In-vivo measurement of swine myocardial resistivity

Recent advances in tailoring stimuli-responsive hybrid scaffolds for cardiac tissue engineering and allied applications

To recapitulate bio-physical properties and functional behaviour of native heart tissues, recent tissue engineering-based approaches are focused on developing smart/stimuli-responsive materials for interfacing cardiac cells. Overcoming the drawbacks of the traditionally used biomaterials, these smart materials portray outstanding mechanical and conductive properties while promoting cell-cell interaction and cell-matrix transduction cues in such excitable tissues. To date, a large number of stimuli-responsive materials have been employed for interfacing cardiac tissues alone or in combination with natural/synthetic materials for cardiac tissue engineering. However, their comprehensive classification and a comparative analysis of the role played by these materials in regulating cardiac cell behaviour and in vivo metabolism are much less discussed. In an attempt to cover the recent advances in fabricating stimuli-responsive biomaterials for engineering cardiac tissues, this review details the role of these materials in modulating cardiomyocyte behaviour, functionality and surrounding matrix properties. Furthermore, concerns and challenges regarding the clinical translation of these materials and the possibility of using such materials for the fabrication of bio-actuators and bioelectronic devices are discussed.

How intramyocardial fat can alter the electric field distribution during Pulsed Field Ablation (PFA): Qualitative findings from computer modeling

Even though the preliminary experimental data suggests that cardiac Pulsed Field Ablation (PFA) could be superior to radiofrequency ablation (RFA) in terms of being able to ablate the viable myocardium separated from the catheter by collagen and fat, as yet there is no formal physical-based analysis that describes the process by which fat can affect the electric field distribution. Our objective was thus to determine the electrical impact of intramyocardial fat during PFA by means of computer modeling. Computer models were built considering a PFA 3.5-mm blunt-tip catheter in contact with a 7-mm ventricular wall (with and without a scar) and a 2-mm epicardial fat layer. High voltage was set to obtain delivered currents of 19, 22 and 25 A. An electric field value of 1000 V/cm was considered as the lethal threshold. We found that the presence of fibrotic tissue in the scar seems to have a similar impact on the electric field distribution and lesion size to that of healthy myocardium only. However, intramyocardial fat considerably alters the electrical field distribution and the resulting lesion shape. The electric field tends to peak in zones with fat, even away from the ablation electrode, so that 'cold points' (i.e. low electric fields) appear around the fat at the current entry and exit points, while 'hot points' (high electric fields) occur in the lateral areas of the fat zones. The results show that intramyocardial fat can alter the electric field distribution and lesion size during PFA due to its much lower electrical conductivity than that of myocardium and fibrotic tissue.

MXene functionalized collagen biomaterials for cardiac tissue engineering driving iPSC-derived cardiomyocyte maturation

Electroconductive biomaterials are gaining significant consideration for regeneration in tissues where electrical functionality is of crucial importance, such as myocardium, neural, musculoskeletal, and bone tissue. In this work, conductive biohybrid platforms were engineered by blending collagen type I and 2D MXene (Ti3C2Tx) and afterwards covalently crosslinking; to harness the biofunctionality of the protein component and the increased stiffness and enhanced electrical conductivity (matching and even surpassing native tissues) that two-dimensional titanium carbide provides. These MXene platforms were highly biocompatible and resulted in increased proliferation and cell spreading when seeded with fibroblasts. Conversely, they limited bacterial attachment (Staphylococcus aureus) and proliferation. When neonatal rat cardiomyocytes (nrCMs) were cultured on the substrates increased spreading and viability up to day 7 were studied when compared to control collagen substrates. Human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were seeded and stimulated using electric-field generation in a custom-made bioreactor. The combination of an electroconductive substrate with an external electrical field enhanced cell growth, and significantly increased cx43 expression. This in vitro study convincingly demonstrates the potential of this engineered conductive biohybrid platform for cardiac tissue regeneration.

Bioelectrical Impedance Spectroscopy for Monitoring Mammalian Cells and Tissues under Different Frequency Domains: A Review

Bioelectrical impedance analysis and bioelectrical impedance spectroscopy (BIA/BIS) of tissues reveal important information on molecular composition and physical structure that is useful in diagnostics and prognostics. The heterogeneity in structural elements of cells, tissues, organs, and the whole human body, the variability in molecular composition arising from the dynamics of biochemical reactions, and the contributions of inherently electroresponsive components, such as ions, proteins, and polarized membranes, have rendered bioimpedance challenging to interpret but also a powerful evaluation and monitoring technique in biomedicine. BIA/BIS has thus become the basis for a wide range of diagnostic and monitoring systems such as plethysmography and tomography. The use of BIA/BIS arises from (i) being a noninvasive and safe measurement modality, (ii) its ease of miniaturization, and (iii) multiple technological formats for its biomedical implementation. Considering the dependency of the absolute and relative values of impedance on frequency, and the uniqueness of the origins of the α-, β-, δ-, and γ-dispersions, this targeted review discusses biological events and underlying principles that are employed to analyze the impedance data based on the frequency range. The emergence of BIA/BIS in wearable devices and its relevance to the Internet of Medical Things (IoMT) are introduced and discussed.

