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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Optical mapping has become an indispensible tool for studying cardiac electrical activity. However, due to the three-dimensional nature of the optical signal, the optical upstroke is significantly longer than the electrical upstroke. This raises the issue of how to accurately determine the activation time on the epicardial surface. The purpose of this study was to establish a link between the optical upstroke and exact surface activation time using computer simulations, with subsequent validation by a combination of microelectrode recordings and optical mapping experiments. To simulate wave propagation and associated optical signals, we used a hybrid electro-optical model. We found that the time of the surface electrical activation (t(E)) within the accuracy of our simulations coincided with the maximal slope of the optical upstroke (t(F)*) for a broad range of optical attenuation lengths. This was not the case when the activation time was determined at 50% amplitude (t(F50)) of the optical upstroke. The validation experiments were conducted in isolated Langendorff-perfused rat hearts and coronary-perfused pig left ventricles stained with either di-4-ANEPPS or the near-infrared dye di-4-ANBDQBS. We found that t(F)* was a more accurate measure of t(E) than was t(F50) in all experimental settings tested (P = 0.0002). Using t(F)* instead of t(F50) produced the most significant improvement in measurements of the conduction anisotropy and the transmural conduction time in pig ventricles.
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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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Clinical reports describe life-threatening cardiac arrhythmias after environmental exposure to carbon monoxide (CO) or accidental CO poisoning. Numerous case studies describe disruption of repolarization and prolongation of the QT interval, yet the mechanisms underlying CO-induced arrhythmias are unknown.
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The dependency of wave velocity in reaction-diffusion (RD) systems on the local front curvature determines not only the stability of wave propagation, but also the fundamental properties of other spatial configurations such as vortices. This Letter gives the first derivation of a covariant eikonal-curvature relation applicable to general RD systems with spatially varying anisotropic diffusion properties, such as cardiac tissue. The theoretical prediction that waves which seem planar can nevertheless possess a nonvanishing geometrical curvature induced by local anisotropy is confirmed by numerical simulations, which reveal deviations up to 20% from the nominal plane wave speed.
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Current techniques to map intramural activation patterns in ex vivo cardiac tissue have limited spatial resolution. Here, we report on the experimental validation of a novel optical technique that was recently proposed to resolve the size and depth of intramural wave fronts using alternating transillumination (AT). AT was achieved by simultaneously mapping the epi- and endocardial surfaces with two synchronized CCD cameras and rapidly alternating LED illumination between both surfaces. Optical phantoms were made based on tissue optical properties measured using a hybrid optical spectrometer. Spherical fluorescent sources (Scarlet microspheres, Invitrogen, UK) of varying sizes were embedded at known depths in the phantoms. Coronary-perfused procine left ventricular slab preparations were stained with DI-4-ANBDQBS (n = 3) and paced at known intramural depths. In phantoms we were able to reliably estimate the depth of the center of fluorescent sources (9.6± 5.4% error), as well as their transmural extent (15.7 ± 11.5% error). In ventricular slabs we were able to localize the sites of origin of intramural excitation waves with a precision of ± 1.6 mm. Transmural conduction velocities were, for the first time, measured optically from the surfaces and found to be 21.0 ± 12.4 cm/s. In conclusion, alternating transillumination is a promising technique for reliable reconstruction of depth and expansion rate of intramural activation wave fronts in cardiac tissue.
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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.
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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.
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Current techniques to map intramural activation patterns in ex vivo cardiac tissue have limited spatial resolution. Here, we report on the experimental validation of a novel optical technique that was recently proposed to resolve the size and depth of intramural wave fronts using alternating transillumination (AT). AT was achieved by simultaneously mapping the epi- and endocardial surfaces with two synchronized CCD cameras and rapidly alternating LED illumination between both surfaces. Optical phantoms were made based on tissue optical properties measured using a hybrid optical spectrometer. Spherical fluorescent sources (Scarlet microspheres, Invitrogen, UK) of varying sizes were embedded at known depths in the phantoms. Coronary-perfused procine left ventricular slab preparations were stained with DI-4-ANBDQBS (n = 3) and paced at known intramural depths. In phantoms we were able to reliably estimate the depth of the center of fluorescent sources (9.6± 5.4% error), as well as their transmural extent (15.7 ± 11.5% error). In ventricular slabs we were able to localize the sites of origin of intramural excitation waves with a precision of ± 1.6 mm. Transmural conduction velocities were, for the first time, measured optically from the surfaces and found to be 21.0 ± 12.4 cm/s. In conclusion, alternating transillumination is a promising technique for reliable reconstruction of depth and expansion rate of intramural activation wave fronts in cardiac tissue.
