Progressive depolarization: a unified hypothesis for defibrillation and fibrillation induction by shocks

Highly Reproducible Physiological Asymmetric Membrane with Freely Diffusing Embedded Proteins in a 3D-Printed Microfluidic Setup

Experimental setups to produce and to monitor model membranes have been successfully used for decades and brought invaluable insights into many areas of biology. However, they all have limitations that prevent the full in vitro mimicking and monitoring of most biological processes. Here, a suspended physiological bilayer-forming chip is designed from 3D-printing techniques. This chip can be simultaneously integrated to a confocal microscope and a path-clamp amplifier. It is composed of poly(dimethylsiloxane) and consists of a ≈100 µm hole, where the horizontal planar bilayer is formed, connecting two open crossed-channels, which allows for altering of each lipid monolayer separately. The bilayer, formed by the zipping of two lipid leaflets, is free-standing, horizontal, stable, fluid, solvent-free, and flat with the 14 types of physiologically relevant lipids, and the bilayer formation process is highly reproducible. Because of the two channels, asymmetric bilayers can be formed by making the two lipid leaflets of different composition. Furthermore, proteins, such as transmembrane, peripheral, and pore-forming proteins, can be added to the bilayer in controlled orientation and keep their native mobility and activity. These features allow in vitro recapitulation of membrane process close to physiological conditions.

Hypothermic neuroprotection during reperfusion following exposure to aglycemia in central white matter is mediated by acidification

Hypoglycemia is a common iatrogenic consequence of type 1 diabetes therapy that can lead to central nervous system injury and even death if untreated. In the absence of clinically effective neuroprotective drugs we sought to quantify the putative neuroprotective effects of imposing hypothermia during the reperfusion phase following aglycemic exposure to central white matter. Mouse optic nerves (MONs), central white matter tracts, were superfused with oxygenated artificial cerebrospinal fluid (aCSF) containing 10 mmol/L glucose at 37°C. The supramaximal compound action potential (CAP) was evoked and axon conduction was assessed as the CAP area. Extracellular lactate was measured using an enzyme biosensor. Exposure to aglycemia, simulated by omitting glucose from the aCSF, resulted in axon injury, quantified by electrophysiological recordings, electron microscopic analysis confirming axon damage, the extent of which was determined by the duration of aglycemia exposure. Hypothermia attenuated injury. Exposing MONs to hypothermia during reperfusion resulted in improved CAP recovery compared with control recovery measured at 37°C, an effect attenuated in alkaline aCSF. Hypothermia decreases pH implying that the hypothermic neuroprotection derives from interstitial acidification. These results have important clinical implications demonstrating that hypothermic intervention during reperfusion can improve recovery in central white matter following aglycemia.

Challenges and advances in atomistic simulations of potassium and sodium ion channel gating and permeation

Ion channels are implicated in many essential physiological events such as electrical signal propagation and cellular communication. The advent of K⁺ and Na⁺ ion channel structure determination has facilitated numerous investigations of molecular determinants of their behaviour. At the same time, rapid development of computer hardware and molecular simulation methodologies has made computational studies of large biological molecules in all-atom representation tractable. The concurrent evolution of experimental structural biology with biomolecular computer modelling has yielded mechanistic details of fundamental processes unavailable through experiments alone, such as ion conduction and ion channel gating. This review is a short survey of the atomistic computational investigations of K⁺ and Na⁺ ion channels, focusing on KcsA and several voltage-gated channels from the K_V and Na_V families, which have garnered many successes and engendered several long-standing controversies regarding the nature of their structure-function relationship. We review the latest advancements and challenges facing the field of molecular modelling and simulation regarding the structural and energetic determinants of ion channel function and their agreement with experimental observations.

A Solid-State Hard Microfluidic-Nanopore Biosensor with Multilayer Fluidics and On-Chip Bioassay/Purification Chamber

Solid-state nanopores are an emerging biosensor for nucleic acid and protein characterization. For use in a clinical setting, solid-state nanopore sensing requires sample preparation and purification, fluid handling, a heating element, electrical noise insulators, and an electrical readout detector, all of which hamper its translation to a point-of-care diagnostic device. A stand-alone microfluidic-based nanopore device is described that combines a bioassay reaction/purification chamber with a solid-state nanopore sensor. The microfluidic device is composed of the high-temperature/solvent resistance Zeonex plastic, formed via micro-machining and heat bonding, enabling the use of both a heat regulator and a magnetic controller. Fluid control through the microfluidic channels and chambers is controlled via fluid port selector valves and allows up-to eight different solutions. Electrical noise measurements and DNA translocation experiments demonstrate the integrity of the device, with performance comparable to a conventional stand-alone nanopore setup. However, the microfluidic-nanopore setup is superior in terms of ease of use. To showcase the utility of the device, single molecule detection of a DNA PCR product, after magnetic bead DNA separation, is accomplished on chip.

