Simulated internal defibrillation in humans using an anatomically realistic three-dimensional finite element model of the thorax

Effect of ventricular myocardium characteristics on the defibrillation threshold

Myocardium characteristics differ markedly among individuals and play an important role in defibrillation threshold. The accuracy of simulation models used in most published studies are still have room to be improved and most of them only discussed the effect of myocardial anisotropy on defibrillation threshold. In our manuscript, a rabbit ventricular finite-element (FE) volume conductor model with high precision was constructed. Ventricular myocardium characteristics include cardiomyocyte coupling and the degree of myocardial anisotropy, which are represented as the value and the ratio of anisotropic conductivity, respectively. Quantitative analysis was performed simultaneously in terms of cardiomyocyte coupling and the degree of myocardial anisotropy. Based on this, the combined effects of these two factors were further discussed. The electric field distributions of shocks and the defibrillation thresholds under different myocardial characteristics were simulated on this model. The simulation results revealed that as the degree of myocardial anisotropy increases, defibrillation threshold increases, and cardiomyocyte decoupling (decrease in electrical conductivity) can considerably increase the defibrillation threshold.

Acute Management of Atrial Fibrillation: From Emergency Department to Cardiac Care Unit

The prevalence of atrial fibrillation is increasing rapidly, resulting in more patients presenting for care in the emergency department and in-hospital settings. To reduce morbidity and mortality, and improve patient quality of life, clinicians working in these settings need to be both current and facile in their approach to management of these patients. Frequent updates to guideline recommendations (based on emerging research) make this challenging for practicing physicians. This article reviews the acute management of atrial fibrillation in the emergency and in-hospital settings, including practical approaches to rhythm and rate control, anticoagulation, and special situations, incorporating the most up-to-date guidelines.

Purkinje-mediated effects in the response of quiescent ventricles to defibrillation shocks

In normal cardiac function, orderly activation of the heart is facilitated by the Purkinje system (PS), a specialized network of fast-conducting fibers that lines the ventricles. Its role during ventricular defibrillation remains unelucidated. Physical characteristics of the PS make it a poor candidate for direct electrical observation using contemporary experimental techniques. This study uses a computer modeling approach to assess contributions by the PS to the response to electrical stimulation. Normal sinus rhythm was simulated and epicardial breakthrough sites were distributed in a manner consistent with experimental results. Defibrillation shocks of several strengths and orientations were applied to quiescent ventricles, with and without PS, and electrical activation was analyzed. All shocks induced local polarizations in PS branches parallel to the field, which led to the rapid spread of excitation through the network. This produced early activations at myocardial sites where tissue was unexcited by the shock and coupled to the PS. Shocks along the apico-basal axis of the heart resulted in a significant abbreviation of activation time when the PS was present; these shocks are of particular interest because the fields generated by internal cardioverter defibrillators tend to have a strong component in the same direction. The extent of PS-induced changes, both temporal and spatial, was constrained by the amount of shock-activated myocardium. Increasing field strength decreased the transmission delay between PS and ventricular tissue at Purkinje-myocardial junctions (PMJs), but this did not have a major effect on the organ-level response. Weaker shocks directly affect a smaller volume of myocardial tissue but easily excite the PS, which makes the PS contribution to far field excitation more substantial than for stronger shocks.

A computer modeling tool for comparing novel ICD electrode orientations in children and adults

BACKGROUND

Use of implantable cardiac defibrillators (ICDs) in children and patients with congenital heart disease is complicated by body size and anatomy. A variety of creative implantation techniques has been used empirically in these groups on an ad hoc basis.

OBJECTIVE

To rationalize ICD placement in special populations, we used subject-specific, image-based finite element models (FEMs) to compare electric fields and expected defibrillation thresholds (DFTs) using standard and novel electrode configurations.

METHODS

FEMs were created by segmenting normal torso computed tomography scans of subjects ages 2, 10, and 29 years and 1 adult with congenital heart disease into tissue compartments, meshing, and assigning tissue conductivities. The FEMs were modified by interactive placement of ICD electrode models in clinically relevant electrode configurations, and metrics of relative defibrillation safety and efficacy were calculated.

