remote tissues, and viscoelastic property identification should be performed in various time scales. Active contraction experiments on LV wall with MI should be conducted to clarify impaired LV pumping function and supply necessary data to the function modelling. Pressure-volume curves of LV with MI during diastole and systole for the human are also desirable to propose and validate constitutive laws for LV walls with MI.

Spatial scaling in multiscale models: methods for coupling agent-based and finite-element models of wound healing

Multiscale models that couple agent-based modeling (ABM) and finite-element modeling (FEM) allow the dynamic simulation of tissue remodeling and wound healing, with mechanical environment influencing cellular behaviors even as tissue remodeling modifies mechanics. One of the challenges in coupling ABM to FEM is that these two domains typically employ grid or element sizes that differ by several orders of magnitude. Here, we develop and demonstrate an interpolation-based method for mapping between ABM and FEM domains of different resolutions that is suitable for linear and nonlinear FEM meshes and balances accuracy with computational demands. We then explore the effects of refining the FEM mesh and the ABM grid in the setting of a fully coupled model. ABM grid refinement studies showed unexpected effects of grid size whenever cells were present at a high enough density for crowding to affect proliferation and migration. In contrast to an FE-only model, refining the FE mesh in the coupled model increased strain differences between adjacent finite elements. Allowing strain-dependent feedback on collagen turnover magnified the effects of regional heterogeneity, producing highly nonlinear spatial and temporal responses. Our results suggest that the choice of both ABM grid and FEM mesh density in coupled models must be guided by experimental data and accompanied by careful grid and mesh refinement studies in the individual domains as well as the fully coupled model.

Model First and Ask Questions Later: Confessions of a Reformed Experimentalist

This paper is an invited perspective written in association with the awarding of the 2018 American Society of Mechanical Engineers Van C. Mow Medal. Inspired by Professor Mow's collaboration with Professor Michael Lai and the role mathematical modeling played in their work on cartilage biomechanics, this article uses our group's work on myocardial infarct healing as an example of the potential value of models in modern experimental biomechanics. Focusing more on the thought process and lessons learned from our studies on infarct mechanics than on the details of the science, this article argues that the complexity of current research questions and the wealth of information already available about almost any cell, tissue, or organ should change how we approach problems and design experiments. In particular, this paper proposes that constructing a mathematical or computational model is now in many cases a critical prerequisite to designing scientifically useful, informative experiments.

Quantification of myocardial infarct area based on TRAFFn relaxation time maps - comparison with cardiovascular magnetic resonance late gadolinium enhancement, T1ρ and T2 in vivo

BACKGROUND

Two days after myocardial infarction (MI), the infarct consists mostly on necrotic tissue, and the myocardium is transformed through granulation tissue to scar in two weeks after the onset of ischemia in mice. In the current work, we determined and optimized cardiovascular magnetic resonance (CMR) methods for the detection of MI size during the scar formation without contrast agents in mice.

METHODS

We characterized MI and remote areas with rotating frame relaxation time mapping including relaxation along fictitious field in nth rotating frame (RAFFn), T1ρ and T2 relaxation time mappings at 1, 3, 7, and 21 days after MI. These results were compared to late gadolinium enhancement (LGE) and Sirius Red-stained histology sections, which were obtained at day 21 after MI.

RESULTS

All relaxation time maps showed significant differences in relaxation time between the MI and remote area. Areas of increased signal intensities after gadolinium injection and areas with increased TRAFF2 relaxation time were highly correlated with the MI area determined from Sirius Red-stained histology sections (LGE: R2 = 0.92, P < 0.01, TRAFF2: R2 = 0.95, P < 0.001). Infarct area determined based on T1ρ relaxation time correlated highly with Sirius Red histology sections (R2 = 0.97, P < 0.01). The smallest overestimation of the LGE-defined MI area was obtained for TRAFF2 (5.6 ± 4.2%) while for T1ρ overestimation percentage was > 9% depending on T1ρ pulse power.

CONCLUSION

T1ρ and TRAFF2 relaxation time maps can be used to determine accurately MI area at various time points in the mouse heart. Determination of MI size based on TRAFF2 relaxation time maps could be performed without contrast agents, unlike LGE, and with lower specific absorption rate compared to on-resonance T1ρ relaxation time mapping.

The selective NLRP3-inflammasome inhibitor MCC950 reduces infarct size and preserves cardiac function in a pig model of myocardial infarction

Aims

Myocardial infarction (MI) triggers an intense inflammatory response that is associated with infarct expansion and is detrimental for cardiac function. Interleukin (IL)-1β and IL-18 are key players in this response and are controlled by the NLRP3-inflammasome. In the current study, we therefore hypothesized that selective inhibition of the NLRP3-inflammasome reduces infarct size and preserves cardiac function in a porcine MI model.

Methods and results

Thirty female landrace pigs were subjected to 75 min transluminal balloon occlusion and treated with the NLRP3-inflammasome inhibitor MCC950 (6 or 3 mg/kg) or placebo for 7 days in a randomized, blinded fashion. After 7 days, 3D-echocardiography was performed to assess cardiac function and Evans blue/TTC double staining was executed to assess the area at risk (AAR) and infarct size (IS). The IS/AAR was lower in the 6 mg/kg group (64.6 ± 8.8%, P = 0.004) and 3 mg/kg group (69.7 ± 7.2%, P = 0.038) compared with the control group (77.5 ± 6.3%). MCC950 treatment markedly preserved left ventricular ejection fraction in treated animals (6 mg/kg 47 ± 8%, P = 0.001; 3 mg/kg 45 ± 7%, P = 0.031; control 37 ± 6%). Myocardial neutrophil influx was attenuated in treated compared with non-treated animals (6 mg/kg 132 ± 72 neutrophils/mm2, P = 0.035; 3 mg/kg 207 ± 210 neutrophils/mm2, P = 0.5; control 266 ± 158 neutrophils/mm2). Myocardial IL-1β levels were dose-dependently reduced in treated animals.

Conclusions

NLRP3-inflammasome inhibition reduces infarct size and preserves cardiac function in a randomized, blinded translational large animal MI model. Hence, NLRP3-inflammasome inhibition may have therapeutic potential in acute MI patients.