Materials and Structures toward Soft Electronics

Soft electronics are intensively studied as the integration of electronics with dynamic nonplanar surfaces has become necessary. Here, a discussion of the strategies in materials innovation and structural design to build soft electronic devices and systems is provided. For each strategy, the presentation focuses on the fundamental materials science and mechanics, and example device applications are highlighted where possible. Finally, perspectives on the key challenges and future directions of this field are presented.

Optimized expression and purification of NavAb provide the structural insight into the voltage dependence

Voltage-gated sodium channels are crucial for electro-signalling in living systems. Analysis of the molecular mechanism requires both fine electrophysiological evaluation and high-resolution channel structures. Here, we optimized a dual expression system of NavAb, which is a well-established standard of prokaryotic voltage-gated sodium channels, for E. coli and insect cells using a single plasmid vector to analyse high-resolution protein structures and measure large ionic currents. Using this expression system, we evaluated the voltage dependence and determined the crystal structures of NavAb wild-type and two mutants, E32Q and N49K, whose voltage dependence were positively shifted and essential interactions were lost in voltage sensor domain. The structural and functional comparison elucidated the molecular mechanisms of the voltage dependence of prokaryotic voltage-gated sodium channels.

Toward a More Efficient Implementation of Antifibrillation Pacing

We devise a methodology to determine an optimal pattern of inputs to synchronize firing patterns of cardiac cells which only requires the ability to measure action potential durations in individual cells. In numerical bidomain simulations, the resulting synchronizing inputs are shown to terminate spiral waves with a higher probability than comparable inputs that do not synchronize the cells as strongly. These results suggest that designing stimuli which promote synchronization in cardiac tissue could improve the success rate of defibrillation, and point towards novel strategies for optimizing antifibrillation pacing.

A multi-criteria evaluation method for assessing the defibrillation outcome of different electrode placements in swine

Compared with clinical and experimental approaches, numerical modeling of defibrillation offers a great opportunity to optimize the defibrillation strategy in a more individualized way. Through numerical simulation of the shock-induce electric field distribution, the outcome of a certain defibrillation shock could be predicted according to several different metrics. In this paper, we propose a novel evaluation method, in which four defibrillation criteria are assigned with separate weighting factors to quantitatively assess the efficiency of a certain defibrillation shock. Three anatomically realistic finite element models of swine were constructed for the evaluation study of 8 electrode pairs in different placements. In addition, corresponding animal experiments were performed to determine the defibrillation threshold of 8 electrode placements. Both computational and experimental results suggest that the clinical recommended anterior-lateral position is the most efficient electrode displacement for transthoracic defibrillation in swine. In conclusion, the good agreement between stimulations and experiments indicates that the present multi-criteria evaluation method would be potentially useful for optimizations of cardiac defibrillation outcome.

Imaging of Ventricular Fibrillation and Defibrillation: The Virtual Electrode Hypothesis

Ventricular fibrillation is the major underlying cause of sudden cardiac death. Understanding the complex activation patterns that give rise to ventricular fibrillation requires high resolution mapping of localized activation. The use of multi-electrode mapping unraveled re-entrant activation patterns that underlie ventricular fibrillation. However, optical mapping contributed critically to understanding the mechanism of defibrillation, where multi-electrode recordings could not measure activation patterns during and immediately after a shock. In addition, optical mapping visualizes the virtual electrodes that are generated during stimulation and defibrillation pulses, which contributed to the formulation of the virtual electrode hypothesis. The generation of virtual electrode induced phase singularities during defibrillation is arrhythmogenic and may lead to the induction of fibrillation subsequent to defibrillation. Defibrillating with low energy may circumvent this problem. Therefore, the current challenge is to use the knowledge provided by optical mapping to develop a low energy approach of defibrillation, which may lead to more successful defibrillation.

A comparison of chronaxies for ventricular fibrillation induction, defibrillation, and cardiac stimulation: unexpected findings and their implications

INTRODUCTION

A low-energy (<or= 4 J) cardioversion shock (LEC) either terminates reentrant ventricular tachycardia (VT) or accelerates it to ventricular fibrillation (VF). Optimization of the duration and amplitude of LEC shocks could improve the success rate of VT termination without VF induction.