RESULTS

Predicted DFTs for standard transvenous configurations were comparable with published results. Although transvenous systems generally predicted lower DFTs, a variety of extracardiac orientations were also predicted to be comparably effective in children and adults. Significant trend effects on DFTs were associated with body size and electrode length. In many situations, small alterations in electrode placement and patient anatomy resulted in significant variation of predicted DFT. We also show patient-specific use of this technique for optimization of electrode placement.

CONCLUSION

Image-based FEMs allow predictive modeling of defibrillation scenarios and predict large changes in DFTs with clinically relevant variations of electrode placement. Extracardiac ICDs are predicted to be effective in both children and adults. This approach may aid both ICD development and patient-specific optimization of electrode placement. Further development and validation are needed for clinical or industrial utilization.

Finite element modelling and simulations in cardiovascular mechanics and cardiology: A bibliography 1993–2004

The paper gives a bibliographical review of the finite element modelling and simulations in cardiovascular mechanics and cardiology from the theoretical as well as practical points of views. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1993 and 2004. At the end of this paper, more than 890 references are given dealing with subjects as: Cardiovascular soft tissue modelling; material properties; mechanisms of cardiovascular components; blood flow; artificial components; cardiac diseases examination; surgery; and other topics.

Towards automated 3D finite element modeling of direct fiber reinforced composite dental bridge

An automated 3D finite element (FE) modeling procedure for direct fiber reinforced dental bridge is established on the basis of computer tomography (CT) scan data. The model presented herein represents a two-unit anterior cantilever bridge that includes a maxillary right incisor as an abutment and a maxillary left incisor as a cantilever pontic bonded by adhesive and reinforced fibers. The study aims at gathering fundamental knowledge for design optimization of this type of innovative composite dental bridges. To promote the automatic level of numerical analysis and computational design of new dental biomaterials, this report pays particular attention to the mathematical modeling, mesh generation, and validation of numerical models. To assess the numerical accuracy and to validate the model established, a convergence test and experimental verification are also presented.

An Epicardial Subxiphoid Implantable Defibrillator Lead:. Superior Effectiveness After Failure of Standard Implants

A single epicardial implantable lead using the subxiphoid approach is described in this article. It consists of a single halo-shaped coil that is implanted under the inferior surface of the heart, including the right and left inferior ventricular surfaces. It has been implanted in four patients who could not be defibrillated with a transvenous system, even with the adjunct use of subcutaneous leads or left chest wall patch. Three of the patients had progressive heart failure due to ischemic myocardiopathy; the fourth patient had a dilated idiopathic myocardiopathy. The approach is simple and appears to be effective due to its ability to encompass the left and right ventricles. This vector seems to significantly lower the threshold for defibrillation, and may offer substantial benefit in the setting of high defibrillation thresholds with conventional leads, or when conventional systems are inadequate to achieve consistent defibrillation.

A comparison of biventricular and conventional transvenous defibrillation: a computational study using patient derived models

Conventional transvenous defibrillation is performed with an ICD using a dual current pathway. The defibrillation energy is delivered from the RV electrode to the superior vena cava (SVC) electrode and the metallic case (CAN) of the ICD. Biventricular defibrillation uses an additional electrode placed in the LV free wall with sequential shocks to create an additional current vector. Clinical studies of biventricular defibrillation have reported a 45% reduction in mean defibrillation threshold (DFT) energy. The aim of the study was to use computational methods to examine the biventricular defibrillation fields together with their corresponding DFTs in a variety of patient derived models and to compare them to simulations of conventional defibrillation. A library of thoracic models derived from nine patients was used to solve for electric field distributions. The defibrillation waveform consisted of a LV --> SVC + CAN monophasic shock followed by a biphasic shock delivered via the RV --> SVC + CAN electrodes. When the initial voltage of the two shocks is the same, the simulations show that the biventricular configuration reduces the mean DFT by 46% (3.5 +/- 1.3 vs 5.5 +/- 2.7 J, P = 0.005). When the leading edge of the biphasic shock is equal to the trailing edge of the monophasic shock, there is no statistically significant difference in the mean DFT (4.9 +/- 1.9 vs 5.5 +/- 2.7 J, P > 0.05) with the DFT decreasing in some patients and increasing in others. These results suggest that patient-specific computational models may be able to identify those patients who would most benefit from a biventricular configuration.