Optimization of large animal MI models; a systematic analysis of control groups from preclinical studies

Large animal models are essential for the development of novel therapeutics for myocardial infarction. To optimize translation, we need to assess the effect of experimental design on disease outcome and model experimental design to resemble the clinical course of MI. The aim of this study is therefore to systematically investigate how experimental decisions affect outcome measurements in large animal MI models. We used control animal-data from two independent meta-analyses of large animal MI models. All variables of interest were pre-defined. We performed univariable and multivariable meta-regression to analyze whether these variables influenced infarct size and ejection fraction. Our analyses incorporated 246 relevant studies. Multivariable meta-regression revealed that infarct size and cardiac function were influenced independently by choice of species, sex, co-medication, occlusion type, occluded vessel, quantification method, ischemia duration and follow-up duration. We provide strong systematic evidence that commonly used endpoints significantly depend on study design and biological variation. This makes direct comparison of different study-results difficult and calls for standardized models. Researchers should take this into account when designing large animal studies to most closely mimic the clinical course of MI and enable translational success.

Enzyme-dependent fluorescence recovery of NADH after photobleaching to assess dehydrogenase activity of isolated perfused hearts

Reduction of NAD⁺ by dehydrogenase enzymes to form NADH is a key component of cellular metabolism. In cellular preparations and isolated mitochondria suspensions, enzyme-dependent fluorescence recovery after photobleaching (ED-FRAP) of NADH has been shown to be an effective approach for measuring the rate of NADH production to assess dehydrogenase enzyme activity. Our objective was to demonstrate how dehydrogenase activity could be assessed within the myocardium of perfused hearts using NADH ED-FRAP. This was accomplished using a combination of high intensity UV pulses to photobleach epicardial NADH. Replenishment of epicardial NADH fluorescence was then imaged using low intensity UV illumination. NADH ED-FRAP parameters were optimized to deliver 23.8 mJ of photobleaching light energy at a pulse width of 6 msec and a duty cycle of 50%. These parameters provided repeatable measurements of NADH production rate during multiple metabolic perturbations, including changes in perfusate temperature, electromechanical uncoupling, and acute ischemia/reperfusion injury. NADH production rate was significantly higher in every perturbation where the energy demand was either higher or uncompromised. We also found that NADH production rate remained significantly impaired after 10 min of reperfusion after global ischemia. Overall, our results indicate that myocardial NADH ED-FRAP is a useful optical non-destructive approach for assessing dehydrogenase activity.

In vivo assessment of regional mechanics post-myocardial infarction: A focus on the road ahead

Cardiovascular disease, particularly the occurrence of myocardial infarction (MI), remains a leading cause of morbidity and mortality (Go et al., Circulation 127: e6-e245, 2013; Go et al. Circulation 129: e28-e292, 2014). There is growing recognition that a key factor for post-MI outcomes is adverse remodeling and changes in the regional structure, composition, and mechanical properties of the MI region itself. However, in vivo assessment of regional mechanics post-MI can be confounded by the species, temporal aspects of MI healing, as well as size, location, and extent of infarction across myocardial wall. Moreover, MI regional mechanics have been assessed over varying phases of the cardiac cycle, and thus, uniform conclusions regarding the material properties of the MI region can be difficult. This review assesses past studies that have performed in vivo measures of MI mechanics and attempts to provide coalescence on key points from these studies, as well as offer potential recommendations for unifying approaches in terms of regional post-MI mechanics. A uniform approach to biophysical measures of import will allow comparisons across studies, as well as provide a basis for potential therapeutic markers.

Sonomicrometry-Based Analysis of Post-Myocardial Infarction Regional Mechanics

Following myocardial infarction (MI), detrimental changes to the geometry, composition, and mechanical properties of the left ventricle (LV) are initiated in a process generally termed adverse post-MI remodeling. Cumulatively, these changes lead to a loss of LV function and are deterministic factors in the progression to heart failure. Proposed therapeutic strategies to target aberrant LV mechanics post-MI have shown potential to stabilize LV functional indices throughout the remodeling process. The in vivo quantification of LV mechanics, particularly within the MI region, is therefore essential to the continued development and evaluation of strategies to interrupt the post-MI remodeling process. The present study utilizes a porcine MI model and in vivo sonomicrometry to characterize MI region stiffness at 14 days post-MI. Obtained results demonstrate a significant dependence of mechanical properties on location and direction within the MI region, as well as cardiac phase. While approaches for comprehensive characterization of LV mechanics post-MI still need to be improved and standardized, our findings provide insight into the issues and complexities that must be considered within the MI region itself.

Can heart function lost to disease be regenerated by therapeutic targeting of cardiac scar tissue?

Myocardial infarction results in scar tissue that cannot actively contribute to heart mechanical function and frequently causes lethal arrhythmias. The healing response after infarction involves inflammation, biochemical signaling, changes in cellular phenotype, activity, and organization, and alterations in electrical conduction due to variations in cell and tissue geometry and alterations in protein expression, organization, and function - particularly in membrane channels. The intensive research focus on regeneration of myocardial tissues has, as of yet, only met with modest success, with no near-term prospect of improving standard-of-care for patients with heart disease. An alternative concept for novel therapeutic approach is the rejuvenation of cardiac electrical and mechanical properties through the modification of scar tissue. Several peptide therapeutics, locally applied genetic therapies, or delivery of genetically modified cells have shown promise in improving the characteristics of the fibrous scar and post-myocardial infarction prognosis in experimental models. This review highlights several factors that contribute to arrhythmogenesis in scar formation and how these might be targeted to regenerate some of the electrical and mechanical function of the post-MI scar.

Modifying the mechanics of healing infarcts: Is better the enemy of good?

Myocardial infarction (MI) is a major source of morbidity and mortality worldwide, with over 7 million people suffering infarctions each year. Heart muscle damaged during MI is replaced by a collagenous scar over a period of several weeks, and the mechanical properties of that scar tissue are a key determinant of serious post-MI complications such as infarct rupture, depression of heart function, and progression to heart failure. Thus, there is increasing interest in developing therapies that modify the structure and mechanics of healing infarct scar. Yet most prior attempts at therapeutic scar modification have failed, some catastrophically. This article reviews available information about the mechanics of healing infarct scar and the functional impact of scar mechanical properties, and attempts to infer principles that can better guide future attempts to modify scar. One important conclusion is that collagen structure, mechanics, and remodeling of healing infarct scar vary so widely among experimental models that any novel therapy should be tested across a range of species, infarct locations, and reperfusion protocols. Another lesson from past work is that the biology and mechanics of healing infarcts are sufficiently complex that the effects of interventions are often counterintuitive; for example, increasing infarct stiffness has little effect on heart function, and inhibition of matrix metalloproteases (MMPs) has little effect on scar collagen content. Computational models can help explain such counterintuitive results, and are becoming an increasingly important tool for integrating known information to better identify promising therapies and design experiments to test them. Moving forward, potentially exciting new opportunities for therapeutic modification of infarct mechanics include modulating anisotropy and promoting scar compaction.

