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OBJECTIVE: The aim of the study is to compare structure tensor imaging (STI) with diffusion tensor imaging (DTI) of the sheep heart (approximately the same size as the human heart). MATERIALS AND METHODS: MRI acquisition on three sheep ex vivo hearts was performed at 9.4 T/30 cm with a seven-element RF coil. 3D FLASH with an isotropic resolution of 150 µm and 3D spin-echo DTI at 600 µm were performed. Tensor analysis, angles extraction and segments divisions were performed on both volumes. RESULTS: A 3D FLASH allows for visualization of the detailed structure of the left and right ventricles. The helix angle determined using DTI and STI exhibited a smooth transmural change from the endocardium to the epicardium. Both the helix and transverse angles were similar between techniques. Sheetlet organization exhibited the same pattern in both acquisitions, but local angle differences were seen and identified in 17 segments representation. DISCUSSION: This study demonstrated the feasibility of high-resolution MRI for studying the myocyte and myolaminar architecture of sheep hearts. We presented the results of STI on three whole sheep ex vivo hearts and demonstrated a good correspondence between DTI and STI.
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Imagem de Tensor de Difusão , Coração , Animais , Coração/diagnóstico por imagem , Ventrículos do Coração/diagnóstico por imagem , Imageamento por Ressonância Magnética , OvinosRESUMO
BACKGROUND: Cardiovascular magnetic resonance (CMR) can through the two methods 3D FLASH and diffusion tensor imaging (DTI) give complementary information on the local orientations of cardiomyocytes and their laminar arrays. METHODS: Eight explanted rat hearts were perfused with Gd-DTPA contrast agent and fixative and imaged in a 9.4T magnet by two types of acquisition: 3D fast low angle shot (FLASH) imaging, voxels 50 × 50 × 50 µm, and 3D spin echo DTI with monopolar diffusion gradients of 3.6 ms duration at 11.5 ms separation, voxels 200 × 200 × 200 µm. The sensitivity of each approach to imaging parameters was explored. RESULTS: The FLASH data showed laminar alignments of voxels with high signal, in keeping with the presumed predominance of contrast in the interstices between sheetlets. It was analysed, using structure-tensor (ST) analysis, to determine the most (v1(ST)), intermediate (v2(ST)) and least (v3(ST)) extended orthogonal directions of signal continuity. The DTI data was analysed to determine the most (e1(DTI)), intermediate (e2(DTI)) and least (e3(DTI)) orthogonal eigenvectors of extent of diffusion. The correspondence between the FLASH and DTI methods was measured and appraised. The most extended direction of FLASH signal (v1(ST)) agreed well with that of diffusion (e1(DTI)) throughout the left ventricle (representative discrepancy in the septum of 13.3 ± 6.7°: median ± absolute deviation) and both were in keeping with the expected local orientations of the long-axis of cardiomyocytes. However, the orientation of the least directions of FLASH signal continuity (v3(ST)) and diffusion (e3(ST)) showed greater discrepancies of up to 27.9 ± 17.4°. Both FLASH (v3(ST)) and DTI (e3(DTI)) where compared to directly measured laminar arrays in the FLASH images. For FLASH the discrepancy between the structure-tensor calculated v3(ST) and the directly measured FLASH laminar array normal was of 9 ± 7° for the lateral wall and 7 ± 9° for the septum (median ± inter quartile range), and for DTI the discrepancy between the calculated v3(DTI) and the directly measured FLASH laminar array normal was 22 ± 14° and 61 ± 53.4°. DTI was relatively insensitive to the number of diffusion directions and to time up to 72 hours post fixation, but was moderately affected by b-value (which was scaled by modifying diffusion gradient pulse strength with fixed gradient pulse separation). Optimal DTI parameters were b = 1000 mm/s(2) and 12 diffusion directions. FLASH acquisitions were relatively insensitive to the image processing parameters explored. CONCLUSIONS: We show that ST analysis of FLASH is a useful and accurate tool in the measurement of cardiac microstructure. While both FLASH and the DTI approaches appear promising for mapping of the alignments of myocytes throughout myocardium, marked discrepancies between the cross myocyte anisotropies deduced from each method call for consideration of their respective limitations.