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The rotating fiber orientation within the cardiac wall substantially affects the electrical propagation and can cause intra-myocardial cusp waves. Numerical simulations have shown that the cusps form in layers where propagation is perpendicular to the fiber orientation and lead to complex wave front morphologies. They can travel across layers and break through at the epi- or endocardial surfaces where they cause apparent accelerations of propagation. The validation of these results remains a major experimental challenge. In the present study, we investigate both computationally and experimentally how intramural cusp waves can be detected using optical imaging. Our simulations show that cusps alter the optical upstroke morphology and can be detected well before they reach the surface (up to 1 mm deep). Experiments in Langendorff-perfused guinea pig hearts are consistent with our numerical findings.
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Voltage-sensitive dyes have become an important tool in visualizing electrical activity in cardiac tissue. However, there are no established methods for assessing the contribution of intramural electrical excitation to recorded optical signals. Here, we develop algorithms to calculate voltage-dependent optical signals from three-dimensional distributions of transmembrane voltage inside the myocardial wall (the forward problem). Optical diffusion theory is applied for different imaging modes including subsurface imaging or epi-illumination, transillumination and coaxial scanning. We use the solutions of the forward problem to assess these imaging methods with respect to their effectiveness in visualizing two types of 3D cardiac activity: electrical point sources and intramural scroll waves initiated at various depths. Simulations were performed both for fluorescent and absorptive voltage-sensitive dyes. In the case of point sources, we focus on the lateral optical resolution, as a function of the source depth. We find that, among the studied methods, fluorescent coaxial scanning yields the best optical resolution (<2.5 mm). In the case of scroll waves we investigate how well the filament, i.e. the organizing center, can be visualized as function of its depth. Our results show that using absorptive transillumination, filaments can be detected up to 3 mm below the recording surface. The presented results provide a powerful tool for the interpretation of experimental data and are the first step towards the development of inverse procedures.
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Until recently, optical mapping of electrical activity in the heart muscle using voltage-sensitive dyes has mainly been applied to subsurface imaging. Here we present a method for the three-dimensional (3D) reconstruction of electrical activity deep inside the myocardial wall. We propose an alternative approach to diffusive optical tomography, based on ideas from binocular vision. Detection and illumination occur on opposite sides of the preparation. Staining with absorptive voltage-sensitive dyes is assumed. Data acquisition follows a paraxial scanning procedure, which modifies coaxial scanning by the introduction of a vector offset between illumination and detection axes. Pairs of 2D images are obtained corresponding to offsets of opposite signs. Those image pairs created by parallax are used as an input for the reconstruction algorithm, whose output is a 3D optical image of intramural electrical excitation. We apply this method to the slab geometry. The procedure was tested for a variety of computer-generated sources including particles, lines, bubbles, and simulated electrophysiological. patterns such as scroll waves. The limitations of the method and possible improvements are discussed.
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Aims Here, we investigate the mechanisms underlying the onset of conduction - related arrhythmias in a three-dimensional (3D) computational model of acute regional ischaemia.Methods Ischaemia was introduced by realistic gradients of potassium, pH, oxygen and electrical coupling in a 3D stab of ventricular tissue using the LRd model. We focused on a specific stage (10-15 min after occlusion) at which an intramural non-conductive ischaemic core (IC) surrounded by a border zone (BZ) has formed.Results At pacing frequencies greater than 4.5 Hz, we observed narrow areas (0.5 mm wide) of 2:1 conduction blocks at the periphery of the IC. As the pacing frequency increased, the area of block widened to 9 mm and gave rise to reentry at the periphery of the BZ. Alternating conduction blocks produced discordant action potential duration (APD) alternans throughout the stab and T-wave alternans in pseudo-ECG. Slowing the recovery of the calcium current broadened the range of pacing frequencies at which blocks were observed. Hyperkalaemia alone was sufficient to induce the alternating blocks.Conclusion Computermodelling predicts that ischaemia-retated arrhythmias are triggered by calcium-mediated alternating conduction blocks in the ischaemic border zone. Alternating conduction blocks lead to intramural reentry and APD atternans. (c) 2005 The European Society of Cardiology. Published by Elsevier Ltd. All rights reserved.