METHODS AND RESULTS

In order to learn how LEC shocks may be optimized, we used an animal model to compare the strength-duration curve for VF induction and the strength-duration curve for cardiac stimulation via the shock coil. Conventional implantable cardioverter-defibrillator (ICD) leads were implanted in 12 narcotized pigs from 20 kg to 25 kg in weight. Stimulation, VF induction, and defibrillation pulses were delivered by custom-designed stimulators at preset pulse durations and amplitudes. The corresponding hyperbolic strength-duration curves were constructed using the least-squares fit method and averaged for all the animals. The mean chronaxie for stimulation via the shock coil of 0.23 ms was significantly shorter than both defibrillation (4.8 ms) and VF induction (3.1 ms) chronaxie values. At a shock duration of 0.3 ms or less, the mean VF-induction threshold amplitude exceeded 300 V.

CONCLUSION

It may be reasonable to study whether LEC pulses from 0.25 ms to 0.30 ms in duration and up to 250 V in amplitude would increase therapeutic yield in VT termination without VF induction in humans. Contrary to the current belief, the discrepancy between defibrillation and stimulation chronaxie is not caused by different electrode size. We postulate that the time constant of the fast sodium channel reactivation may be the underlying reason.

Molecular basis of inhibitory peptide maurotoxin recognizing Kv1.2 channel explored by ZDOCK and molecular dynamic simulations

Inhibitory peptide-channel interactions have been utilized to characterize both channels and peptides; however, the fundamental basis for these interactions remains elusive. Here, combined computation methods were employed to study the specific binding of maurotoxin (MTX) peptide to Kv1.2 channel. In the first stage, numerous predicted complexes were generated by docking an ensemble of all 35 NMR conformations of MTX to Kv1.2 channel with ZDOCK program. Then the resulted complexes were clustered and classified into four main binding modes, based on experimental information and interaction energy analysis after the energy minimization and molecular dynamics (MD) simulations. By examining the stability of the plausible candidates through unrestrained MD simulations and calculation of the binding free energies, a final reasonable MTX-Kv1.2 complex was identified, with an overall high degree of correlation between the calculation and experiment on mutational effects. In the obtained complex structure model, MTX mainly used its beta-sheet domains to associate the channel mouth instead of the well-recognized functionally important S5P linkers of Kv1.2 channel. Structure analysis characterized that the most essential Tyr(32) residue of MTX was surrounded by a "pocket" formed by many nonpolar and polar residues of Kv1.2 channel, and revealed a pore-blocking Lys(23) and an important Lys(7) stabilized by strong electrostatic interactions with Asp(379) of Kv1.2. Furthermore, a stepwise structural arrangement for both ligand and receptor was found to accompany the tighter interaction of MTX into the target channel. The starting conformation of MTX, the side-chain conformation of the most important residue Tyr(32), and proper introduction of flexibility for candidate complexes were demonstrated to be considerably important factors for obtaining the final reasonable complex structure model. All these findings should not only be helpful for identifying more plausible K(+) channel-inhibitory peptide complex structures, but also provide intrinsically valuable structural biology information to interpret binding affinities, specificities, and diversity of K(+) channel-nature toxin interactions.