Probe Contact Force Monitoring during Conductivity Measurements of the Left Atrial Appendage to Support the Design of Novel Diagnostic and Therapeutic Procedures

The electrical properties of many biological tissues are freely available from the INRC and the IT'IS databases. However, particularly in lower frequency ranges, few studies have investigated the optimal measurement protocol or the key confounders that need to be controlled, monitored, and reported. However, preliminary work suggests that the contact force of the measurement probe on the tissue sample can affect the measurements. The aim of this paper is to investigate the conductivity change due to the probe contact force in detail. Twenty ex vivo bovine heart samples are used, and conductivity measurements are taken in the Left Atrial Appendage, a common target for medical device developments. The conductivity measurements reported in this work (between 0.14 S/m and 0.24 S/m) align with the literature. The average conductivity is observed to change by -21% as the contact force increases from 2 N to 10 N. In contrast, in conditions where the fluid concentration in the measurement area is expected to be lower, very small changes are observed (less than 2.5%). These results suggest that the LAA conductivity is affected by the contact force due to the fluid concentration in the tissue. This work suggests that contact force should be controlled for in all future experiments.

In-Silico Modeling to Compare Radiofrequency-Induced Thermal Lesions Created on Myocardium and Thigh Muscle

Beating heart (BH) and thigh muscle (TM) are two pre-clinical models aimed at studying the lesion sizes created by radiofrequency (RF) catheters in cardiac ablation. Previous experimental results have shown that thermal lesions created in the TM are slightly bigger than in the BH. Our objective was to use in-silico modeling to elucidate some of the causes of this difference. In-silico RF ablation models were created using the Arrhenius function to estimate lesion size under different energy settings (25 W/20 s, 50 W/6 s and 90 W/4 s) and parallel, 45° and perpendicular catheter positions. The models consisted of homogeneous tissue: myocardium in the BH model and striated muscle in the TM model. The computer results showed that the lesion sizes were generally bigger in the TM model and the differences depended on the energy setting, with hardly any differences at 90 W/4 s but with differences of 1 mm in depth and 1.5 m in width at 25 W/20 s. The higher electrical conductivity of striated muscle (0.446 S/m) than that of the myocardium (0.281 S/m) is possibly one of the causes of the higher percentage of RF energy delivered to the tissue in the TM model, with differences between models of 2–5% at 90 W/4 s, ~9% at 50 W/6 s and ~10% at 25 W/20 s. Proximity to the air–blood interface (just 2 cm from the tissue surface) artificially created in the TM model to emulate the cardiac cavity had little effect on lesion size. In conclusion, the TM-based experimental model creates fairly similar-sized lesions to the BH model, especially in high-power short-duration ablations (50 W/6 s and 90 W/4 s). Our computer results suggest that the higher electrical conductivity of striated muscle could be one of the causes of the slightly larger lesions in the TM model.

Computer modeling of radiofrequency cardiac ablation: 30 years of bioengineering research

This review begins with a rationale of the importance of theoretical, mathematical and computational models for radiofrequency (RF) catheter ablation (RFCA). We then describe the historical context in which each model was developed, its contribution to the knowledge of the physics of RFCA and its implications for clinical practice. Next, we review the computer modeling studies intended to improve our knowledge of the biophysics of RFCA and those intended to explore new technologies. We describe the most important technical details of the implementation of mathematical models, including governing equations, tissue properties, boundary conditions, etc. We discuss the utility of lumped element models, which despite their simplicity are widely used by clinical researchers to provide a physical explanation of how RF power is absorbed in different tissues. Computer model verification and validation are also discussed in the context of RFCA. The article ends with a section on the current limitations, i.e. aspects not yet included in state-of-the-art RFCA computer modeling and on future work aimed at covering the current gaps.

Effect of anisotropy in myocardial electrical conductivity on lesion characteristics during radiofrequency cardiac ablation: a numerical study

BACKGROUND

Traditional computer simulation studies of radiofrequency catheter ablation (RFCA) usually neglect the anisotropy in myocardial electrical conductivity (MEC), which is likely an essential factor in governing the ablation outcome. Here, a numerical study of lesion characteristics during RFCA based on an anatomy-based model incorporating fiber orientation was performed to investigate the anisotropy in MEC.

METHODS

A three-dimensional thorax model including atria, blood, connective tissue, muscle, fat, and skin was constructed. The myocardial fiber was established through a rule-based method (RBM) based on the anatomical structure of the heart. The anisotropic MEC were 0.40 and 0.28 S m^-1 in longitudinal and transverse directions, respectively. The ablation result was compared with the isotropic scenario where the isotropic MEC was the average of the anisotropic conductivities as 0.34 S m^-1.

RESULTS

The complexity of fiber architecture varied with that of the local anatomical structure. At RF power of 20 W for 30 s, the tissue temperature and lesion volume were reduced by 2.8 ± 0.1% and 6.9 ± 0.5%, respectively, under anisotropic MEC around the ostium of the pulmonary vein and left atrial appendage. Those for the posterior wall and roof of the left atrium, and the inside of the superior vena cava were 1.9 ± 0.3% and 5.6 ± 1.2%, respectively.

CONCLUSIONS

Anisotropy in MEC has a greater reduction effect on lesion volume than on tissue temperature during RFCA; this effect tends to be restrained at positions with more uniform fiber distributions and can be enhanced where significant variation in fiber architecture occurred.

The rationale and emergence of electroconductive biomaterial scaffolds in cardiac tissue engineering

The human heart possesses minimal regenerative potential, which can often lead to chronic heart failure following myocardial infarction. Despite the successes of assistive support devices and pharmacological therapies, only a whole heart transplantation can sufficiently address heart failure. Engineered scaffolds, implantable patches, and injectable hydrogels are among the most promising solutions to restore cardiac function and coax regeneration; however, current biomaterials have yet to achieve ideal tissue regeneration and adequate integration due a mismatch of material physicochemical properties. Conductive fillers such as graphene, carbon nanotubes, metallic nanoparticles, and MXenes and conjugated polymers such as polyaniline, polypyrrole, and poly(3,4-ethylendioxythiophene) can possibly achieve optimal electrical conductivities for cardiac applications with appropriate suitability for tissue engineering approaches. Many studies have focused on the use of these materials in multiple fields, with promising effects on the regeneration of electrically active biological tissues such as orthopedic, neural, and cardiac tissue. In this review, we critically discuss the role of heart electrophysiology and the rationale toward the use of electroconductive biomaterials for cardiac tissue engineering. We present the emerging applications of these smart materials to create supportive platforms and discuss the crucial role that electrical stimulation has been shown to exert in maturation of cardiac progenitor cells.