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An analytical description of the asymptotic wave front, as well as of cusp waves in cardiac tissue was presented. The motion of cusp waves, based on the assumption that they occur at the intersection of asymptotic solutions was investigated. It was observed that the asymptotic solutions are determined purely by the fiber organization within the cardiac wall, independent of the excitable properties of cardiac tissue. It was suggested that the asymptotic wave front propagates at the speed along the best aligned fiber, independent of its curvature.
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We have investigated the effect of blocking the Ca2+-channel on the transition from fibrillation to tachycardia in simulations in an anatomical model of the human ventricles, using a previously developed model of human ventricular cells where ventricular fibrillation was obtained by the process of spiral wave breakup. We show that blocking the Ca2+-current by 75% can convert fibrilliation into a periodic regime with a single stable spiral waves, which anchored to an anatomical obstacle. We show that the observed effects were due to a flattening of the restitution curve, which prevented the generation of wave breaks and stabilized the activation patterns.
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A study was conducted on the effect of biphasic restitution on spiral wave dynamics in a model that fits experimentally measured restitution curves of human ventricular tissue and reproduces basic properties of huyman cardiac action potential. A tissue with biphasic restitution curve two types of spiral breakup was found. In addition, it was found that increasing the slope of the descending part or the amplitude of the biphasic part of the restitution curve increases the meandering of the spiral wave, due to the repeated occurrence of conduction blocks near the spiral wave tip.
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Recirculation of excitation, or re-entry, is one of the most important mechanisms of life-threatening cardiac arrhythmias and fibrillation. Modelling these phenomena requires large scale computations in two and three-dimensional slabs of cardiac tissue. Because of computational constraints, most of the studies use simplified (non-ionic) models of cardiac tissue, which are electrophysiologically less accurate than the detailed ionic models. In this paper, we propose a method to modify ionic models of cardiac tissue into an intermediate class of models, which are almost as efficient for computations as simplified models, and retain most of the properties of the original ionic models, such as the shape of the action potential, the restitution of action potential duration and of the conduction velocity, as well as unchanged description of most of the ionic currents.
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Recent experimental and theoretical results have stressed the importance of modeling studies of reentrant arrhythmias in cardiac tissue and at the whole heart level. We introduce a six-variable model obtained by a reformulation of the Priebe-Beuckelmann model of a single human ventricular cell. The reformulated model is 4.9 times faster for numerical computations and it is more stable than the original model. It retains the action potential shape at various frequencies, restitution of action potential duration, and restitution of conduction velocity. We were able to reproduce the main properties of epicardial, endocardial, and M cells by modifying selected ionic currents. We performed a simulation study of spiral wave behavior in a two-dimensional sheet of human ventricular tissue and showed that spiral waves have a frequency of 3.3 Hz and a linear core of similar to50-mm diameter that rotates with an average frequency of 0.62 rad/s. Simulation results agreed with experimental data. In conclusion, the proposed model is suitable for efficient and accurate studies of reentrant phenomena in human ventricular tissue.
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We study the effect of blocking the L-type Ca(2+)-channel on fibrillation in simulations in two-dimensional (2D) isotropic sheets of ventricular tissue and in a three-dimensional anisotropic anatomical model of human ventricles, using a previously developed model of human ventricular cells. Ventricular fibrillation (VF) was obtained as a result of spiral wave breakup and consisted of a varying number of chaotically wandering wavelets activating tissue at a frequency of about 6.0 Hz. We show that blocking the Ca(2+)-current by 75% can convert ventricular fibrillation into a periodic regime with a small number of stable spiral waves, ranging from six in 2D sheets of 25 x 25 cm to a single spiral in the anatomical model of human ventricles. The dominant frequency during this process changed to about 10.0 Hz in the 2D simulations, but to only 5.0 Hz in the whole heart simulations where a single spiral wave anchored around an anatomical obstacle. We show that the observed effects were due to a flattening of the electrical restitution curve, which prevented the generation of wave breaks and stabilized the activation patterns.
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