Novel Electrode Design for Potentially Painless Internal Defibrillation Also Allows for Successful External Defibrillation

Electrical Stimulation for Low-Energy Termination of Cardiac Arrhythmias: a Review

Cardiac arrhythmias are a leading cause of morbidity and mortality in the developed world, estimated to be responsible for hundreds of thousands of deaths annually. Our understanding of the electrophysiological mechanisms of such arrhythmias has grown since they were formally characterized in the late nineteenth century, and this has led to the development of numerous devices and therapies that have markedly improved outcomes for patients affected by such conditions. Despite these advancements, the application of a single large shock remains the clinical standard for treating deadly tachyarrhythmias. Such defibrillating shocks are undoubtedly effective in terminating such arrhythmias; however, they are applied without forewarning, contributing to the patient's stress and anxiety; they can be intensely painful; and they can have adverse psychological and physiological effects on patients. In recent years, there has been interest in developing defibrillation protocols that can terminate arrhythmias without crossing the human pain threshold for energy delivery, generally estimated to be between 0.1 and 1 J. In this article, we review existing literature on the development of such low-energy defibrillation methods and their underlying mechanisms, in an attempt to broadly describe the current landscape of these technologies.

A Novel Defibrillation Tool: Percutaneously Delivered, Partially Insulated Epicardial Defibrillation

INTRODUCTION

Epicardial defibrillation systems currently require surgical access. We aimed to develop a percutaneous defibrillation system with partially-insulated epicardial coils to focus electrical energy on the myocardium and prevent or minimize extra-cardiac stimulation.

METHODS

We tested 2 prototypes created for percutaneous introduction into the pericardial space via a steerable sheath. This included a partially-insulated defibrillation coil and a defibrillation mesh with a urethane balloon acting as an insulator to the face of the mesh not in contact with the epicardium. The average energy associated with a chance of successful defibrillation 75% of the time (ED75) was calculated for each experiment.

RESULTS

Of 16 animal experiments, 3 pig experiments had malfunctioning mesh prototypes such that results were unreliable; these were excluded. Therefore, 13 animal experiments were analyzed - 6 canines (29.8±4.0kg); 7 pigs (41.1±4.4kg). The overall ED75 was 12.8±6.7J (10.9±9.1J for canines; 14.4±3.9J in pigs [P=0.37]). The lowest ED75 obtained in canines was 2.5J while in pigs it was 9.5J. The lowest energy resulting in successful defibrillation was 2J in canines and 5J in pigs. There was no evidence of coronary vessel injury or trauma to extra-pericardial structures.

CONCLUSION

Percutaneous, epicardial defibrillation using a partially insulated coil is feasible and appears to be associated with low defibrillation thresholds. Focusing insulation may limit extra-cardiac stimulation and potentially lower energy requirements for efficient defibrillation.

Measuring defibrillator surface potentials: The validation of a predictive defibrillation computer model

Implantable cardioverter defibrillators (ICDs) are commonly used to reduce the risk in patients with life-threatening arrhythmias, however, clinicians have little systematic guidance to place the device, especially in cases of unusual anatomy. We have previously developed a computational model that evaluates the efficacy of a delivered shock as a clinical and research aid to guide ICD placement on a patient specific basis. We report here on progress to validate this model with measured ICD surface potential maps from patients undergoing ICD implantation and testing for defibrillation threshold (DFT). We obtained body surface potential maps of the defibrillation pulses by adapting a limited lead selection and potential estimation algorithm to deal with the limited space for recording electrodes. Comparison of the simulated and measured potential maps of the defibrillation shock yielded similar patterns, a typical correlation greater than 0.9, and a relative error less than 15%. Comparison of defibrillation thresholds also showed accurate prediction of the simulations. The high agreement of the potential maps and DFTs suggests that the predictive simulation generates realistic potential values and can accurately predict DFTs in patients. These validation results pave the way for use of this model in optimization studies prior to device implantation.

Tetanizing prepulse: A novel strategy to mitigate implantable cardioverter-defibrillator shock-related pain

BACKGROUND

Skeletal muscle activation has been implicated as the source of pain associated with implantable cardioverter-defibrillator shocks. We hypothesized that the skeletal muscle response to defibrillatory shocks could be attenuated with a tetanizing prepulse immediately before biphasic shock delivery.

OBJECTIVE

The purpose of this study was to test the ability of tetanizing prepulses to reduce the skeletal muscle activation associated with defibrillation.

METHODS

Seven adult pigs were studied. A left ventricular coil and subcutaneous dummy can in the right thorax were used to deliver either pure biphasic waveforms or test waveforms consisting of a tetanizing pulse of high-frequency alternating current (HFAC) ramped to an amplitude of 5-100 V over 0.25-1 second, immediately followed by a biphasic shock of approximately 9 J (ramped HFAC and biphasic [rHFAC+B]). We used limb acceleration and rate of force development as surrogate measures of pain. Test and control waveforms were delivered in sinus rhythm and induced ventricular fibrillation to test defibrillation efficacy.