The dilemma of ICD implant testing

Ventricular fibrillation (VF) has been induced at implantable cardioverter defibrillator (ICD) implant to ensure reliable sensing, detection, and defibrillation. Despite its risks, the value was self-evident for early ICDs: failure of defibrillation was common, recipients had a high risk of ventricular tachycardia (VT) or VF, and the only therapy for rapid VT or VF was a shock. Today, failure of defibrillation is rare, the risk of VT/VF is lower in some recipients, antitachycardia pacing is applied for fast VT, and vulnerability testing permits assessment of defibrillation efficacy without inducing VF in most patients. This review reappraises ICD implant testing. At implant, defibrillation success is influenced by both predictable and unpredictable factors, including those related to the patient, ICD system, drugs, and complications. For left pectoral implants of high-output ICDs, the probability of passing a 10 J safety margin is approximately 95%, the probability that a maximum output shock will defibrillate is approximately 99%, and the incidence of system revision based on testing is < or = 5%. Bayes' Theorem predicts that implant testing identifies < or = 50% of patients at high risk for unsuccessful defibrillation. Most patients who fail implant criteria have false negative tests and may undergo unnecessary revision of their ICD systems. The first-shock success rate for spontaneous VT/VF ranges from 83% to 93%, lower than that for induced VF. Thus, shocks for spontaneous VT/VF fail for reasons that are not evaluated at implant. Whether system revision based on implant testing improves this success rate is unknown. The risks of implant testing include those related to VF and those related to shocks alone. The former may be due to circulatory arrest alone or the combination of circulatory arrest and shocks. Vulnerability testing reduces risks related to VF, but not those related to shocks. Mortality from implant testing probably is 0.1-0.2%. Overall, VF should be induced to assess sensing in approximately 5% of ICD recipients. Defibrillation or vulnerability testing is indicated in 20-40% of recipients who can be identified as having a higher-than-usual probability of an inadequate defibrillation safety margin based on patient-specific factors. However, implant testing is too risky in approximately 5% of recipients and may not be worth the risks in 10-30%. In 25-50% of ICD recipients, testing cannot be identified as either critical or contraindicated.

Optimizing defibrillation waveforms for ICDs

While no simple electrical descriptor provides a good measure of defibrillation efficacy, the waveform parameters that most directly influence defibrillation are voltage and duration. Voltage is a critical parameter for defibrillation because its spatial derivative defines the electrical field that interacts with the heart. Similarly, waveform duration is a critical parameter because the shock interacts with the heart for the duration of the waveform. Shock energy is the most often cited metric of shock strength and an ICD's capacity to defibrillate, but it is not a direct measure of shock effectiveness. Despite the physiological complexities of defibrillation, a simple approach in which the heart is modeled as passive resistor-capacitor (RC) network has proved useful for predicting efficient defibrillation waveforms. The model makes two assumptions: (1) The goal of both a monophasic shock and the first phase of a biphasic shock is to maximize the voltage change in the membrane at the end of the shock for a given stored energy. (2) The goal of the second phase of a biphasic shock is to discharge the membrane back to the zero potential, removing the charge deposited by the first phase. This model predicts that the optimal waveform rises in an exponential upward curve, but such an ascending waveform is difficult to generate efficiently. ICDs use electronically efficient capacitive-discharge waveforms, which require truncation for effective defibrillation. Even with optimal truncation, capacitive-discharge waveforms require more voltage and energy to achieve the same membrane voltage than do square waves and ascending waveforms. In ICDs, the value of the shock output capacitance is a key intermediary in establishing the relationship between stored energy-the key determinant of ICD size-and waveform voltage as a function of time, the key determinant of defibrillation efficacy. The RC model predicts that, for capacitive-discharge waveforms, stored energy is minimized when the ICD's system time constant taus equals the cell membrane time constant taum, where taus is the product of the output capacitance and the resistance of the defibrillation pathway. Since the goal of phase two is to reverse the membrane charging effect of phase one, there is no advantage to additional waveform phases. The voltages and capacitances used in commercial ICDs vary widely, resulting in substantial disparities in waveform parameters. The development of present biphasic waveforms in the 1990s resulted in marked improvements in defibrillation efficacy. It is unlikely that substantial improvement in defibrillation efficacy will be achieved without radical changes in waveform design.

Using the Upper Limit of Vulnerability to Assess Defibrillation Efficacy at Implantation of ICDs

The upper limit of vulnerability (ULV) is the weakest shock strength at or above which ventricular fibrillation (VF) is not induced when the shock is delivered during the vulnerable period. The ULV, a measurement made in regular rhythm, provides an estimate of the minimum shock strength required for reliable defibrillation that is as accurate or more accurate than the defibrillation threshold (DFT). The ULV hypothesis of defibrillation postulates a mechanistic relationship between the ULV-measured during regular rhythm-and the minimum shock strength that defibrillates reliably. Vulnerability testing can be applied at implantable cardioverter defibrillator (ICD) implant to confirm a clinically adequate defibrillation safety margin without inducing VF in 75%-95% of ICD recipients. Alternatively, the ULV provides an accurate patient-specific safety margin with a single fibrillation-defibrillation episode. Programming first ICD shocks based on patient-specific measurements of ULV rather than programming routinely to maximum output shortens charge time and may reduce the probability of syncope as ICDs age and charge times increase. Because the ULV is more reproducible than the DFT, it provides greater statistical power for clinical research with fewer episodes of VF. Limited evidence suggests that vulnerability testing is safer than conventional defibrillation testing.