Why Is Infarct Expansion Such an Elusive Therapeutic Target?

Myocardial infarct expansion has been associated with an increased risk of infarct rupture and progression to heart failure, motivating therapies such as infarct restraint and polymer injection that aim to limit infarct expansion. However, an exhaustive review of quantitative studies of infarct remodeling reveals that only half found chronic in-plane expansion, and many reported in-plane compaction. Using a finite element model, we demonstrate that the balance between scar stiffening due to collagen accumulation and increased wall stresses due to infarct thinning can produce either expansion or compaction in the pressurized heart-potentially explaining variability in the literature-and that loaded dimensions are much more sensitive to changes in thickness than in stiffness. Our analysis challenges the concept that in-plane expansion is a central feature of post-infarction remodeling; rather, available data suggest that radial thinning is the dominant process during infarct healing and may be an attractive therapeutic target.

Physiological Implications of Myocardial Scar Structure

Once myocardium dies during a heart attack, it is replaced by scar tissue over the course of several weeks. The size, location, composition, structure, and mechanical properties of the healing scar are all critical determinants of the fate of patients who survive the initial infarction. While the central importance of scar structure in determining pump function and remodeling has long been recognized, it has proven remarkably difficult to design therapies that improve heart function or limit remodeling by modifying scar structure. Many exciting new therapies are under development, but predicting their long-term effects requires a detailed understanding of how infarct scar forms, how its properties impact left ventricular function and remodeling, and how changes in scar structure and properties feed back to affect not only heart mechanics but also electrical conduction, reflex hemodynamic compensations, and the ongoing process of scar formation itself. In this article, we outline the scar formation process following a myocardial infarction, discuss interpretation of standard measures of heart function in the setting of a healing infarct, then present implications of infarct scar geometry and structure for both mechanical and electrical function of the heart and summarize experiences to date with therapeutic interventions that aim to modify scar geometry and structure. One important conclusion that emerges from the studies reviewed here is that computational modeling is an essential tool for integrating the wealth of information required to understand this complex system and predict the impact of novel therapies on scar healing, heart function, and remodeling following myocardial infarction.

Infiltration and sustenance of viability of cells by amphiphilic biosynthetic biodegradable hydrogels

Amphiphilic biosynthetic hydrogels comprising natural polysaccharide alginate (I) and synthetic polyester polypropylene fumarate (II) units were prepared by crosslinking the copolymer of I and II with calcium ion and vinyl monomers viz, 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA), butyl methacrylate (BMA) and N,N'-methylene bisacrylamide (NMBA). Three fast degradable hydrogels, ALPF-MMA, ALPF-HEMA and ALPF-BMA and one slow degradable hydrogel ALPF-NMBA were prepared. These hydrogels are amphiphilic and able to hold sufficient amount of proteins on their surfaces. All these hydrogels are found to be hemocompatible, cytocompatible and genocompatible. ALPF-NMBA promotes infiltration of L929 fibroblasts and 3D growth of H9c2 cardiomyoblasts and long-term viability.

Coupled agent-based and finite-element models for predicting scar structure following myocardial infarction

Following myocardial infarction, damaged muscle is gradually replaced by collagenous scar tissue. The structural and mechanical properties of the scar are critical determinants of heart function, as well as the risk of serious post-infarction complications such as infarct rupture, infarct expansion, and progression to dilated heart failure. A number of therapeutic approaches currently under development aim to alter infarct mechanics in order to reduce complications, such as implantation of mechanical restraint devices, polymer injection, and peri-infarct pacing. Because mechanical stimuli regulate scar remodeling, the long-term consequences of therapies that alter infarct mechanics must be carefully considered. Computational models have the potential to greatly improve our ability to understand and predict how such therapies alter heart structure, mechanics, and function over time. Toward this end, we developed a straightforward method for coupling an agent-based model of scar formation to a finite-element model of tissue mechanics, creating a multi-scale model that captures the dynamic interplay between mechanical loading, scar deformation, and scar material properties. The agent-based component of the coupled model predicts how fibroblasts integrate local chemical, structural, and mechanical cues as they deposit and remodel collagen, while the finite-element component predicts local mechanics at any time point given the current collagen fiber structure and applied loads. We used the coupled model to explore the balance between increasing stiffness due to collagen deposition and increasing wall stress due to infarct thinning and left ventricular dilation during the normal time course of healing in myocardial infarcts, as well as the negative feedback between strain anisotropy and the structural anisotropy it promotes in healing scar. The coupled model reproduced the observed evolution of both collagen fiber structure and regional deformation following coronary ligation in the rat, and suggests that fibroblast alignment in the direction of greatest stretch provides negative feedback on the level of anisotropy in a scar forming under load. In the future, this coupled model may prove useful in computational design and screening of novel therapies to influence scar formation in mechanically loaded tissues.

Pathological mechanism for delayed hyperenhancement of chronic scarred myocardium in contrast agent enhanced magnetic resonance imaging

Objectives

To evaluate possible mechanism for delayed hyperenhancement of scarred myocardium by investigating the relationship of contrast agent (CA) first pass and delayed enhancement patterns with histopathological changes.

Materials and Methods

Eighteen pigs underwent 4 weeks ligation of 1 or 2 diagonal coronary arteries to induce chronic infarction. The hearts were then removed and perfused in a Langendorff apparatus. The hearts firstly experienced phosphorus 31 MR spectroscopy. The hearts in group I (n = 9) and II (n = 9) then received the bolus injection of Gadolinium diethylenetriamine pentaacetic acid (0.05 mmol/kg) and gadolinium-based macromolecular agent (P792, 15 µmol/kg), respectively. First pass T₂^* MRI was acquired using a gradient echo sequence. Delayed enhanced T₁ MRI was acquired with an inversion recovery sequence. Masson's trichrome and anti- von Willebrand Factor (vWF) staining were performed for infarct characterization.