Improved lung edema monitoring with coronary vein pacing leads: a simulation study

This computer simulation study compared the ability of left ventricular coronary vein (LV) pacemaker leads against right ventricular (RV) and right atrial (RA) leads to monitor lung edema using electrical impedance measurements. MRI images were used to construct electrical models of the thorax. Four lead configurations were tested with increases of pulmonary edema, intravascular fluids and heart dilation. The impedance changes observed at end systole with severe lung edema were 8.5%, 11.2%, 12.3% and 26.8% for the RA, RV, RV coil and LV configurations, respectively. Sensitivities in ohms per litre of lung fluid were 19.15, 19.15, 25.07 and 52.11 for the same configurations. The impedance changes for intravascular fluid overload with constant lung status were 1%, 1.3%, 9.2% and 6.4% while the sensitivities were 2, 2, 17 and 11 ohms per litre of intravascular fluid, respectively. Regional analysis of the thoracic sources of impedance revealed a high sensitivity near pacing electrodes and generator, and a low sensitivity to the right lung and all pulmonary vessels. Simulations showed that LV leads have a threefold advantage in sensitivity when monitoring lung edema in comparison to conventional RV leads. To monitor vascular and lung fluids independently, combined impedance configurations may be used. Regional sensitivities must be taken into account for proper clinical interpretation of impedance changes.

Biphasic waveform external defibrillation thresholds for spontaneous ventricular fibrillation secondary to acute ischemia

OBJECTIVES

The goal of this study was to determine if the defibrillation threshold (DFT) after spontaneous ventricular fibrillation (VF) secondary to acute ischemia differs from the DFT for electrically induced VF in the absence of ischemia in anesthetized, closed-chest dogs and pigs.

BACKGROUND

The efficacy of external defibrillators has been tested mainly in animals and humans using E-VF, yet external defibrillators are often used in patients to halt S-VF.

METHODS

Protocol 1: biphasic truncated exponential (BTE) waveform shocks were delivered through electrodes placed in an anterior-anterior (A-A) position (left and right lateral thorax) in nine dogs. After measuring the E-VF DFT, acute ischemia was induced with an angioplasty balloon in either the left anterior descending or left circumflex coronary artery, and the S-VF DFT was determined. Protocol 2: in a group of 12 pigs, the E-VF DFT and S-VF DFT were determined for electrodes in the A-A position and in the anterior-posterior position (A-P). Protocol 3: the E-VF DFT was determined in seven pigs. Then up to three shocks 1.5x the E-VF DFT were delivered to S-VF. If defibrillation did not occur, a step-up protocol was used until defibrillation occurred.

RESULTS

Protocol 1: the DFT for E-VF was 65 +/- 28 J (mean +/- SD) compared with 226 +/- 97 J for S-VF, p < 0.05. Protocol 2: the DFT was 152 +/- 58 J for E-VF and 315 +/- 123 J for S-VF for A-A electrodes. The DFT was 100 +/- 43 J for E-VF and 206 +/- 114 J for S-VF for A-P electrodes. Protocol 3: 11/37 shocks of strength 1.5x E-VF DFT (182 +/- 40 J) stopped the arrhythmia. The episodes of S-VF not halted by these shocks required energy levels of up to 400 J for defibrillation.

CONCLUSIONS

External defibrillation of S-VF induced by acute ischemia requires significantly more energy than VF induced by 60-Hz current in the absence of ischemia. A safety margin >1.5x the DFT for electrically induced VF may be necessary in BTE external defibrillators to defibrillate S-VF.

Improvement of defibrillation efficacy and quantification of activation patterns during ventricular fibrillation in a canine heart failure model

BACKGROUND

Little is known about the effects of heart failure (HF) on the defibrillation threshold (DFT) and the characteristics of activation during ventricular fibrillation (VF).