A computational model of open-irrigated radiofrequency catheter ablation accounting for mechanical properties of the cardiac tissue

Radiofrequency catheter ablation (RFCA) is an effective treatment for cardiac arrhythmias. Although generally safe, it is not completely exempt from the risk of complications. The great flexibility of computational models can be a major asset in optimizing interventional strategies if they can produce sufficiently precise estimations of the generated lesion for a given ablation protocol. This requires an accurate description of the catheter tip and the cardiac tissue. In particular, the deformation of the tissue under the catheter pressure during the ablation is an important aspect that is overlooked in the existing literature, which resorts to a sharp insertion of the catheter into an undeformed geometry. As the lesion size depends on the power dissipated in the tissue and the latter depends on the percentage of the electrode surface in contact with the tissue itself, the sharp insertion geometry has the tendency to overestimate the lesion obtained, which is a consequence of the tissue temperature rise overestimation. In this paper, we introduce a full 3D computational model that takes into account the tissue elasticity and is able to capture tissue deformation and realistic power dissipation in the tissue. Numerical results in FEniCS-HPC are provided to validate the model against experimental data and to compare the lesions obtained with the new model and with the classical ones featuring a sharp electrode insertion in the tissue.

A viscoelastic adhesive epicardial patch for treating myocardial infarction

Acellular epicardial patches that treat myocardial infarction by increasing the mechanical integrity of damaged left ventricular tissues exhibit widely scattered therapeutic efficacy. Here, we introduce a viscoelastic adhesive patch, made of an ionically crosslinked transparent hydrogel, that accommodates the cyclic deformation of the myocardium and outperforms most existing acellular epicardial patches in reversing left ventricular remodelling and restoring heart function after both acute and subacute myocardial infarction in rats. The superior performance of the patch results from its relatively low dynamic modulus, designed at the so-called 'gel point' via finite-element simulations of left ventricular remodelling so as to balance the fluid and solid properties of the material.

Electroconductive Biohybrid Collagen/Pristine Graphene Composite Biomaterials with Enhanced Biological Activity

Electroconductive substrates are emerging as promising functional materials for biomedical applications. Here, the development of biohybrids of collagen and pristine graphene that effectively harness both the biofunctionality of the protein component and the increased stiffness and enhanced electrical conductivity (matching native cardiac tissue) obtainable with pristine graphene is reported. As well as improving substrate physical properties, the addition of pristine graphene also enhances human cardiac fibroblast growth while simultaneously inhibiting bacterial attachment (Staphylococcus aureus). When embryonic-stem-cell-derived cardiomyocytes (ESC-CMs) are cultured on the substrates, biohybrids containing 32 wt% graphene significantly increase metabolic activity and cross-striated sarcomeric structures, indicative of the improved substrate suitability. By then applying electrical stimulation to these conductive biohybrid substrates, an enhancement of the alignment and maturation of the ESC-CMs is achieved. While this in vitro work has clearly shown the potential of these materials to be translated for cardiac applications, it is proposed that these graphene-based biohybrid platforms have potential for a myriad of other applications-particularly in electrically sensitive tissues, such as neural and neural and musculoskeletal tissues.

Sensor spacing affects the tissue impedance spectra of rabbit ventricular epicardium

This study was designed to test the hypothesis that a complex composite impedance spectra develops when stimulation and recording of cardiac muscle with sufficiently fine spatial resolution in a four-electrode configuration is used. With traditional (millimeter scale) separations, the ratio between the recorded interstitial central potential difference and total supplied interstitial current is constant at all frequencies. This occurs because the fraction of supplied current that redistributes to the intracellular compartment depends on effective membrane resistance between electrodes, which is low, to a much greater extent than effective membrane capacitance. The spectra should therefore change with finer separations at which effective membrane resistance increases, as supplied current will remain primarily interstitial at lower frequencies and redistribute between compartments at higher frequencies. To test this hypothesis, we built arrays with sensors separated (d) by 804 μm, 452 μm, and 252 μm; positioned those arrays across myocyte axes on rabbit ventricular epicardium; and resolved spectra in terms of resistivity (ρt) and reactivity (χt) over the 10 Hz to 4,000 Hz range. With all separations, we measured comparable spectra with predictions from passive membrane simulations that used a three-dimensional structural framework in which intracellular, interstitial, and membrane properties were prescribed based on the limited data available from the literature. At the finest separation, we found mean ρt at 100 Hz and 4,000 Hz that lowered from 395 Ω-cm to 236 Ω-cm, respectively, with maximal mean χt of 160 Ω-cm. This experimental confirmation of spectra development in whole heart experiments is important because such development is central to achieve measurements of intracellular and interstitial passive electrical properties in cardiac electrophysiological experiments using only interstitial access.

A structural framework for interpretation of four-electrode microimpedance spectra in cardiac tissue

Renewed interest in the four-electrode method for identification of passive electrical properties in cardiac tissue has been sparked by a recognition that measurements made with sensors in close proximity are frequency dependent. Therefore, resolution of four-electrode microimpedance spectra (4EMS) may provide an opportunity for routine identification of passive electrical properties for the interstitial and intracellular compartments using only interstitial access. The present study documents a structural framework in which the tissue resistivity (ρt) and reactivity (xt) that comprise spectra are computed using interstitial and intracellular microimpedance distributions that account for differences in compartment size, anisotropic electrical properties in each compartment and electrode separations. We used this framework to consider 4EMS development with relatively wide (d=1 mm) and fine (d=250 μm) electrode separations and sensors oriented along myocyte axes, across myocyte axes and intermediate between those axes.