Results

Wash-in of both kinds of CA caused the sharp and dramatic T₂^* signal decrease of scarred myocardium similar to that of normal myocardium. Myocardial blood flow and microvessel density were significantly recovered in 4-week-old scar tissue. Steady state distribution volume (ΔR₁ relaxation rate) of Gd-DTPA was markedly higher in scarred myocardium than in normal myocardium, whereas ΔR₁ relaxation rate of P792 did not differ significantly between scarred and normal myocardium. The ratio of extracellular volume to the total water volume was significantly greater in scarred myocardium than in normal myocardium. Scarred myocardium contained massive residual capillaries and dilated vessels. Histological stains indicated the extensively discrete matrix deposition and lack of cellular structure in scarred myocardium.

Conclusions

Collateral circulation formation and residual vessel effectively delivered CA into scarred myocardium. However, residual vessel without abnormal hyperpermeability allowed Gd-DTPA rather than P792 to penetrate into extravascular compartment. Discrete collagen fiber meshwork and loss of cellularity enlarged extracellular space accessible to Gd-DTPA, resulting in the delayed hyper-enhanced scar.

Admittance-based pressure-volume loops versus gold standard cardiac magnetic resonance imaging in a porcine model of myocardial infarction

A novel admittance‐based pressure–volume system (AS) has recently been developed and introduced. Thus far, the new technique has been validated predominantly in small animals. In large animals it has only been compared to three‐dimensional echocardiography (3DE) where the AS showed to overestimate left ventricular (LV) volumes. To fully determine the accuracy of this device, we compared the AS with gold standard cardiac magnetic resonance imaging (CMRI) in a porcine model of chronic myocardial infarction (MI). Fourteen pigs were subjected to 90 min closed chest balloon occlusion of the left anterior descending artery. After 8 weeks of follow up, pigs were consecutively subjected to LV volume measurements by the AS, CMRI, and 3DE under general anesthesia. The AS overestimated end diastolic volume (EDV; +20.9 ± 30.6 mL, P = 0.024) and end systolic volume (ESV; +17.7 ± 29.4 mL, P = 0.042) but not ejection fraction (EF; +2.46 ± 6.16%, P = NS) compared to CMRI. Good correlations of EDV (R = 0.626, P = 0.017) and EF (R = 0.704, P = 0.005) between the AS and CMRI were observed. EF measured by the AS and 3DE also correlated significantly (R = 0.624, P = 0.030). After subjection of pigs to MI, the AS very moderately overestimates LV volumes and shows accurate measurements for EF compared to CMRI. This makes the AS a useful tool to determine cardiac function and dynamic changes in large animal models of cardiac disease.

Is the novel admittance‐based pressure–volume loop system reliable for the assessment of left ventricular volumes compared to gold standard cardiac magnetic resonance imaging in a porcine model of myocardial infarction? In the postinfarction remodeled heart, admittance‐based pressure–volume loop measurements accurately measure ejection fraction and very moderately overestimate end diastolic and end systolic volumes compared to gold standard cardiac magnetic resonance imaging, making it a very useful technique for cardiac function assessment in experimental studies.

Post myocardial infarction of the left ventricle: the course ahead seen by cardiac MRI

In the last decades, cardiac magnetic resonance imaging (MRI) has gained acceptance in cardiology community as an accurate and reproducible diagnostic imaging modality in patients with ischemic heart disease (IHD). In particular, in patients with acute myocardial infarction (MI) cardiac MRI study allows a comprehensive assessment of the pattern of ischemic injury in term of reversible and irreversible damage, myocardial hemorrhage and microvascular obstruction (MVO). Myocardial salvage index, derived by quantification of myocardium (area) at risk and infarction, has become a promising surrogate end-point increasingly used in clinical trials testing novel or adjunctive reperfusion strategies. Early post-infarction, the accurate and reproducible quantification of myocardial necrosis, along with the characterization of ischemic myocardial damage in its diverse components, provides important information to predict post-infarction left ventricular (LV) remodeling, being useful for patients stratification and management. Considering its non-invasive nature, cardiac MRI suits well for investigating the time course of infarct healing and the changes occurring in peri-infarcted (adjacent) and remote myocardium, which ultimately promote the geometrical, morphological and functional abnormalities of the entire left ventricle (global LV remodeling). The current review will focus on the cardiac MRI utility for a comprehensive evaluation of patients with acute and chronic IHD with particular regard to post-infarction remodeling.

Pim-1 mediated signaling during the process of cardiac remodeling following myocardial infarction in ovine hearts

The serine/threonine kinase Pim-1 was recently identified as a cardiomyocyte survival regulator downstream of Akt. The present study aims to examine Pim-1 activity and its association with the post MI remodeling myocardium in a clinically relevant large animal model. Apical myocardial infarction of approximately 25% left ventricular mass was created in an ovine model. Regional post-infarction deformation of the left ventricle was monitored by sonomicrometry and quantified using areal remodeling strain (i.e., areal expansion). Myocardial tissues were harvested at 12weeks from the adjacent and remote regions of the infarct for analysis of Pim-1 mediated survival signaling proteins as well as apoptotic activity. The cDNA coding sequences of two ovine Pim-1 kinase isoforms, 44 and 33kDa, were identified. Both isoforms were detected in heart tissue and the overall Pim-1 expression was found to be tightly controlled at multiple molecular levels. Pim-1 as well as the Pim-1 mediated survival signaling proteins Bcl-2, Bcl-xL, and phospho-Bad (Ser112) were upregulated in the adjacent region at 12weeks post-infarction and their expression correlated positively with the degree of the remodeling, which was accompanied by significant upregulations of the PP2A/BAD mediated apoptotic signaling proteins. However these upregulations were imbalanced, such that p-BAD (Ser112)/BAD decreased in the adjacent region of the infarcted hearts. Apoptotic activity also increased with remodeling strain. Despite an observed intrinsic upregulation of survival proteins, the imbalanced activation of apoptotic pathways resulted in evident apoptosis in the adjacent region.

ULTRAMINI-ABSTRACT

Pim-1 mediated survival signaling in myocardial tissues from infarcted ovine hearts was studied. It was shown that the adjacent region of the infarct experienced higher remodeling strain and exhibited increased levels of Pim-1 and related anti-apoptotic proteins. Despite this elevation of survival activity, however, the imbalanced activation of PP2A/BAD mediated apoptotic pathway resulted in evident apoptosis in the adjacent region.

Salvage assessment with cardiac MRI following acute myocardial infarction underestimates potential for recovery of systolic strain

OBJECTIVES

Our aim was to evaluate the relationship between the degree of salvage following acute ST elevation myocardial infarction (STEMI) and subsequent reversible contractile dysfunction using cardiac magnetic resonance (CMR) imaging.