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Meios de Contraste/administração & dosagem , Imagem de Tensor de Difusão/métodos , Gadolínio DTPA/administração & dosagem , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Imageamento por Ressonância Magnética/métodos , Miócitos Cardíacos/citologia , Animais , Preparação de Coração Isolado , Masculino , Contração Miocárdica , Valor Preditivo dos Testes , Ratos Wistar , Função Ventricular EsquerdaRESUMO
INTRODUCTION: Artificial intelligence (AI) encompasses a wide range of algorithms with risks when used to support decisions about diagnosis or treatment, so professional and regulatory bodies are recommending how they should be managed. AREAS COVERED: AI systems may qualify as standalone medical device software (MDSW) or be embedded within a medical device. Within the European Union (EU) AI software must undergo a conformity assessment procedure to be approved as a medical device. The draft EU Regulation on AI proposes rules that will apply across industry sectors, while for devices the Medical Device Regulation also applies. In the CORE-MD project (Coordinating Research and Evidence for Medical Devices), we have surveyed definitions and summarize initiatives made by professional consensus groups, regulators, and standardization bodies. EXPERT OPINION: The level of clinical evidence required should be determined according to each application and to legal and methodological factors that contribute to risk, including accountability, transparency, and interpretability. EU guidance for MDSW based on international recommendations does not yet describe the clinical evidence needed for medical AI software. Regulators, notified bodies, manufacturers, clinicians and patients would all benefit from common standards for the clinical evaluation of high-risk AI applications and transparency of their evidence and performance.
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Inteligência Artificial , Software , Humanos , Algoritmos , União Europeia , Inquéritos e QuestionáriosRESUMO
It has been shown by histology that cardiac myocytes are organized into laminae and this structure is important in function, both influencing the spread of electrical activation and enabling myocardial thickening in systole by laminar sliding. We have carried out high-spatial resolution three-dimensional MRI of the ventricular myolaminae of the entire volume of the isolated rat heart after contrast perfusion [dimeglumine gadopentate (Gd-DTPA)]. Four ex vivo rat hearts were perfused with Gd-DTPA and fixative and high-spatial resolution MRI was performed on a 9.4T MRI system. After MRI, cryosectioning followed by histology was performed. Images from MRI and histology were aligned, described, and quantitatively compared. In the three-dimensional MR images we directly show the presence of laminae and demonstrate that these are highly branching and are absent from much of the subepicardium. We visualized these MRI volumes to demonstrate laminar architecture and quantitatively demonstrated that the structural features observed are similar to those imaged in histology. We showed qualitatively and quantitatively that laminar architecture is similar in the four hearts. MRI can be used to image the laminar architecture of ex vivo hearts in three dimensions, and the images produced are qualitatively and quantitatively comparable with histology. We have demonstrated in the rat that: 1) laminar architecture is consistent between hearts; 2) myolaminae are absent from much of the subepicardium; and 3) although localized orthotropy is present throughout the myocardium, tracked myolaminae are branching structures and do not have a discrete identity.
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Meios de Contraste , Gadolínio DTPA , Coração/anatomia & histologia , Imageamento por Ressonância Magnética , Animais , Fixadores , Formaldeído , Interpretação de Imagem Assistida por Computador , Imageamento Tridimensional , Masculino , Perfusão , Ratos , Ratos Wistar , Reprodutibilidade dos Testes , Fixação de TecidosRESUMO
Pulmonary hypertension provokes right heart failure and arrhythmias. Better understanding of the mechanisms underlying these arrhythmias is needed to facilitate new therapeutic approaches for the hypertensive, failing right ventricle (RV). The aim of our study was to identify the mechanisms generating arrhythmias in a model of RV failure induced by pulmonary hypertension. Rats were injected with monocrotaline to induce either RV hypertrophy or failure or with saline (control). ECGs were measured in conscious, unrestrained animals by telemetry. In isolated hearts, electrical activity was measured by optical mapping and myofiber orientation by diffusion tensor-MRI. Sarcoplasmic reticular Ca(2+) handling was studied in single myocytes. Compared with control animals, the T-wave of the ECG was prolonged and in three of seven heart failure animals, prominent T-wave alternans occurred. Discordant action potential (AP) alternans occurred in isolated failing hearts and Ca(2+) transient alternans in failing myocytes. In failing hearts, AP duration and dispersion were increased; conduction velocity and AP restitution were steeper. The latter was intrinsic to failing single myocytes. Failing hearts had greater fiber angle disarray; this correlated with AP duration. Failing myocytes had reduced sarco(endo)plasmic reticular Ca(2+)-ATPase activity, increased sarcoplasmic reticular Ca(2+)-release fraction, and increased Ca(2+) spark leak. In hypertrophied hearts and myocytes, dysfunctional adaptation had begun, but alternans did not develop. We conclude that increased electrical and structural heterogeneity and dysfunctional sarcoplasmic reticular Ca(2+) handling increased the probability of alternans, a proarrhythmic predictor of sudden cardiac death. These mechanisms are potential therapeutic targets for the correction of arrhythmias in hypertensive, failing RVs.