A new approach for resolution of complex tissue impedance spectra in hearts

This study was designed to test the feasibility of using sinusoidal approximation in combination with a new instrumentation approach to resolve complex impedance (uCI) spectra from heart preparations. To assess that feasibility, we applied stimuli in the 10-4000 Hz range and recorded potential differences (uPDs) in a four-electrode configuration that allowed identification of probe constants (Kp) during calibration that were in turn used to measure total tissue resistivity ρt from rabbit ventricular epicardium. Simultaneous acquisition of a signal proportional to the supplied current (Vstim) with uPD allowed identification of the V- I ratio needed for ρt measurement, as well as the phase shift from Vstim to uPD needed for uCI spectra resolution. Performance with components integrated to reduce noise in cardiac electrophysiologic experiments, in particular, and provide accurate electrometer-based measurements, in general, was first characterized in tests using passive loads. Load tests showed accurate uCI recovery with mean uPD SNRs between 10 (1) and 10 (3) measured with supplied currents as low as 10 nA. Comparable performance characteristics were identified during calibration of nine arrays built with 250 μm Ag/AgCl electrodes, with uCIs that matched analytic predictions and no apparent effect of frequency ( F = 0.12, P = 0.99). The potential ability of parasitic capacitance in the presence of the electrode-electrolyte interface associated with the small sensors to influence the uCI spectra was therefore limited by the instrumentation. Resolution of uCI spectra in rabbit ventricle allowed measurement of ρt = 134 ± 53 Ω· cm. The rapid identification available with this strategy provides an opportunity for new interpretations of the uCI spectra to improve quantification of disease-, region-, tissue-, and species-dependent intercellular uncoupling in hearts.

An iterative method for problems with multiscale conductivity

A model with its conductivity varying highly across a very thin layer will be considered. It is related to a stable phantom model, which is invented to generate a certain apparent conductivity inside a region surrounded by a thin cylinder with holes. The thin cylinder is an insulator and both inside and outside the thin cylinderare filled with the same saline. The injected current can enter only through the holes adopted to the thin cylinder. The model has a high contrast of conductivity discontinuity across the thin cylinder and the thickness of the layer and the size of holes are very small compared to the domain of the model problem. Numerical methods for such a model require a very fine mesh near the thin layer to resolve the conductivity discontinuity. In this work, an efficient numerical method for such a model problem is proposed by employing a uniform mesh, which need not resolve the conductivity discontinuity. The discrete problem is then solved by an iterative method, where the solution is improved by solving a simple discrete problem with a uniform conductivity. At each iteration, the right-hand side is updated by integrating the previous iterate over the thin cylinder. This process results in a certain smoothing effect on microscopic structures and our discrete model can provide a more practical tool for simulating the apparent conductivity. The convergence of the iterative method is analyzed regarding the contrast in the conductivity and the relative thickness of the layer. In numerical experiments, solutions of our method are compared to reference solutions obtained from COMSOL, where very fine meshes are used to resolve the conductivity discontinuity in the model. Errors of the voltage in L² norm follow O(h) asymptotically and the current density matches quitewell those from the reference solution for a sufficiently small mesh size h. The experimental results present a promising feature of our approach for simulating the apparent conductivity related to changes in microscopic cellular structures.

Magnetoacoustic imaging of electrical conductivity of biological tissues at a spatial resolution better than 2 mm

Magnetoacoustic tomography with magnetic induction (MAT-MI) is an emerging approach for noninvasively imaging electrical impedance properties of biological tissues. The MAT-MI imaging system measures ultrasound waves generated by the Lorentz force, having been induced by magnetic stimulation, which is related to the electrical conductivity distribution in tissue samples. MAT-MI promises to provide fine spatial resolution for biological tissue imaging as compared to ultrasound resolution. In the present study, we first estimated the imaging spatial resolution by calculating the full width at half maximum (FWHM) of the system point spread function (PSF). The actual spatial resolution of our MAT-MI system was experimentally determined to be 1.51 mm by a parallel-line-source phantom with Rayleigh criterion. Reconstructed images made from tissue-mimicking gel phantoms, as well as animal tissue samples, were consistent with the morphological structures of the samples. The electrical conductivity value of the samples was determined directly by a calibrated four-electrode system. It has been demonstrated that MAT-MI is able to image the electrical impedance properties of biological tissues with better than 2 mm spatial resolution. These results suggest the potential of MAT-MI for application to early detection of small-size diseased tissues (e.g. small breast cancer).

Ion mobility imaging and contrast mechanism of apparent conductivity in MREIT

Magnetic resonance electrical impedance tomography (MREIT) aims to produce high-resolution cross-sectional images of conductivity distribution inside the human body. Injected current into an imaging object induces a distribution of internal magnetic flux density, which is measured by using an MRI scanner. We can reconstruct a conductivity image based on its relation with the measured magnetic flux density. In this paper, we explain the contrast mechanism in MREIT by performing and analyzing a series of numerical simulations and imaging experiments. We built a stable conductivity phantom including a hollow insulating cylinder with holes. Filling both inside and outside the hollow cylinder with the same saline, we controlled ion mobilities to create a conductivity contrast without being affected by the ion diffusion process. From numerical simulations and imaging experiments, we found that slopes of induced magnetic flux densities change with hole diameters and therefore conductivity contrasts. Associating the hole diameter with apparent conductivity of the region inside the hollow cylinder with holes, we could experimentally validate the contrast mechanism in MREIT. Interpreting reconstructed apparent conductivity images of the phantom as ion mobility images, we discuss the meaning of the apparent conductivity seen by a certain probing method. In designing MREIT imaging experiments, the ion mobility imaging method using the proposed stable conductivity phantom will enable us to estimate a distinguishable conductivity contrast for a given set of imaging parameters.