Role of intramural virtual electrodes in shock-induced activation of left ventricle: Optical measurements from the intact epicardial surface

BACKGROUND

According to one hypothesized mechanism of defibrillation, shocks directly excite the bulk of ventricular myocardium in the excitable state due to intramural virtual electrodes; however, this hypothesis has not been examined in intact myocardium.

OBJECTIVES

The purpose of this study was examine the role of intramural virtual electrodes in shock-induced activation of intact left ventricular (LV) tissue.

METHODS

Twelve isolated porcine LV preparations were stained with a transmembrane potential (V(m))-sensitive dye by two methods: (1) surface staining and (2) global staining via coronary perfusion. Shocks (E approximately 0.8-48 V/cm, duration = 10 ms) were applied across the wall from epicardium to endocardium during diastole via transparent electrodes. Shock-induced V(m) responses were measured optically from the intact epicardial surface after surface staining and global staining.

RESULTS

Surface-staining recordings demonstrated different V(m) responses to cathodal and anodal shocks. Whereas cathodal shocks caused depolarization and rapid activation of the epicardial surface, anodal shocks induced hyperpolarization and delayed surface activation. In contrast, global-staining V(m) responses to cathodal and anodal shocks were qualitatively similar. Both responses were characterized by activation with small latency and rapid propagation. Weak shocks of both polarities induced monotonic action potential upstrokes; stronger shocks induced nonmonotonic upstrokes with two rising phases at shock onset and end. Such features of global-staining V(m) responses as make activation of the epicardium by anodal shocks and the nonmonotonic action potential upstrokes can be explained by the presence of subepicardial intramural virtual electrodes.

CONCLUSION

These data suggest that shocks induce intramural virtual electrodes that directly excite LV tissue and account for the shape of optical V(m) responses recorded from the epicardial surface.

Present Understanding of Shock Polarity for Internal Defibrillation: The Obvious and Non-Obvious Clinical Implications

BACKGROUND

Uncertainty about the best electrode configuration has combined with the programming flexibility in modern implantable cardioverter-defibrillators (ICDs) to result in routine polarity reversal during an implant to deal with a high defibrillation threshold (DFT). We feel that this practice is not always supported by the clinical data and the present scientific understanding of defibrillation.

METHOD

A meta-analysis of the clinical studies on ICD shock polarity was performed. Subgroup analyses were also performed to test the impact of high DFTs, various tilts, and the use of the hot can electrode. A review of the basic research surrounding the effects of polarity in defibrillation is also presented.

RESULTS

A total of 224 patients were studied. The use of an anodal right ventricular (RV) coil lowers the mean DFT by 14.8% (P = 0.00001). It provides thresholds equal to or lower than cathodal defibrillation in 83% of patients. The fraction of patients with lower anodal DFTs was 94/224 versus 38/224 for cathodal polarity. This phenomenon may be explained by virtual electrode effects. In particular, anodal electrodes tend to produce collapsing wavefronts while cathodal electrodes tend to produce expanding proarrhythmic wavefronts.

CONCLUSION

In an ICD implant, the RV coil should be the anode. Furthermore, DFT testing beginning with cathodal defibrillation is most likely unnecessary and needlessly extends the procedure's duration and increases the risks for the patient.

A Latin American registry of implantable cardioverter defibrillators: the ICD-LABOR study

OBJECTIVE

Despite the progress that has been reached in emergency medical systems and resuscitation, sudden cardiac death (SCD) continues to be the major cause of the death, and remains a significant public health problem. In this publication we are reporting our Latin American experience in the secondary prevention of SCD, by means of an ongoing registry involving seven Latin American countries and 770 patients.

METHODS

Every individual within the present registry to date has presented with antecedents of aborted sudden death or cardiac arrest due to ventricular tachycardia or ventricular fibrillation. Patients included have fulfilled the Class I indication for implantable cardioverter defibrillator (ICD) and they were implanted with a Biotronik ICD (all models). The study was not sponsored by Biotronik, nor did they have access to the data. A specific protocol was designed for implantation and follow-up of patients. The database was completely registered through the Internet and a personal password was assigned to each group of investigators. The primary end point was death from all causes. Secondary end points were SCD and death due to congestive heart failure (CHF).