RESULTS

Defibrillation threshold energy was indistinguishable between rHFAC+B and pure biphasic shocks. Peak acceleration and rate of force development were reduced by 72% ± 7% and 71% ± 22%, respectively, with a 25-V, 1-second rHFAC+B waveform compared with pure biphasic shocks. Notably, rHFAC+B with a 9-J biphasic shock produced significantly less skeletal muscle activation than a 0.1-J pure biphasic shock.

CONCLUSION

A putative source of implantable cardioverter-defibrillator shock-related pain can be mitigated using a tetanizing prepulse followed by biphasic shock. Human studies will be required to assess true pain reduction with this approach.

Towards electroporation based treatment planning considering electric field induced muscle contractions

The electric field threshold for muscle contraction is two orders of magnitudes lower than that for electroporation. Current electroporation treatment planning and electrode design studies focus on optimizing the delivery of electroporation electric fields to the targeted tissue. The goal of one part of this study was to investigate the relation between the volumes of tissue that experience electroporation electric fields in a targeted tissue volume and the volumes of tissue that experience muscle contraction inducing electric fields around the electroporated tissue volume, (V(MC)), during standard electroporation procedures and for various electroporation electrodes designs. The numerical analysis shows that conventional electroporation protocols and electrode design can generate muscle contraction inducing electric fields in surprisingly large volumes of non-target tissue, around the electroporation treated tissue. In studying various electrode configurations, we found that electrode placement in a structure we refer to as a "Current Cage" can substantially reduce the volume of non-target tissue exposed to electric fields above the muscle contraction threshold. In an experimental study on a tissue phantom we compare a commercial two parallel needle electroporation system with the Current Cage design. While tissue electroporated volumes were similar, V(MC) of tissue treated using the Current Cage design electrodes was an order of magnitude smaller than that using a commercially available system. An important aspect of the entire study is that it suggests the benefit of including the calculations of V(MC) for planning of electroporation based treatments such as DNA vaccination, electrochemotherapy and irreversible electroporation.

Switchable Faraday shielding with application to reducing the pain of internal cardiac defibrillation while permitting external defibrillation

Switchable Faraday shielding is desirable in situations where electric field shielding is required at certain times and undesirable at other times. In this study, electrostatic finite element modeling was used to assess the effect of different shield geometries on the leakage of an internally applied field and penetration of an externally applied field. "Switching OFF" the shield by electrically disconnecting shield faces from each other was shown to significantly increase external field penetration. Applying this model to defibrillation, we looked at the effect of spacing and size of shield panels to maximize the ability to deliver an external defibrillation shock to the heart when shield panels are disconnected while providing acceptably low leakage of internal defibrillation shocks to avoid painful skeletal muscle capture when shield panels are connected. This analysis may be useful for designing internal defibrillator electrodes that preserve the efficacy of internal and external defibrillation while avoiding the significant morbidity associated with painful defibrillator shocks. Similar analysis could also guide optimizing the switchable Faraday shielding concept for other applications.

Low-energy multistage atrial defibrillation therapy terminates atrial fibrillation with less energy than a single shock

BACKGROUND

Implantable device therapy of atrial fibrillation (AF) is limited by pain from high-energy shocks. We developed a low-energy multistage defibrillation therapy and tested it in a canine model of AF.

METHODS AND RESULTS

AF was induced by burst pacing during vagus nerve stimulation. Our novel defibrillation therapy consisted of 3 stages: stage (ST) 1 (1-4 low-energy biphasic [BP] shocks), ST2 (6-10 ultralow-energy monophasic [MP] shocks), and ST3 (antitachycardia pacing). First, ST1 testing compared single or multiple MP and BP shocks. Second, several multistage therapies were tested: ST1 versus ST1+ST3 versus ST1+ST2+ST3. Third, 3 shock vectors were compared: superior vena cava to distal coronary sinus, proximal coronary sinus to left atrial appendage, and right atrial appendage to left atrial appendage. The atrial defibrillation threshold (DFT) of 1 BP shock was <1 MP shock (0.55 ± 0.1 versus 1.38 ± 0.31 J, P=0.003). Two to 3 BP shocks terminated AF with lower peak voltage than 1 BP or 1 MP shock and with lower atrial DFT than 4 BP shocks. Compared with ST1 therapy alone, ST1+ST3 lowered the atrial DFT moderately (0.51 ± 0.46 versus 0.95 ± 0.32 J, P=0.036), whereas 3-stage therapy (ST1+ST2+ST3) dramatically lowered the atrial DFT (0.19 ± 0.12 versus 0.95 ± 0.32 J for ST1 alone, P=0.0012). Finally, the 3-stage therapy was equally effective for all studied vectors.

CONCLUSIONS

Three-stage electrotherapy significantly reduces the AF DFT and opens the door to low-energy atrial defibrillation at or below the pain threshold.