Magnetoacoustic imaging of human liver tumor with magnetic induction

Magnetoacoustic tomography with magnetic induction (MAT-MI) is an imaging technique under development to achieve imaging of electrical impedance contrast in biological tissues with spatial resolution close to ultrasound imaging. However, previously reported MAT-MI experimental results are obtained either from low salinity gel phantoms, or from normal animal tissue samples. In this study, we report the experimental study on the performance of the MAT-MI imaging method for imaging in vitro human liver tumor tissue. The present promising experimental results suggest the feasibility of MAT-MI to image electrical impedance contrast between the cancerous tissue and its surrounding normal tissues.

Imaging biological tissues with electrical conductivity contrast below 1 S m by means of magnetoacoustic tomography with magnetic induction

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a recently introduced imaging modality for noninvasive electrical impedance imaging, with ultrasound imaging resolution and a contrast reflecting the electrical conductivity properties of tissues. However, previous MAT-MI systems can only image samples that are much more conductive than real human or animal tissues. To image real biological tissue samples, a large-current-carrying coil that can give stronger magnetic stimulations and stronger MAT-MI acoustic signals is employed in this study. The conductivity values of all the tissue samples employed in this study are also directly measured using a well calibrated four-electrode system. The experimental results demonstrated the feasibility to image biological tissues with electrical conductivity contrast below 1.0 S∕m using the MAT-MI technique with safe level of electromagnetic energy applied to tissue samples.

Effect of electrode thermal conductivity in cardiac radiofrequency catheter ablation: a computational modeling study

PURPOSE

Radiofrequency (RF ablation) is the treatment of choice for certain types of cardiac arrhythmias. Recent studies have suggested that using gold instead of platinum as the electrode material for cardiac catheter ablation leads to larger thermal lesions due to its higher thermal conductivity. In this study we created computer models to compare the effects of different electrode materials on lesion dimensions using different catheters, insertion depths, and flow rates.

MATERIALS AND METHODS

Finite element method (FEM) models of two cardiac ablation electrodes (7Fr, length 4 mm and 8Fr, length 10 mm) made of platinum, gold, and copper were created with tissue insertion depths of 0.75, 1.25, and 2.5 mm. Convective cooling was applied to the electrode and tissue based on measurements from previous studies at different flow rates. RF ablations were simulated with both temperature control and constant power control algorithms to determine temperature profiles after 60 s.

RESULTS

With the constant power algorithm there was no difference in lesion dimensions between the electrode materials over the range of parameters. With the temperature control algorithm, lesion width and depth were only marginally larger ( approximately 0.1-0.7 mm) with the gold and copper electrodes compared to the platinum electrode for all parameter combinations.

CONCLUSION

Our computer modelling results show only minor increases in thermal lesion dimensions with electrode materials of higher thermal conductivity. These observed differences likely do not provide a significant advantage during clinical procedures.

Electrical conductivity measurement of excised human metastatic liver tumours before and after thermal ablation

We measured the ex vivo electrical conductivity of eight human metastatic liver tumours and six normal liver tissue samples from six patients using the four electrode method over the frequency range 10 Hz to 1 MHz. In addition, in a single patient we measured the electrical conductivity before and after the thermal ablation of normal and tumour tissue. The average conductivity of tumour tissue was significantly higher than normal tissue over the entire frequency range (from 4.11 versus 0.75 mS cm(-1) at 10 Hz, to 5.33 versus 2.88 mS cm(-1) at 1 MHz). We found no significant correlation between tumour size and measured electrical conductivity. While before ablation tumour tissue had considerably higher conductivity than normal tissue, the two had similar conductivity throughout the frequency range after ablation. Tumour tissue conductivity changed by +25% and -7% at 10 Hz and 1 MHz after ablation (0.23-0.29 at 10 Hz, and 0.43-0.40 at 1 MHz), while normal tissue conductivity increased by +270% and +10% at 10 Hz and 1 MHz (0.09-0.32 at 10 Hz and 0.37-0.41 at 1 MHz). These data can potentially be used to differentiate tumour from normal tissue diagnostically.

Sequential activation of a segmented ground pad reduces skin heating during radiofrequency tumor ablation: optimization via computational models

Radiofrequency (RF) ablation has become an accepted treatment modality for unresectable tumors. The need for larger ablation zones has resulted in increased RF generator power. Skin burns due to ground pad heating are increasingly limiting further increases in generator power, and thus, ablation zone size. We investigated a method for reducing ground pad heating in which a commercial ground pad is segmented into multiple ground electrodes, with sequential activation of ground electrode subsets. We created finite-element method computer models of a commercial ground pad (14 x 23 cm) and compared normal operation of a standard pad to sequential activation of a segmented pad (two to five separate ground electrode segments). A constant current of 1 A was applied for 12 min in all simulations. Time periods during sequential activation simulations were adjusted to keep the leading edge temperatures at each ground electrode equal. The maximum temperature using standard activation of the commercial pad was 41.7 degrees C. For sequential activation of a segmented pad, the maximum temperature ranged from 39.3 degrees C (five segments) to 40.9 degrees C (two segments). Sequential activation of a segmented ground pad resulted in lower tissue temperatures. This method may reduce the incidence of ground pad burns and enable the use of higher power generators during RF tumor ablation.