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Arritmias Cardíacas/fisiopatologia , Insuficiência Cardíaca/etiologia , Insuficiência Cardíaca/fisiopatologia , Hipertensão Pulmonar/complicações , Disfunção Ventricular Direita/fisiopatologia , Potenciais de Ação/fisiologia , Animais , Cálcio/metabolismo , ATPases Transportadoras de Cálcio/metabolismo , Eletrocardiografia , Masculino , Modelos Animais , Miócitos Cardíacos/patologia , Ratos , Ratos Wistar , Retículo Sarcoplasmático/metabolismoRESUMO
Mechanical stimulation has important consequences for myocardial function. However, this stimulation and the response to it, is not uniform. The right ventricle is thinner walled and operates at lower pressure than the left ventricle. Within the ventricles, differences in the orientation of myocardial fibres exist. These differences produce inhomogeneity in the stress and strain between and across the ventricles. Possibly as a result of these variations in mechanical stimulation, there are well characterised inhomogeneities in gene expression and protein function within the ventricular myocardium, for example in the transient outward K+ current and its associated Kv channels. Perhaps not surprisingly, it is becoming apparent that gradients of expression and function exist for proteins that are intimately involved in the response to mechanical stimulation in the heart, for example in the left ventricle of the rat there is a transmural gradient in mRNA and current density of the mechanosensitive two-pore domain K+ channel TREK-1 (ENDO>EPI). In healthy hearts it is assumed that these gradients are important for normal function and therefore that their disruption in diseased myocardium is involved in the dysfunction that occurs.
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Coração/fisiologia , Mecanotransdução Celular/fisiologia , Miocárdio/metabolismo , Canais de Potássio de Domínios Poros em Tandem/fisiologia , Animais , Expressão Gênica , Ratos , Função VentricularRESUMO
BACKGROUND: Ventricular arrhythmias are frequent in patients with repaired tetralogy of Fallot (rTOF), but their origin and underlying mechanisms remain unclear. In this study, the involvement of left ventricular (LV) electrical and structural remodeling was assessed in an animal model mimicking rTOF sequelae. METHODS: Piglets underwent a tetralogy of Fallot repair-like surgery (n=6) or were sham operated (Sham, n=5). Twenty-three weeks post-surgery, cardiac function was assessed in vivo by magnetic resonance imaging. Electrophysiological properties were characterized by optical mapping. LV fibrosis and connexin-43 localization were assessed on histological sections and protein expression assessed by Western Blot. RESULTS: Right ventricular dysfunction was evident, whereas LV function remained unaltered in rTOF pigs. Optical mapping showed longer action potential duration on the rTOF LV epicardium and endocardium. Epicardial conduction velocity was significantly reduced in the longitudinal direction in rTOF LVs but not in the transverse direction compared with Sham. An elevated collagen content was found in LV basal and apical sections from rTOF pigs. Moreover, a trend for connexin-43 lateralization with no change in protein expression was found in the LV of rTOFs. Finally, rTOF LVs had a lower threshold for arrhythmia induction using incremental pacing protocols. CONCLUSIONS: We found an arrhythmogenic substrate with prolonged heterogeneous action potential duration and reduced conduction velocity in the LV of rTOF pigs. This remodeling precedes LV dysfunction and is likely to contribute to ventricular arrhythmias and sudden cardiac death in patients with rTOF.