METHODS

Thirty-four patients underwent CMR examination 1-7 days after primary percutaneous coronary intervention (PPCI) for acute STEMI with follow-up at 1 year. The ischaemic area-at-risk (AAR) was assessed with T2-weighted imaging and myocardial necrosis with late gadolinium enhancement. Myocardial strain was quantified with complementary spatial modulation of magnetisation (CSPAMM) tagging.

RESULTS

Ischaemic segments with poor (<25 %) or intermediate (26-50 %) salvage index were associated with worse Eulerian circumferential (Ecc) strain immediately post-PPCI (-9.1 % ± 0.6, P = 0.033 and -11.8 % ± 1.3, P = 0.003, respectively) than those with a high (51-100 %) salvage index (-14.4 % ± 1.3). Mean strain in ischaemic myocardium improved between baseline and follow-up (-10.1 % ± 0.5 vs. -16.2 % ± 0.5 %, P < 0.0001). Segments with poor salvage also showed an improvement in strain by 1 year (-9.1 % ± 0.6 vs. -15.3 % ± 0.6, P = 0.033) although they remained the most functionally impaired.

CONCLUSIONS

Partial recovery of peak systolic strain following PPCI is observed even when apparent salvage is less than 25 %. Late gadolinium enhancement (LGE) may not equate to irreversibly injured myocardium and salvage assessment performed within the first week of revascularisation may underestimate the potential for functional recovery.

KEY POINTS

• MRI can measure how much myocardium is damaged after a heart attack. • Heart muscle that appears initially non-viable may sometimes partially recover. • Enhancement around the edges of infarcts may resolve over time. • Evaluating new cardio-protective treatments with MRI requires appreciation of its limitations.

Mechanical regulation of fibroblast migration and collagen remodelling in healing myocardial infarcts

Effective management of healing and remodelling after myocardial infarction is an important problem in modern cardiology practice. We have recently shown that the level of infarct anisotropy is a critical determinant of heart function following a large anterior infarction, which suggests that therapeutic gains may be realized by controlling infarct anisotropy. However, factors regulating infarct anisotropy are not well understood. Mechanical, structural and chemical guidance cues have all been shown to regulate alignment of fibroblasts and collagen in vitro, and prior studies have proposed that each of these cues could regulate anisotropy of infarct scar tissue, but understanding of fibroblast behaviour in the complex environment of a healing infarct is lacking. We developed an agent-based model of infarct healing that accounted for the combined influence of these cues on fibroblast alignment, collagen deposition and collagen remodelling. We pooled published experimental data from several sources in order to determine parameter values, then used the model to test the importance of each cue for predicting collagen alignment measurements from a set of recent cryoinfarction experiments. We found that although chemokine gradients and pre-existing matrix structures had important effects on collagen organization, a response of fibroblasts to mechanical cues was critical for correctly predicting collagen alignment in infarct scar. Many proposed therapies for myocardial infarction, such as injection of cells or polymers, alter the mechanics of the infarct region. Our modelling results suggest that such therapies could change the anisotropy of the healing infarct, which could have important functional consequences. This model is therefore a potentially important tool for predicting how such interventions change healing outcomes.

Regional mechanics determine collagen fiber structure in healing myocardial infarcts

Following myocardial infarction, the mechanical properties of the healing infarct are an important determinant of heart function and the risk of progression to heart failure. In particular, mechanical anisotropy (having different mechanical properties in different directions) in the healing infarct can preserve pump function of the heart. Based on reports of different collagen structures and mechanical properties in various animal models, we hypothesized that differences in infarct size, shape, and/or location produce different patterns of mechanical stretch that guide evolving collagen fiber structure. We tested the effects of infarct shape and location using a combined experimental and computational approach. We studied mechanics and collagen fiber structure in cryoinfarcts in 53 Sprague-Dawley rats and found that regardless of shape or orientation, cryoinfarcts near the equator of the left ventricle stretched primarily in the circumferential direction and developed circumferentially aligned collagen, while infarcts at the apex stretched similarly in the circumferential and longitudinal directions and developed randomly oriented collagen. In a computational model of infarct healing, an effect of mechanical stretch on fibroblast and collagen alignment was required to reproduce the experimental results. We conclude that mechanical environment determines collagen fiber structure in healing myocardial infarcts. Our results suggest that emerging post-infarction therapies that alter regional mechanics will also alter infarct collagen structure, offering both potential risks and novel therapeutic opportunities.

Myocardial area at risk after ST-elevation myocardial infarction measured with the late gadolinium enhancement after scar remodeling and T2-weighted cardiac magnetic resonance imaging

To evaluate the myocardial area at risk (AAR) measured by the endocardial surface area (ESA) method on late gadolinium enhancement (LGE) cardiovascular magnetic resonance (CMR) when applied after scar remodeling (3 months after index infarction) compared to T2-weighted CMR imaging. One hundred and sixty nine patients with ST-elevation myocardial infarction, treated with primary percutaneous coronary intervention, underwent one CMR within 1 week after index treatment to determine the AAR with T2-weighted imaging and a second scan 3 months after to measure AAR with the ESA method. There was a moderate correlation between the two methods (r = 0.86; P < 0.001). The AAR was significantly higher measured with T2-weighted imaging than with the ESA methods (32 ± 11% of left ventricle (LV) vs. 26 ± 10%LV; P < 0.001). The mean difference was 6 ± 6%LV. Furthermore, the mean difference between the two methods was statistical higher in the patients with myocardial salvage index ≥0.90 than in the remaining patients (9 ± 8%LV vs. 6 ± 5%LV; P = 0.02). The ESA method performed after scar remodeling (3 months following STEMI) yields significantly lower AAR's and myocardial salvage indices compared to the T2-weighted method. Therefore, T2-weighted CMR plus LGE is the method of choice to assess AAR and myocardial salvage index using CMR. However, the ESA method is an easy and valid method for determining AAR, which can be used in settings where T2-weighted imaging has not been obtained in the acute phase.

Remodeling of the infarct territory in the time course of infarct healing in humans

OBJECT

To analyze the remodeling processes of the infarct territory in the time course of infarct healing.