Contrasting effects of convective flow on catheter ablation lesion size: cryo versus radiofrequency energy

BACKGROUND

Cryoablation has now become an alternative to treat many cardiac arrhythmias, and may be the treatment of choice in some patient populations. We compared the effects of convective flow on large-tip cryo and radiofrequency (RF) lesions dimensions.

METHODS

Cryoablation and RF ablation were performed on porcine heart sections in a saline bath with varying directed flow rates. Cryoablation was performed for 4 minutes on 50 tissue pieces with tip temperature controlled at -80 degrees C. RF ablation was performed on 50 tissue pieces for 60 seconds at 60 degrees C tip temperature. The pieces were placed in culture media for 24 hours, and then sectioned, stained, and measured.

RESULTS

Cryoablation and RF lesion sizes varied significantly with flow such that higher flow rates produced smaller cryoablation lesions and larger RF lesions (mean cryoablation volumes: 854 +/- 402, 808 +/- 217, 781 +/- 217, 359 +/- 114, and 292 +/- 117 mm(3), and mean RF volumes: 211 +/- 35, 304 +/- 79, 439 +/- 125, 525 +/- 187, and 597 +/- 126 mm(3) for 0, 1, 2, 3, and 5 L/min flow rates, respectively, P < 0.0005). Trabeculated pieces had larger cryoablation lesions and smaller RF lesions than nontrabeculated ones at higher flow rate (P < 0.005). Cryoablation lesion volume increased as the time to reach -80 degrees C decreased (r(2)= 0.72).

CONCLUSION

In contrast to RF ablation, cryoablation lesion size is smaller at high flow rates, and larger at low flow rates due to the warming effects of local convective flow. The effects of high flow are reduced in areas of trabeculation, and the time to reach -80 degrees C predicts cryoablation lesion size.

Convective cooling affects cardiac catheter cryoablation and radiofrequency ablation in opposite directions

Recently, cryoablation has received increased attention as a safer alternative to radiofrequency (RF) ablation. The purpose of this study was to compare the effect of convective cooling at physiologic flow rates on RF lesion size and on cryo lesion size. Porcine hearts were sectioned into 40 pieces and placed in a temperature-controlled saline bath (37 degrees C) with varying directed flow rates (0, 1, 2 and 3 L/min). Large-tip cryoablation (8 mm tip) was performed for 4 minutes on 20 tissue sections at -80 degrees C tip temperature. Large-tip RF catheter ablation (10 mm tip) was performed at 60 degrees C target temperature for 1 minute on 20 tissue sections. For each catheter, flow rates were randomized between applications. The tissue pieces were placed in culture medium for 24 hrs, sectioned, stained and measured to determine lesion depth, width and volume. Average lesion geometry was estimated from the data. Lesion dimensions were dependent on the flow rate for RF ablation with larger lesions at higher flow rates (mean volumes: 211+/-35, 304+/-79, 439+/-125 and 525.7+/-187 mm3 for 0, 1, 2 and 3 L/min flow rate, respectively, p<0.01). Also for cryoablation lesion size varied significantly with flow, such that lower flow rates produced larger lesions (mean volumes: 855+/-402, 809+/-218, 658+/-91 and 360+/-14 mm3 for 0, 1, 2 and 3 L/min flow rate respectively, p<0.01). While RF ablation creates larger lesions at high flow rates (3 L/min), cryoablation creates larger lesions at low flow rates (0-1 L/min, p<0.05).

Volume catheter parallel conductance varies between end-systole and end-diastole

In order for the conductance catheter system to accurately measure instantaneous cardiac blood volume, it is necessary to determine and remove the contribution from parallel myocardial tissue. In previous studies, the myocardium has been treated as either purely resistive or purely capacitive when developing methods to estimate the myocardial contribution. We propose that both the capacitive and the resistive properties of the myocardium are substantial, and neither should be ignored. Hence, the measured result should be labeled admittance rather than conductance. We have measured the admittance (magnitude and phase angle) of the left ventricle in the mouse, and have shown that it is measurable and increases with frequency. Further, this more accurate technique suggests that the myocardial contribution to measured admittance varies between end-systole and end-diastole, contrary to previous literature. We have tested these hypotheses both with numerical finite-element models for a mouse left ventricle constructed from magnetic resonance imaging images, and with in vivo admittance measurements in the murine left ventricle. Finally, we propose a new method to determine the instantaneous myocardial contribution to the measured left ventricular admittance that does not require saline injection or other intervention to calibrate.

Convective cooling effect on cooled-tip catheter compared to large-tip catheter radiofrequency ablation

BACKGROUND

Both actively cooled-tip and large-tip catheters are currently available clinically to create large endomyocardial lesions during application of radiofrequency (RF) energy. The purpose of this study was to compare the effect of convective cooling at physiologic flow rates on RF lesion size using both actively cooled and large-tip catheters.

METHODS

Porcine hearts were sectioned into 72 pieces and placed in a temperature-controlled saline bath (37 degrees C) with varying directed flow rates (0, 1, 2, and 3 L/min). Cooled-tip RF ablation (4 mm tip) was performed for 1 minute on 36 tissue sections with power manually titrated to keep tip temperature below 40 degrees C. Large-tip catheter ablation (10 mm tip) was performed at 65 degrees C target temperature for 1 minute on 36 tissue sections. For each catheter, flow rates were randomized between applications. The tissue pieces were sectioned and measured to determine lesion depth, width, and volume.