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Arritmias Cardíacas/etiologia , Procedimentos Cirúrgicos Cardíacos/efeitos adversos , Ventrículos do Coração/fisiopatologia , Tetralogia de Fallot/cirurgia , Função Ventricular Esquerda , Remodelação Ventricular , Potenciais de Ação , Animais , Animais Recém-Nascidos , Arritmias Cardíacas/diagnóstico por imagem , Arritmias Cardíacas/metabolismo , Arritmias Cardíacas/fisiopatologia , Conexina 43/metabolismo , Modelos Animais de Doenças , Fibrose , Frequência Cardíaca , Ventrículos do Coração/diagnóstico por imagem , Ventrículos do Coração/metabolismo , Imageamento por Ressonância Magnética , Sus scrofa , Tetralogia de Fallot/fisiopatologia , Fatores de Tempo , Imagens com Corantes Sensíveis à VoltagemRESUMO
The architecture of the heart remains controversial despite extensive effort and recent advances in imaging techniques. Several opposing and non-mutually compatible models have been proposed to explain cardiac structure, and these models, although limited, have advanced the study and understanding of heart structure, function and development. We describe key areas of similarity and difference, highlight areas of contention and point to the important limitations of these models. Recent research in animal models on the nature, geometry and interaction of cardiac sheet structure allows unification of some seemingly conflicting features of the structural models. Intriguingly, evidence points to significant inter-individual structural variability (within constrained limits) in the canine, leading to the concept of a continuum (or distribution) of cardiac structures. This variability in heart structure partly explains the ongoing debate on myocardial architecture. These developments are used to construct an integrated description of cardiac structure unifying features of fibre, sheet and band architecture that provides a basis for (i) explaining cardiac electromechanics, (ii) computational simulations of cardiac physiology and (iii) designing interventions.
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Coração/anatomia & histologia , Modelos Cardiovasculares , Animais , Cães , Ventrículos do Coração/anatomia & histologia , Humanos , Imageamento por Ressonância Magnética/métodos , Microscopia Confocal/métodos , Contração Miocárdica/fisiologia , Miócitos Cardíacos/citologia , Miofibrilas , Terminologia como AssuntoRESUMO
OBJECTIVE: The growing adult population with surgically corrected tetralogy of Fallot (TOF) is at risk of arrhythmias and sudden cardiac death. We sought to investigate the contribution of right ventricular (RV) structural and electrophysiological remodelling to arrhythmia generation in a preclinical animal model of repaired TOF (rTOF). METHODS AND RESULTS: Pigs mimicking rTOF underwent cardiac MRI functional characterisation and presented with pulmonary regurgitation, RV hypertrophy, dilatation and dysfunction compared with Sham-operated animals (Sham). Optical mapping of rTOF RV-perfused wedges revealed a significant prolongation of RV activation time with slower conduction velocities and regions of conduction slowing well beyond the surgical scar. A reduced protein expression and lateralisation of Connexin-43 were identified in rTOF RVs. A remodelling of extracellular matrix-related gene expression and an increase in collagen content that correlated with prolonged RV activation time were also found in these animals. RV action potential duration (APD) was prolonged in the epicardial anterior region at early and late repolarisation level, thus contributing to a greater APD heterogeneity and to altered transmural and anteroposterior APD gradients in rTOF RVs. APD remodelling involved changes in Kv4.3 and MiRP1 expression. Spontaneous arrhythmias were more frequent in rTOF wedges and more complex in the anterior than in the posterior RV. CONCLUSION: Significant remodelling of RV conduction and repolarisation properties was found in pigs with rTOF. This remodelling generates a proarrhythmic substrate likely to facilitate re-entries and to contribute to sudden cardiac death in patients with rTOF.
Assuntos
Arritmias Cardíacas/etiologia , Procedimentos Cirúrgicos Cardíacos/efeitos adversos , Sistema de Condução Cardíaco/fisiopatologia , Tetralogia de Fallot/cirurgia , Função Ventricular Direita , Remodelação Ventricular , Potenciais de Ação , Animais , Arritmias Cardíacas/metabolismo , Arritmias Cardíacas/fisiopatologia , Conexina 43/metabolismo , Modelos Animais de Doenças , Sistema de Condução Cardíaco/metabolismo , Frequência Cardíaca , Imageamento por Ressonância Magnética , Canais de Potássio Shal/metabolismo , Sus scrofa , Tetralogia de Fallot/complicações , Tetralogia de Fallot/metabolismo , Tetralogia de Fallot/fisiopatologia , Fatores de Tempo , Imagens com Corantes Sensíveis à VoltagemRESUMO
Mathematical modeling of excitation-contraction coupling (ECC) in ventricular cardiac myocytes is a multiscale problem, and it is therefore difficult to develop spatially detailed simulation tools. ECC involves gradients on the length scale of 100 nm in dyadic spaces and concentration profiles along the 100 µm of the whole cell, as well as the sub-millisecond time scale of local concentration changes and the change of lumenal Ca(2+) content within tens of seconds. Our concept for a multiscale mathematical model of Ca(2+) -induced Ca(2+) release (CICR) and whole cardiomyocyte electrophysiology incorporates stochastic simulation of individual LC- and RyR-channels, spatially detailed concentration dynamics in dyadic clefts, rabbit membrane potential dynamics, and a system of partial differential equations for myoplasmic and lumenal free Ca(2+) and Ca(2+)-binding molecules in the bulk of the cell. We developed a novel computational approach to resolve the concentration gradients from dyadic space to cell level by using a quasistatic approximation within the dyad and finite element methods for integrating the partial differential equations. We show whole cell Ca(2+)-concentration profiles using three previously published RyR-channel Markov schemes.