MATERIALS AND METHODS

Serial late enhancement (LE) studies were performed in 30 patients following reperfused myocardial infarction (MI) in the first and second week post-MI and after 3 months. To characterize infarct remodeling over time, the following variables were derived and analyzed in a blinded fashion: Infarct size (IS, in mm(3)), maximum infarct thickness (IT(max), mm), mean infarct thickness (IT(mean), mm) and the variability of infarct thickness (VIT=IT(max)/IT(mean)). Further, a new parameter for the assessment of infarct remodeling, the infarct extent (IE, mm(2)) was computed. IE quantifies IS in two dimensions along the heart's circumferential and longitudinal directions. IS was divided by the IE to obtain IT(mean).

RESULTS

Overall infarct thickness was highly variable. Infarct shrinkage due to infarct thinning and IE reduction was found in the first months of healing. IS, IT(mean) and IT(max) significantly decreased during follow-up. There was a less consistent change of the IE: IE decreased in 75% of all infarcts from the first week up to 3 months post-MI, whereas 25% of infarcts expanded. Infarct thinning was found in almost all patients (92%), hence occurring in patients with infarct expansion and in patients without infarct expansion.

CONCLUSION

Infarct thinning and-to a lesser extent-IE reduction, contribute to infarct shrinkage in the time course of infarct healing. Infarct thinning may occur without infarct expansion.

Dynamic changes of edema and late gadolinium enhancement after acute myocardial infarction and their relationship to functional recovery and salvage index

BACKGROUND

Changes in the myocardium in acute ischemia are dynamic and complex, and the characteristics of myocardial tissue on cardiovascular magnetic resonance in the acute setting are not fully defined. We investigated changes in edema and late gadolinium enhancement (LGE) with serial imaging early after acute myocardial infarction, relating these to global and segmental myocardial function at 6 months.

METHODS AND RESULTS

Cardiovascular magnetic resonance scans were performed on 30 patients with ST-elevation--myocardial infarction treated by primary percutaneous coronary intervention at each of 4 time points: 12 to 48 hours; 5 to 7 days; 14 to 17 days; and 6 months. All patients showed edema at 24 hours. The mean volume of edema (% left ventricle) was 37±16 at 24 hours and 39±17 at 1 week, with a reduction to 24±13 (P<0.01) by 2 weeks. Myocardial segments with edema also had increased signal on LGE at 24 hours (κ=0.77; P<0.001). The volume of LGE decreased significantly between 24 hours and 6 months (27±15% versus 22±12%; P=0.002). Of segments showing LGE at 24 hours, 50% showed resolution by 6 months. In segments with such a reduction in LGE, 65% also showed improved wall motion (P<0.0001). The area of LGE measured at 6 months correlated more strongly with troponin at 48 hours (r=0.9; P<0.01) than LGE at 24 hours (r=0.7). The difference in LGE between 24 hours and 6 months had profound effects on the calculation of salvage index (26±21% at 24 hours versus 42±23% at 6 months; P=0.02).

CONCLUSIONS

Myocardial edema is maximal and constant over the first week after myocardial infarction, providing a stable window for the retrospective evaluation of area at risk. By contrast, myocardial areas with high signal intensity in LGE images recede over time with corresponding recovery of function, indicating that acutely detected LGE does not necessarily equate with irreversible injury and may severely underestimate salvaged myocardium.

Changes in left ventricular function and remodeling after myocardial infarction in hypothyroid rats

It has been shown that hypothyroidism may lead to delayed wound healing after experimental myocardial infarction (MI) in rats and increased infarct size in dogs. However, the long-term effect of hypothyroidism on left ventricular (LV) remodeling after MI has not been determined. Adult female Sprague-Dawley rats with and without surgical thyroidectomy (TX) were used in the study. Four weeks after TX, MI or sham MI was performed on TX and non-TX rats. Rats from all groups were examined 4 wk later. Four weeks after TX, hypothyroid-induced LV dysfunction was confirmed by echocardiography. In terminal experiments 4 wk after MI, TX sham-MI rats showed smaller hearts and impaired LV function compared with non-TX sham-MI controls. TX + MI rats showed smaller hearts with bigger infarct areas, higher LV end-diastolic pressures, and greater impairment of relaxation (−dP/d t) compared with non-TX MI rats. Relative changes after MI between TX and non-TX rats for most other hemodynamic and echocardiographic indexes were similar. These results suggest that preexisting hypothyroidism exaggerates post-MI remodeling and worsens LV function, particularly diastolic function.

c-Ski, Smurf2, and Arkadia as regulators of TGF-β signaling: new targets for managing myofibroblast function and cardiac fibrosisThis article is one of a selection of papers published in a special issue celebrating the 125th anniversary of the Faculty of Medicine at the University of Manitoba

Recent studies demonstrate the critical role of the extracellular matrix in the organization of parenchymal cells in the heart. Thus, an understanding of the modes of regulation of matrix production by cardiac myofibroblasts is essential. Transforming growth factor β (TGF-β) signaling is transduced through the canonical Smad pathway, and the involvement of this pathway in matrix synthesis and other processes requires precise control. Inhibition of Smad signaling may be achieved at the receptor level through the targeting of the TGF-β type I receptors with an inhibitory Smad7 / Smurf2 complex, or at the transcriptional level through c-Ski / receptor-Smad / co-mediator Smad4 interactions. Conversely, Arkadia protein intensifies TGF-β-induced effects by marking c-Ski and inhibitory Smad7 for destruction. The study of these TGF-β mediators is essential for future treatment of fibrotic disease, and this review highlights recent relevant findings that may impact our understanding of cardiac fibrosis.

Slowing of cardiomyocyte Ca2+ release and contraction during heart failure progression in postinfarction mice

Deterioration of cardiac contractility during congestive heart failure (CHF) is believed to involve decreased function of individual cardiomyocytes and may include reductions in contraction magnitude and/or kinetics. We examined the progression of in vivo and in vitro alterations in contractile function in CHF mice and investigated underlying alterations in Ca(2+) homeostasis. Following induction of myocardial infarction (MI), mice with CHF were examined at early (1 wk post-MI) and chronic (10 wk post-MI) stages of disease development. Sham-operated mice served as controls. Global and local left ventricle function were assessed by echocardiography in sedated animals ( approximately 2% isoflurane). Excitation-contraction coupling was examined in cardiomyocytes isolated from the viable septum. CHF progression between 1 and 10 wk post-MI resulted in increased mortality, development of hypertrophy, and deterioration of global left ventricular function. Local function in the noninfarcted myocardium also declined, as posterior wall shortening velocity was reduced in chronic CHF (1.2 +/- 0.1 vs. 1.9 +/- 0.2 cm/s in sham). Parallel alterations occurred in isolated cardiomyocytes since contraction and Ca(2+) transient time to peak values were prolonged in chronic CHF (115 +/- 6 and 158 +/- 11% sham values, respectively). Surprisingly, contraction and Ca(2+) transient magnitudes in CHF were larger than sham values at both time points, resulting from increased sarcoplasmic reticulum Ca(2+) content and greater Ca(2+) influx via L-type channels. We conclude that, in mice with CHF following myocardial infarction, declining myocardial function involves slowing of cardiomyocyte contraction without reduction in contraction magnitude. Corresponding alterations in Ca(2+) transients suggest that slowing of Ca(2+) release is a critical mediator of CHF progression.