METHODS AND RESULTS

HF was induced by rapid right ventricular (RV) pacing for at least 3 weeks in 6 dogs. Another 6 dogs served as controls. Catheter defibrillation electrodes were placed in the RV apex, the superior vena cava, and the great cardiac vein (CV). An active can coupled to the superior vena cava electrode served as the return for the RV and CV electrodes. DFTs were determined before and during HF for a shock through the RV electrode with and without a smaller auxiliary shock through the CV electrode. VF activation patterns were recorded in HF and control animals from 21x24 unipolar electrodes spaced 2 mm apart on the ventricular epicardium. Using these recordings, we computed a number of quantitative VF descriptors. DFT was unchanged in the control dogs. DFT energy was increased 79% and 180% (with and without auxiliary shock, respectively) in HF compared with control dogs. During but not before HF, DFT energy was significantly lowered (21%) by addition of the auxiliary shock. The VF descriptors revealed marked VF differences between HF and control dogs. The differences suggest decreased excitability and an increased refractory period during HF. Most, but not all, descriptors indicate that VF was less complex during HF, suggesting that VF complexity is multifactorial and cannot be expressed by a scalar quantity.

CONCLUSIONS

HF increases the DFT. This is partially reversed by an auxiliary shock. HF markedly changes VF activation patterns.

Defibrillation efficacy of different electrode placements in a human thorax model

The objective of this study was to measure the defibrillation threshold (DFT) associated with different electrode placements using a three-dimensional anatomically realistic finite element model of the human thorax. Coil electrodes (Endotak DSP, model 125, Guidant/CPI) were placed in the RV apex along the lateral wall (RV), withdrawn 10 mm away from the RV apex along the lateral wall (RVprox), in the RV apex along the anterior septum (RVseptal), and in the SVC. An active pulse generator (can) was placed in the subcutaneous prepectoral space. Five electrode configurations were studied: RV-->SVC, RVprox-->SVC, RVSEPTAL-->SVC, RV-->Can, and RV-->SVC + Can. DFTs are defined as the energy required to produce a potential gradient of at least 5 V/cm in 95% of the ventricular myocardium. DFTs for RV-->SVC, RVprox-->SVC, RVseptal-->SVC, RV-->Can, and RV-->SVC + Can were 10, 16, 7, 9, and 6 J, respectively. The DFTs measured at each configuration fell within one standard deviation of the mean DFTs reported in clinical studies using the Endotak leads. The relative changes in DFT among electrode configurations also compared favorably. This computer model allows measurements of DFT or other defibrillation parameters with several different electrode configurations saving time and cost of clinical studies.

Impact of transvenous lead position on active-can ICD defibrillation: a computer simulation study

Optimizing lead placement in transvenous defibrillation remains central to the clinical aspects of the defibrillation procedure. Studies involving superior vena cava (SVC) return electrodes have found that left ventricular (LV) leads or septal positioning of the right ventricular (RV) lead minimizes the voltage defibrillation threshold (VDFT) in endocardial lead-->SVC defibrillation systems. However, similar studies have not been conducted for active-can configurations. The goal of this study was to determine the optimal lead position to minimize the VDFT for systems incorporating an active can. This study used a high resolution finite element model of a human torso that includes the fiber architecture of the ventricular myocardium to find the role of lead positioning in a transvenous LEAD-->can defibrillation electrode system. It was found that, among single lead systems, posterior positioning of leads in the right ventricle lowers VDFTs appreciably. Furthermore, a septal location of leads resulted in lower VDFTs than free-wall positioning. Increasing the number of leads, and thus the effective lead surface area in the right ventricle also resulted in lower VDFTs. However, the lead configuration that resulted in the lowest VDFTs is a combination of mid-cavity right ventricle lead and a mid-cavity left ventricle lead. The addition of a left ventricular lead resulted in a reduction in the size of the low gradient regions and a change of its location from the left ventricular free wall to the septal wall.