RESULTS

Lesion dimensions were independent of the flow rate for the cooled-tip catheter (mean volumes: 382.0 +/- 121.6, 419.9 +/- 133.4, 375.9 +/- 169.1, and 346.7 +/- 173.4 mm(3) for 0, 1, 2, and 3 L/min flow rate, respectively, P = 0.78). For the large-tip catheter, lesion size varied significantly with flow, such that higher flow rates produced larger lesions (mean volumes: 120.7 +/- 50.7, 256.5 +/- 97.9, 393.4 +/- 149.9, and 548.9 +/- 157.0 mm(3) for 0, 1, 2, and 3 L/min flow rate respectively, P < 0.001)

CONCLUSION

During RF ablation, blood flow rate significantly affects lesion size for large-tip but not cooled-tip catheters. At low flow rates (0-1 L/min) cooled-tip catheters create larger lesions, while at high flow rates (3 L/min) large-tip catheters create larger lesions.

Estimation of Cardiac Bidomain Parameters from Extracellular Measurement: Two Dimensional Study

Cardiac tissue conductivity measurements can be used to assess the electrical substrate underlying normal and abnormal wavefront propagation. We describe a method of solving the inverse cardiac bidomain model to estimate average longitudinal and transverse intra and extra-cellular conductivities and fiber angle relative to an electrode array placed arbitrarily on the epi- or endocardial surface. A Newton-Raphson reconstruction method and two Tikhonov-type regularizations were able to stably identify conductivities and fiber angles in tissue models having anisotropies similar to those in real cardiac tissue. The reconstruction methods were tested with data from increasingly realistic two dimensional cardiac bidomain models and performed well both when measurement noise was added, and when simulated experimental and forward model matching was diminished. This approach may be a suitable basis for continuous monitoring of myocardial condition in-vivo via a catheter based electrode array.

On the passive cardiac conductivity

In order to relate the structure of cardiac tissue to its passive electrical conductivity, we created a geometrical model of cardiac tissue on a cellular scale that encompassed myocytes, capillaries, and the interstitial space that surrounds them. A special mesh generator was developed for this model to create realistically shaped myocytes and interstitial space with a controlled degree of variation included in each model. In order to derive the effective conductivities, we used a finite element model to compute the currents flowing through the intracellular and extracellular space due to an externally applied electrical field. The product of these computations were the effective conductivity tensors for the intracellular and extracellular spaces. The simulations of bi-domain conductivities for healthy tissue resulted in an effective intracellular conductivity of 0.16S/m (longitudinal) and 0.005 S/m (transverse) and an effective extracellular conductivity of 0.21S/m (longitudinal) and 0.06 S/m (transverse). The latter values are within the range of measured values reported in literature. Furthermore, we anticipate that this method can be used to simulate pathological conditions for which measured data is far more sparse.

Ischemia-induced arrhythmia: the role of connexins, gap junctions, and attendant changes in impulse propagation

Sudden cardiac death accounts for more than half of all cardiovascular deaths in the US, and a large proportion of these deaths are attributed to ischemia-induced ventricular fibrillation. As such, the mechanisms underlying the initiation and maintenance of these lethal rhythms are of significant clinical and scientific interest. In large animal hearts, regional ischemia induces two phases of ventricular arrhythmia. The first phase (1A) occurs between 5 and 7 min after arrest of perfusion. This phase is associated with membrane depolarization, a mild intracellular and extracellular acidification and a small membrane depolarization. A second phase (1B) of ventricular arrhythmia occurs between 20 and 30 minutes after arrest of perfusion. This phase occurs at a time when ischemia-induced K+ and pH changes are relatively stable. The arrhythmia is presumed to relate to the process of cell-to-cell electrical uncoupling because a rapid increase of tissue impedance precedes the onset of the arrhythmia. Of note is that tissue resistance is primarily determined by the conductance properties of the gap junctions accounting for cell-to-cell coupling. Impulse propagation in heart is determined by active and passive membrane properties. An important passive cable property that is modulated by ischemia is intercellular resistance and is determined primarily by gap junctional conductance. As such changes in Impulse propagation during myocardial ischemia are determined by contemporaneous changes in active and passive membrane properties. Cellular K loss, intracellular and extracellular acidosis and membrane depolarization are important factors decreasing excitatory currents, while the collapse of the extracellular compartment and cell-to-cell electrical uncoupling increase the resistance to current flow. The time-course of cellular coupling is closely linked to a number of physiological processes including depletion of ATP, and accumulation of intracellular Ca2+. Hence, interventions such as ischemic preconditioning attenuate the effect of subsequent ischemia, delay the onset of cell-to-cell electrical uncoupling and likewise delay the onset of ischemia-induced arrhythmia.

Electrical impedance along connective tissue planes associated with acupuncture meridians

Background

Acupuncture points and meridians are commonly believed to possess unique electrical properties. The experimental support for this claim is limited given the technical and methodological shortcomings of prior studies. Recent studies indicate a correspondence between acupuncture meridians and connective tissue planes. We hypothesized that segments of acupuncture meridians that are associated with loose connective tissue planes (between muscles or between muscle and bone) visible by ultrasound have greater electrical conductance (less electrical impedance) than non-meridian, parallel control segments.

Methods

We used a four-electrode method to measure the electrical impedance along segments of the Pericardium and Spleen meridians and corresponding parallel control segments in 23 human subjects. Meridian segments were determined by palpation and proportional measurements. Connective tissue planes underlying those segments were imaged with an ultrasound scanner. Along each meridian segment, four gold-plated needles were inserted along a straight line and used as electrodes. A parallel series of four control needles were placed 0.8 cm medial to the meridian needles. For each set of four needles, a 3.3 kHz alternating (AC) constant amplitude current was introduced at three different amplitudes (20, 40, and 80 μAmps) to the outer two needles, while the voltage was measured between the inner two needles. Tissue impedance between the two inner needles was calculated based on Ohm's law (ratio of voltage to current intensity).