Left ventricular remodeling can be predicted with left ventricular volume response during dobutamine echocardiography after acute myocardial infarction

OBJECTIVES

This study was performed to evaluate the significance of left ventricular (LV) volume response during dobutamine stress echocardiography (DSE) in the prediction of LV volume change during follow-up (F/U) in patients with acute myocardial infarction (AMI).

METHODS

Forty-five patients with AMI (male 39, age 57+/-10 y, anterior myocardial infarction [MI] 29) underwent DSE 6+/-4 d after AMI. Revascularization of the infarct-related artery was performed if severe stenosis was present. A F/U echocardiography was performed 7.5+/-3.4 mo after DSE. The LV end-diastolic volume (EDV) and end-systolic volume (ESV) using the modified Simpson's method were measured at baseline echocardiography, low-dose (10 microg x kg(-1) x min(-1)) DSE, and F/U echocardiography.

RESULTS

Patients were divided into 2 groups; Group I (n = 21) with an abnormal response (<10% decrease) in LVEDV during low-dose DSE; Group II (n = 24) with a normal response (> or =10% decrease) in LVEDV during low-dose DSE. At F/U echocardiography, the (%) change of LVEDV was significantly different between the 2 groups (-2.0+/-16.7 versus - 22.6+/-24.7%, p<0.01). Using multivariate analysis, the response of LVEDV (%) at low-dose DSE was the only significant independent predictor of the change of LVEDV (%) during F/U (y = 0.85 x - 0.03, r = 0.63, p<0.001).

CONCLUSIONS

The response of LVEDV during DSE can be used as a predictor for the LV volume change after AMI.

Three-dimensional mapping of mechanical activation patterns, contractile dyssynchrony and dyscoordination by two-dimensional strain echocardiography: rationale and design of a novel software toolbox

Background

Dyssynchrony of myocardial deformation is usually described in terms of variability only (e.g. standard deviations SD's). A description in terms of the spatio-temporal distribution pattern (vector-analysis) of dyssynchrony or by indices estimating its impact by expressing dyscoordination of shortening in relation to the global ventricular shortening may be preferential. Strain echocardiography by speckle tracking is a new non-invasive, albeit 2-D imaging modality to study myocardial deformation.

Methods

A post-processing toolbox was designed to incorporate local, speckle tracking-derived deformation data into a 36 segment 3-D model of the left ventricle. Global left ventricular shortening, standard deviations and vectors of timing of shortening were calculated. The impact of dyssynchrony was estimated by comparing the end-systolic values with either early peak values only (early shortening reserve ESR) or with all peak values (virtual shortening reserve VSR), and by the internal strain fraction (ISF) expressing dyscoordination as the fraction of deformation lost internally due to simultaneous shortening and stretching. These dyssynchrony parameters were compared in 8 volunteers (NL), 8 patients with Wolff-Parkinson-White syndrome (WPW), and 7 patients before (LBBB) and after cardiac resynchronization therapy (CRT).

Results

Dyssynchrony indices merely based on variability failed to detect differences between WPW and NL and failed to demonstrate the effect of CRT. Only the 3-D vector of onset of shortening could distinguish WPW from NL, while at peak shortening and by VSR, ESR and ISF no differences were found. All tested dyssynchrony parameters yielded higher values in LBBB compared to both NL and WPW. CRT reduced the spatial divergence of shortening (both vector magnitude and direction), and improved global ventricular shortening along with reductions in ESR and dyscoordination of shortening expressed by ISF.

Conclusion

Incorporation of local 2-D echocardiographic deformation data into a 3-D model by dedicated software allows a comprehensive analysis of spatio-temporal distribution patterns of myocardial dyssynchrony, of the global left ventricular deformation and of newer indices that may better reflect myocardial dyscoordination and/or impaired ventricular contractile efficiency. The potential value of such an analysis is highlighted in two dyssynchronous pathologies that impose particular challenges to deformation imaging.

Serial evaluations of myocardial infarct size after alcohol septal ablation in hypertrophic cardiomyopathy and effects of the changes on clinical status and left ventricular outflow pressure gradients

Alcohol septal ablation (ASA) as a treatment for obstructive hypertrophic cardiomyopathy produces septal infarction. There is a concern that such infarcts could be detrimental. Changes in the size of these infarcts by serial perfusion testing have not been studied. We performed resting serial-gated single-photon emission computed tomographic myocardial perfusion imaging in 30 patients (age 51+/-17 years, 57% were women) who had ASA between September 2003 and March 2007 before, 2+/-0.8 days (early), and 8.4+/-6.9 months (late) after ASA. Patients were also followed clinically and with serial 2-dimensional echocardiography. New York Heart Association class decreased from 3.50+/-0.51 before to 1.14+/-0.36 (p<0.0001) 3 months after ASA. The left ventricular (LV) outflow gradient (by Doppler echocardiography) decreased from 63+/-32 mm Hg before to 28+/-23 mm Hg after ASA (p<0.005). None of the patients had perfusion defects at rest before ASA. After ASA, perfusion defect size, involving the basal septum, decreased from 9.4+/-5.8% early to 5.2+/-4.2% of LV myocardium late after ASA (p<0.001). There were no changes in LV size and ejection fraction after ASA. In conclusion, ASA produces small basal ventricular septal infarcts (resting perfusion abnormality) involving<10% of the LV myocardium (including ventricular septum). There is a significant reduction in the perfusion abnormality late after ASA without an increase in LV outflow obstruction or recurrence of symptoms.