RESUMO
Surgical repair of Tetralogy of Fallot (TOF) is highly successful but may be complicated in adulthood by arrhythmias, sudden death, and right ventricular or biventricular dysfunction. To better understand the molecular and cellular mechanisms of these delayed cardiac events, a chronic animal model of postoperative TOF was studied using microarrays to perform cardiac transcriptomic studies. The experimental study included 12 piglets (7 rTOF and 5 controls) that underwent surgery at age 2 months and were further studied after 23 (+/- 1) weeks of postoperative recovery. Two distinct regions (endocardium and epicardium) from both ventricles were analyzed. Expression levels from each localization were compared in order to decipher mechanisms and signaling pathways leading to ventricular dysfunction and arrhythmias in surgically repaired TOF. Several genes were confirmed to participate in ventricular remodeling and cardiac failure and some new candidate genes were described. In particular, these data pointed out FRZB as a heart failure marker. Moreover, calcium handling and contractile function genes (SLN, ACTC1, PLCD4, PLCZ), potential arrhythmia-related genes (MYO5B, KCNA5), and cytoskeleton and cellular organization-related genes (XIRP2, COL8A1, KCNA6) were among the most deregulated genes in rTOF ventricles. To our knowledge, this is the first comprehensive report on global gene expression profiling in the heart of a long-term swine model of repaired TOF.
Assuntos
Regulação da Expressão Gênica , Miocárdio/metabolismo , Miocárdio/patologia , Tetralogia de Fallot/genética , Tetralogia de Fallot/cirurgia , Animais , Doença Crônica , Modelos Animais de Doenças , Endocárdio/patologia , Perfilação da Expressão Gênica , Ontologia Genética , Redes Reguladoras de Genes , Estudos de Associação Genética , Ventrículos do Coração/metabolismo , Ventrículos do Coração/patologia , Análise de Sequência com Séries de Oligonucleotídeos , Especificidade de Órgãos/genética , Reação em Cadeia da Polimerase em Tempo Real , Reprodutibilidade dos Testes , Sus scrofaRESUMO
Pathological cardiac fibrosis is important in predisposing the heart to arrhythmia and mechanical failure. The regional distribution of fibrosis is often described qualitatively and quantitatively in histological studies of animal hearts after staining collagen with specific colored stains. Currently this description is often piecemeal, as it lacks rigorous spatial registration, matching and methodological standardization between animals and between study groups. We propose a strategy for the quantification of regional fibrosis using the American Heart Association (AHA) cardiac segmentation model. We quantify fibrosis after whole heart 3D histological reconstruction in one normal rat heart and in one rat heart in right heart failure induced by monocrotaline. We then assess the minimum spaced histological sampling which allows for accurate assessment of regional fibrosis. We show that using every section of a set of 5 µm serial sections quantifies regional right ventricular fibrosis, with highly significant (p <; 0.001) differences between heart failure and control hearts. We show that the absolute error of collagen quantification is low when sections are taken spaced by up to 100 µm (error 5.7±5.8%). Likewise, absolute error associated with sectioning starting position is low for sections spaced up to 100 µm (error 13.3±17.2%). Above 100 µm section spacing quantification error is large (tending to 50%) and error associated with sectioning starting position is large (tending to 60%).