Results

At the Pericardium location, mean tissue impedance was significantly lower at meridian segments (70.4 ± 5.7 Ω) compared with control segments (75.0 ± 5.9 Ω) (p = 0.0003). At the Spleen location, mean impedance for meridian (67.8 ± 6.8 Ω) and control segments (68.5 ± 7.5 Ω) were not significantly different (p = 0.70).

Conclusion

Tissue impedance was on average lower along the Pericardium meridian, but not along the Spleen meridian, compared with their respective controls. Ultrasound imaging of meridian and control segments suggested that contact of the needle with connective tissue may explain the decrease in electrical impedance noted at the Pericardium meridian. Further studies are needed to determine whether tissue impedance is lower in (1) connective tissue in general compared with muscle and (2) meridian-associated vs. non meridian-associated connective tissue.

A six-ring probe for monitoring conductivity changes

This paper presents the construction of a six-ring probe for monitoring immittance changes. The spatial sensitivity of the probe is defined. This is used to examine the uniqueness of the probe in terms of its application to monitoring conductivity changes. A spatial distribution of the sensitivity is presented for isotropic and anisotropic cases. The latter case is restricted only to anisotropy met when measuring muscles, i.e. diagonal anisotropy. Theoretical calculations performed using the finite element method were verified experimentally using a specially developed measuring system. An example of in vivo measurements is included.

Transmural versus nontransmural in situ electrical impedance spectrum for healthy, ischemic, and healed myocardium

Electrical properties of myocardial tissue are anisotropic due to the complex structure of the myocardial fiber orientation and the distribution of gap junctions. For this reason, measured myocardial impedance may differ depending on the current distribution and direction with respect to myocardial fiber orientation and, consequently, according to the measurement method. The objective of this study is to compare the specific impedance spectra of the myocardium measured using two different methods. One method consisted of transmural measurements using an intracavitary catheter and the other method consisted of nontransmural measurements using a four-needle probe inserted into the epicardium. Using both methods, we provide the in situ specific impedance spectrum (magnitude and phase angle) of normal, ischemic, and infarcted pig myocardium tissue from 1 kHz to 1 MHz. Magnitude spectra showed no significant differences between the measurement techniques. However, the phase angle spectra showed significant differences for normal and ischemic tissues according to the measurement technique. The main difference is encountered after 60 min of acute ischeimia in the phase angle spectrum. Healed myocardial tissue showed a small and flat phase angle spectrum in both methods due tothe low content of cells in the transmural infarct scar. In conclusion, both transmural and nontransmural measurements of phase angle spectrum allow the differentiation among normal, ischemic, and infarcted tissue.

Measurement of thoracic current flow in pigs for the study of defibrillation and cardioversion

Although defibrillation has been in clinical use for more than 50 years, the complete current flow distribution inside the body during a defibrillation procedure has never been directly measured. This is due to the lack of appropriate imaging technology to noninvasively monitor the current flow inside the body. The current density imaging (CDI) technique, using a magnetic resonance (MR) imager, provides a new approach to this problem [Scott et al. (1991)]. CDI measures the local magnetic field generated by the current and calculates the current density by computing its curl. In this study, CDI was used to measure current density at all points within a postmortem pig torso during an electrical current application through defibrillation electrodes. Furthermore, current flow information was visualized along the chest wall and within the chest cavity using streamline analysis. As expected, some of the highest current densities were observed in the chest wall. However, current density distribution varied significantly from one region to another, possibly reflecting underlying heterogeneous tissue conductivity and anisotropy. Moreover, the current flow analysis revealed many complex and unexpected current flow patterns that have never been observed before. This study has, for the first time, noninvasively measured the volume current measurement inside the pig torso.

In vitro measurement of myocardial impedivity anisotropy with a miniature rectangular tube

Due to rapid change of fiber orientation, it is difficult to measure myocardial impedivity separately in a longitudinal or transverse fiber direction without mutual influence in the two directions. Previously published values of the longitudinal and the transverse myocardial impedivity were derived indirectly from measurements that mixed the impedivity in all directions. Those values are questionable because the derivations were based on a simplified uniform myocardial fiber model. In this paper, a miniature rectangular tube was devised to facilitate direct measurement of myocardial impedivity in a uniform fiber direction. The average transverse-to-longitudinal ratio of the measured in vitro swine myocardial impedivity was about 1.66 from 1 Hz to 1 kHz and dropped to 1.25 at 1 MHz. The result is important for accurate modeling of the electrical property of myocardium in biomedical research of radio-frequency cardiac catheter ablation.

Error analysis of tissue resistivity measurement

We identified the error sources in a system for measuring tissue resistivity at eight frequencies from 1 Hz to 1 MHz using the four-terminal method. We expressed the measured resistivity with an analytical formula containing all error terms. We conducted practical error measurements with in-vivo and bench-top experiments. We averaged errors at all frequencies for all measurements. The standard deviations of error of the quantization error of the 8-bit digital oscilloscope with voltage averaging, the nonideality of the circuit, the in-vivo motion artifact and electrical interference combined to yield an error of +/- 1.19%. The dimension error in measuring the syringe tube for measuring the reference saline resistivity added +/- 1.32% error. The estimation of the working probe constant by interpolating a set of probe constants measured in reference saline solutions added +/- 0.48% error. The difference in the current magnitudes used during the probe calibration and that during the tissue resistivity measurement caused +/- 0.14% error. Variation of the electrode spacing, alignment, and electrode surface property due to the insertion of electrodes into the tissue caused +/- 0.61% error. We combined the above errors to yield an overall standard deviation error of the measured tissue resistivity of +/- 1.96%.