Myocardial Matrix Remodeling and the Matrix Metalloproteinases: Influence on Cardiac Form and Function

It is now becoming apparent that dynamic changes occur within the interstitium that directly contribute to adverse myocardial remodeling following myocardial infarction (MI), with hypertensive heart disease and with intrinsic myocardial disease such as cardiomyopathy. Furthermore, a family of matrix proteases, the matrix metalloproteinases (MMPs) and the tissue inhibitors of MMPs (TIMPs), has been recognized to play an important role in matrix remodeling in these cardiac disease states. The purpose of this review is fivefold: 1) to examine and redefine the myocardial matrix as a critical and dynamic entity with respect to the remodeling process encountered with MI, hypertension, or cardiomyopathic disease; 2) present the remarkable progress that has been made with respect to MMP/TIMP biology and how it relates to myocardial matrix remodeling; 3) to evaluate critical translational/clinical studies that have provided a cause-effect relationship between alterations in MMP/TIMP regulation and myocardial matrix remodeling; 4) to provide a critical review and analysis of current diagnostic, prognostic, and pharmacological approaches that utilized our basic understanding of MMP/TIMPs in the context of cardiac disease; and 5) most importantly, to dispel the historical belief that the myocardial matrix is a passive structure and supplant this belief that the regulation of matrix protease pathways such as the MMPs and TIMPs will likely yield a new avenue of diagnostic and therapeutic strategies for myocardial remodeling and the progression to heart failure.

Strain-related regional alterations of calcium-handling proteins in myocardial remodeling

BACKGROUND

Cardiac remodeling has been shown to have deleterious effects at both the global and local levels. The objective of this study is to investigate the role of strain in the initiation of structural and functional changes of myocardial tissue and its relation to alteration of calcium-handling proteins during cardiac remodeling after myocardial infarction.

METHODS

Sixteen sonomicrometry transducers were placed in the left ventricular free wall of 9 sheep to measure the regional strain in the infarct, adjacent, and remote myocardial regions. Hemodynamic, echocardiographic, and sonomicrometry data were collected before myocardial infarction, after infarction, and 2, 6, and 8 weeks after infarction. Regional myocardial tissues were collected for calcium-handling proteins at the end study.

RESULTS

At time of termination, end-systolic strains in 3 regionally distinct zones (remote, adjacent, and infarct) of myocardium were measured to be -14.65 +/- 1.13, -5.11 +/- 0.60 (P < or = .05), and 0.92 +/- 0.56 (P < or = .05), respectively. The regional end-systolic strain correlated strongly with the abundance of 2 major calcium-handling proteins: sarcoplasmic reticulum Ca2+ adenosine triphosphatase subtype 2a (r2 = 0.68, P < or = .05) and phospholamban (r2 = 0.50, P < or = .05). A lesser degree of correlation was observed between the systolic strain and the abundance of sodium/calcium exchanger type 1 protein (r2 = 0.17, P < or = .05).

CONCLUSIONS

Regional strain differences can be defined in the different myocardial regions during postinfarction cardiac remodeling. These differences in regional strain drive regionally distinct alterations in calcium-handling protein expression.

Scarred myocardium imposes additional burden on remote viable myocardium despite a reduction in the extent of area with late contrast MR enhancement

Magnetic resonance imaging (MRI) can simultaneously detect and quantify myocardial dysfunction and shrinkage in contrast-enhanced areas postinfarction. This ability permits the investigation of our hypothesis that transformation of infracted myocardium to scarred tissue imposes additional burdens on peri-infarcted and remote myocardium. Pigs (n = 8) were subjected to reperfused infarction. Gd-DOTA-enhanced inversion recovery gradient echo sequence (IR-GRE) imaging was performed 3 days and 8 weeks postinfarction. Global and regional left ventricular (LV) function was evaluated by cine MRI. Triphenyltetrazolium chloride (TTC) stain was used to delineate infarction while hematoxylin and eosin (H & E) and Masson's trichrome stains were used to characterize remodeled myocardium. Late contrast-enhanced MRIs showed a decrease in the extent of enhanced areas from 17 +/- 2% at 3 days to13 +/- 1% LV mass at 8 weeks. TTC infarction size was 12 +/- 1% LV mass. Cine MRIs showed expansion in dysfunctional area due to unfavorable remodeling, ischemia, or strain. Ejection fraction was reduced in association with increased end-diastolic and end-systolic volumes. Scarred myocardium contained collagen fibers and remodeled thick-walled vessels embedded in collagen. Sequential MRI showed greater LV dysfunction despite the shrinkage in extent of enhanced areas 2 months postinfarction. The integration of late enhancement and cine MRI incorporates anatomical and functional evaluation of remodeled hearts.

Thickening of the infarcted wall by collagen injection improves left ventricular function in rats: a novel approach to preserve cardiac function after myocardial infarction

OBJECTIVES

We determined whether collagen implantation could thicken the infarcted left ventricular (LV) wall and improve LV function.

BACKGROUND

We hypothesized that thickening the infarcted wall by using collagen might result in some benefits that are similar to what previously had been reported when the infarcted wall was thickened with cells.

METHODS

Fischer rats with one-week-old myocardial infarcts were injected with collagen or saline (100 microl) into the scar (n = 12 each group). Six weeks later, LV angiograms, hemodynamics, and regional myocardial blood flow were assessed. The hearts were processed for measurements of postmortem LV volume and histology.

RESULTS

Collagen injection significantly increased scar thickness (719 +/- 26 microm) compared with the saline-treated group (440 +/- 34 microm, p = 2.6 x 10(-6)). By LV angiography, stroke volume was significantly larger in the collagen-treated group (163 +/- 8 microl) than in the saline-treated group (129 +/- 6 microl, p = 0.005), and LV ejection fraction was also greater in the collagen-treated group (48.4 +/- 1.8%) than in the saline-treated group (40.7 +/- 1.0%, p = 0.002). Analysis of regional wall motion demonstrated paradoxical systolic bulging in 5 of 10 saline-treated rats that averaged 20.3 +/- 2.6% of the LV diastolic circumference, but in none of the 11 collagen-treated rats (p = 0.012). The LV end-diastolic and end-systolic volumes were 319 +/- 12 microl and 190 +/- 7 microl in the saline-treated group, respectively. There was a trend for larger LV end-diastolic volumes (343 +/- 23 microl), but smaller end-systolic volumes (180 +/- 16 microl) in the collagen-treated group.

CONCLUSIONS

This study shows that collagen injection thickens an infarct scar and improves LV stroke volume and ejection fraction, and prevents paradoxical systolic bulging after myocardial infarction.