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Insuficiência Cardíaca/diagnóstico , Animais , Colágeno/metabolismo , Fibrose , Insuficiência Cardíaca/metabolismo , Ventrículos do Coração/metabolismo , Ventrículos do Coração/patologia , Interpretação de Imagem Assistida por Computador , Imageamento Tridimensional , Masculino , Ratos WistarRESUMO
DT-MRI has been widely used to quantify myocardial fiber and laminar orientations. These structural orientations influence both the spread of excitation and the reorganization of the myocardium during contraction and are altered in disease states. Studies have sought to validate DT-MRI but questions remain about the accuracy of the method and its sensitivity to the time post-fixation and imaging parameters, including b-value, number of diffusion directions and image voxel size. The advent of high-spatial resolution ex vivo MRI and structure tensor (ST) analysis provides a means of direct validation of DT-MRI and assessment of sensitivity to the b-value, the number of diffusion directions and the image voxel size. We find that, with the fixation method we used, structure does not change with time (up to 72 hours). We show that DT-MRI and ST/HR-MRI are markedly similar measures of fiber orientation but DT-MRI and ST are much less similar measures of laminar orientation. DT-MRI performance is not sensitive to the number of directions, with similar structural orientations measured with 6 or 12 directions. Likewise, DT-MRI performance is generally insensitive to b-value, but laminar measurement is moderately more accurate at b = 500 than for higher b-values.
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Miocárdio/patologia , Animais , Imagem de Difusão por Ressonância Magnética , Imagem de Tensor de Difusão , Masculino , Ratos Wistar , Fatores de Tempo , Fixação de TecidosRESUMO
BACKGROUND: The cardiac conduction system consists of the sinus node, nodal extensions, atrioventricular (AV) node, penetrating bundle, bundle branches, and Purkinje fibers. Node-like AV ring tissue also exists at the AV junctions, and the right and left rings unite at the retroaortic node. The study aims were to (1) construct a 3-dimensional anatomical model of the AV rings and retroaortic node, (2) map electrical activation in the right ring and study its action potential characteristics, and (3) examine gene expression in the right ring and retroaortic node. METHODS AND RESULTS: Three-dimensional reconstruction (based on magnetic resonance imaging, histology, and immunohistochemistry) showed the extent and organization of the specialized tissues (eg, how the AV rings form the right and left nodal extensions into the AV node). Multiextracellular electrode array and microelectrode mapping of isolated right ring preparations revealed robust spontaneous activity with characteristic diastolic depolarization. Using laser microdissection gene expression measured at the mRNA level (using quantitative PCR) and protein level (using immunohistochemistry and Western blotting) showed that the right ring and retroaortic node, like the sinus node and AV node but, unlike ventricular muscle, had statistically significant higher expression of key transcription factors (including Tbx3, Msx2, and Id2) and ion channels (including HCN4, Cav3.1, Cav3.2, Kv1.5, SK1, Kir3.1, and Kir3.4) and lower expression of other key ion channels (Nav1.5 and Kir2.1). CONCLUSIONS: The AV rings and retroaortic node possess gene expression profiles similar to that of the AV node. Ion channel expression and electrophysiological recordings show the AV rings could act as ectopic pacemakers and a source of atrial tachycardia.
Assuntos
Sistema de Condução Cardíaco/metabolismo , Potenciais de Ação/fisiologia , Animais , Nó Atrioventricular/anatomia & histologia , Nó Atrioventricular/metabolismo , Nó Atrioventricular/fisiologia , Fascículo Atrioventricular/anatomia & histologia , Fascículo Atrioventricular/metabolismo , Fascículo Atrioventricular/fisiologia , Sistema de Condução Cardíaco/anatomia & histologia , Sistema de Condução Cardíaco/fisiologia , Modelos Anatômicos , Proteoma , Ramos Subendocárdicos/anatomia & histologia , Ramos Subendocárdicos/metabolismo , Ramos Subendocárdicos/fisiologia , Ratos , Nó Sinoatrial/anatomia & histologia , Nó Sinoatrial/metabolismo , Nó Sinoatrial/fisiologia , TranscriptomaRESUMO
Micro-computed tomography (micro-CT) has been widely used to generate high-resolution 3-D tissue images from small animals nondestructively, especially for mineralized skeletal tissues. However, its application to the analysis of soft cardiovascular tissues has been limited by poor inter-tissue contrast. Recent ex vivo studies have shown that contrast between muscular and connective tissue in micro-CT images can be enhanced by staining with iodine. In the present study, we apply this novel technique for imaging of cardiovascular structures in canine hearts. We optimize the method to obtain high-resolution X-ray micro-CT images of the canine atria and its distinctive regions-including the Bachmann's bundle, atrioventricular node, pulmonary arteries and veins-with clear inter-tissue contrast. The imaging results are used to reconstruct and segment the detailed 3-D geometry of the atria. Structure tensor analysis shows that the arrangement of atrial fibers can also be characterized using the enhanced micro-CT images, as iodine preferentially accumulates within the muscular fibers rather than in connective tissues. This novel technique can be particularly useful in nondestructive imaging of 3-D cardiac architectures from large animals and humans, due to the combination of relatively high speed ( ~ 1 h/per scan of the large canine heart) and high voxel resolution (36 µm) provided. In summary, contrast micro-CT facilitates fast and nondestructive imaging and segmenting of detailed 3-D cardiovascular geometries, as well as measuring fiber orientation, which are crucial in constructing biophysically detailed computational cardiac models.
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Coração/anatomia & histologia , Coração/diagnóstico por imagem , Imageamento Tridimensional/métodos , Compostos de Iodo/química , Modelos Cardiovasculares , Microtomografia por Raio-X/métodos , Animais , Meios de Contraste/química , Vasos Coronários/anatomia & histologia , Vasos Coronários/diagnóstico por imagem , Cães , FemininoRESUMO
Mechanisms underlying the genesis of re-entrant substrate for atrial fibrillation (AF) in the pulmonary veins (PVs) and left atrium (LA) are not well understood. We develop a biophysically detailed computational model for the PVs and surrounding LA tissue. The model integrates canine PV and LA single cell electrophysiology with the respective 3D tissue geometry and fiber orientation reconstructed from micro-CT data. The model simulations demonstrate that a combination of tissue anisotropy and electrical heterogeneity between the PVs and LA causes a break-down of normal electrical excitation wave-fronts. This leads to the generation of a high-frequency re-entrant source near the PV sleeves. Evidence of such sources have been seen clinically in AF patients. In summary, our modeling results provide new insights into the arrhythmogenic mechanisms of re-entrant excitation waves underlying AF.
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Fibrilação Atrial/fisiopatologia , Simulação por Computador , Modelos Cardiovasculares , Veias Pulmonares/fisiopatologia , Animais , Fibrilação Atrial/patologia , Cães , Átrios do Coração/patologia , Átrios do Coração/fisiopatologia , Humanos , Veias Pulmonares/patologiaRESUMO
The ventricular myocardium has a structure of branching laminae through which course regularly orientated myofibers, an architecture important in excitation and contraction. Quantifying this architecture is vital for understanding normal and disease states in the heart and for assessing their impact on electrical function. These data are also highly important in the construction of scientifically and clinically useful computer models of cardiac electrical behavior. Detailed structural information has previously been obtained from serial imaging. In this work we assess the potential for high-resolution (HR) MRI as a means to furnish useful myoarchitecture and compare and contrast this approach with the growing use of DT. Using rat hearts, we conclude that both approaches have strengths and weaknesses, however, HR-MRI may provide a consistently more robust picture of the myoarchitecture in small hearts.
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Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imagem Cinética por Ressonância Magnética/métodos , Miocárdio/citologia , Animais , Masculino , Ratos , Ratos Wistar , Reprodutibilidade dos Testes , Sensibilidade e EspecificidadeRESUMO
Computational models of human atrial cells, tissues and atria have been developed. Cell models, for atrial wall, crista terminalis, appendage, Bachmann's bundle and pectinate myocytes are characterised by action potentials, ionic currents and action potential duration (APD) restitution. The principal effect of the ion channel remodelling of persistent atrial fibrillation (AF), and a mutation producing familial AF, was APD shortening at all rates. Electrical alternans was abolished by the modelled action of Dronedarone. AF induced gap junctional remodelling slows propagation velocity at all rates. Re-entrant spiral waves in 2-D models are characterised by their frequency, wavelength, meander and stability. For homogenous models of normal tissue, spiral waves self-terminate, due to meander to inexcitable boundaries, and by dissipation of excitation. AF electrical remodelling in these homogenous models led to persistence of spiral waves, and AF fibrotic remodelling to their breakdown into fibrillatory activity. An anatomical model of the atria was partially validated by the activation times of normal sinus rhythm. The use of tissue geometry from clinical MRI, and tissue anisotropy from ex vivo diffusion tensor magnetic resonance imaging is outlined. In the homogenous model of normal atria, a single scroll breaks down onto spatio-temporal irregularity (electrical fibrillation) that is self-terminating; while in the AF remodelled atria the fibrillatory activity is persistent. The persistence of electrical AF can be dissected in the model in terms of ion channel and intercellular coupling processes, that can be modified pharmacologically; the effects of anatomy, that can be modified by ablation; and the permanent effects of fibrosis, that need to be prevented.
Assuntos
Antiarrítmicos/farmacologia , Arritmias Cardíacas/tratamento farmacológico , Função Atrial/efeitos dos fármacos , Simulação por Computador , Modelos Cardiovasculares , Biologia de Sistemas , Potenciais de Ação , Arritmias Cardíacas/genética , Arritmias Cardíacas/patologia , Arritmias Cardíacas/fisiopatologia , Imagem de Tensor de Difusão , Fibrose , Predisposição Genética para Doença , Átrios do Coração/efeitos dos fármacos , Átrios do Coração/patologia , Átrios do Coração/fisiopatologia , Humanos , Mutação , Fenótipo , Fatores de Tempo , Interface Usuário-ComputadorRESUMO
Transmural myocardial activation is influenced by myocardial structure, including structural differences between the compacta (Cta) and the trabeculata (Tta), although this has not been fully explained. Hearts from rats were Langendorff perfused, stained with DI-4-ANEPPS, the apex was cut off and fluorescence acquired from the exposed short-axis surface. The hearts were stimulated at 160 ms cycle length at the anterior, lateral, posterior left ventricle (LV) and septal sub-epicardial sites. Conduction velocity perpendicular to the wave front orientation was measured in each pixel using a gradient-based approach. After optical mapping the cut surface was imaged using a light microscope and the extent of the Cta and Tta mapped and validated against 50 u, m isotropic MRI images. We used a 3D rat ventricle computational model, with architecture obtained from 200 u, m isotropic diffusion tensor MRI and kinetics from the modified Pandit model to determine the relative roles of fibers and sheets on propagation. We show in the experimental study that circumferential propagation around the LV cavity is fast in the Cta: 63.2±19.5 and is slower in the Tta: 32.7±11.0(∗) (mean ± s.d cms-1, ∗ p < 0.01 by two sample t test). In the simulation study the pattern and velocity are not replicated in an isotropic model (I), are partially replicated in a simulation study including fiber anisotropy (A) and is more fully replicated in orthotropic (O) ventricles (fiber and sheet anisotropy), where the circumferential propagation velocity is, I: Cta: 54.2±3.9; Tta:54.3±3.9; A: Cta:43.6±3.2; Tta: 40.6±6.6; O: Cta: 63.2±19.5; Tta: 32.7±11.9(∗). We show that sheet orientation is important in understanding activation differences between Cta and Tta.
Assuntos
Potenciais de Ação/fisiologia , Sistema de Condução Cardíaco/fisiologia , Função Ventricular Esquerda/fisiologia , Imagens com Corantes Sensíveis à Voltagem/métodos , Animais , Anisotropia , Masculino , Ratos , Ratos WistarRESUMO
Reaction-diffusion computational models of cardiac electrophysiology require both dynamic excitation models that reconstruct the action potentials of myocytes as well as datasets of cardiac geometry and architecture that provide the electrical diffusion tensor D, which determines how excitation spreads through the tissue. We illustrate an experimental pipeline we have developed in our laboratories for constructing and validating such datasets. The tensor D changes with location in the myocardium, and is determined by tissue architecture. Diffusion tensor magnetic resonance imaging (DT-MRI) provides three eigenvectors e(i) and eigenvalues λ(i) at each voxel throughout the tissue that can be used to reconstruct this architecture. The primary eigenvector e(1) is a histologically validated measure of myocyte orientation (responsible for anisotropic propagation). The secondary and tertiary eigenvectors (e(2) and e(3)) specify the directions of any orthotropic structure if λ(2) is significantly greater than λ(3)-this orthotropy has been identified with sheets or cleavage planes. For simulations, the components of D are scaled in the fibre and cross-fibre directions for anisotropic simulations (or fibre, sheet and sheet normal directions for orthotropic tissues) so that simulated conduction velocities match values from optical imaging or plunge electrode experiments. The simulated pattern of propagation of action potentials in the models is partially validated by optical recordings of spatio-temporal activity on the surfaces of hearts. We also describe several techniques that enhance components of the pipeline, or that allow the pipeline to be applied to different areas of research: Q ball imaging provides evidence for multi-modal orientation distributions within a fraction of voxels, infarcts can be identified by changes in the anisotropic structure-irregularity in myocyte orientation and a decrease in fractional anisotropy, clinical imaging provides human ventricular geometry and can identify ischaemic and infarcted regions, and simulations in human geometries examine the roles of anisotropic and orthotropic architecture in the initiation of arrhythmias.