RESULTS

The etiology of cardiac disease was found to be predominantly coronary artery disease (CAD) 39.7% (306 patients), followed by Chagas disease (ChD), 26.1% (201 patients), and idiopathic dilated cardiomyopathy (DCM), 17% (131 patients). Any remaining pathologies were included as miscellaneous 13.2% (101 patients). In 31 patients (4%) the etiology was unknown. The age did not differ within the principal pathologies, but was significantly older than the miscellaneous group (62.0 +/- 11.3 years vs 48.2 +/- 18.9 years, P < 0.0001). The follow-up period was 27 +/- 25 months (1-113 months) for the whole group. The mortality in functional classes I-II was significantly lower than mortality for functional classes III-IV (relative risk 1.46, CI 95%, P < 0.0001). Mean left ventricular ejection fraction (LVEF) for the whole group was 37.7 +/- 14.3%. Male LVEF was 36.1 +/- 14.1% and female LVEF was 42.2 +/- 13.8% P < 0.0001. During the follow-up period, 130 deaths were reported (global mortality 16.9 +/- 9.7%), out of which 84 (64.6%) were attributed to cardiac causes (10.9 +/- 5.1% of the total population). The annual adjusted cardiac mortality was 5.2 +/- 1.72% (range 3.5-7.0%). Among cardiac deaths the most common cause was progressive heart failure, 48 patients (57%) including 3 patients with pulmonary embolism. The second main cause of cardiac death was SCD, 36 patients (43%), including 4 patients with electrical storm and 3 patients with electromechanical dissociation after multiple shock therapy treatments.

CONCLUSIONS

Despite the differences in terms of pathologies between the ICD-LABOR (Latin American bioelectronic ongoing registry) and randomized ICD trials, a parallel evolution in all cause mortality and cardiac mortality was observed. Independent risk factors for mortality included age >70 years, male gender, NYHA III/IV, and ejection fraction <0.30. The etiology of heart disease (Chagas vs Coronary Disease) was not found to be a risk factor.

Delayed Success in Termination of Three‐Dimensional Reentry:

INTRODUCTION

Defibrillation shocks slightly stronger than cardioversion threshold may defibrillate not immediately but after a transient period of postshock activity (delayed success). The effect of a defibrillation shock is that it polarizes the tissue, primarily at the surfaces; therefore, surface polarization may play an important role at near-threshold shock intensities.

METHODS AND RESULTS

We numerically investigate the effect of a monophasic transmural electrical shock on a three-dimensional (3D) reentrant wave (scroll wave). For simplicity, we assume uniform polarization of the epicardial and endocardial surfaces. We demonstrate that the effect of surface polarization alone is sufficient to induce delayed termination of self-sustained activity (3-4 beats after the shock). In agreement with experimental observations, both successful and failed shocks cause prolongation of the action potentials on the depolarized side and shortening on the hyperpolarized side, while at the same time inducing a shift from a reentrant to a focal activation pattern. Our simulations suggest that the outcome of the shock is determined by its effect on the shape of the scroll wave's center of rotation (filament). We propose a simple rule to predict the postshock filament shape that allows us to make accurate predictions of success and failure of a termination attempt.

CONCLUSION

Surface polarization due to an electrical shock can terminate a reentrant scroll wave. This mechanism may explain the phenomenon of delayed success in defibrillation.

Defibrillation causes immediate cardiac dilation in humans

Defibrillation Causes Dilation.

INTRODUCTION

Prior studies in isolated heart tissue have shown both excitation and deexcitation to be the primary mechanism of defibrillation. This article presents the first evidence in man of deexcitation immediately following defibrillation by tracking the heart's mechanical response.

METHODS AND RESULTS

The geometric changes of the ventricular chambers were measured before and after defibrillation in seven human subjects receiving an implantable cardioverter defibrillator (ICD). The ICD was used to produce approximately three episodes of ventricular fibrillation and defibrillation in each subject. Twenty-two two-dimensional echocardiographic images of the right ventricle (RV) and 11 images of the left ventricle (LV) were recorded and analyzed at 30 frames per second. Just over 2 seconds of each episode were digitized, beginning half a second before the defibrillation shock. Individual frames were analyzed to yield cross-sectional, ventricular chamber area as a function of time. Immediately following defibrillation, ventricular chambers dilated with significant fractional area increase (RV: 1.58 +/- 0.25, LV: 1.10 +/- 0.06), with peak dilation at 194 +/- 114 msec.

CONCLUSION

Defibrillation causes a rapid increase in ventricular chamber area due to relaxation of the myocardium, suggesting that defibrillation synchronizes the cardiac cells to the deexcited state in man.

Effect of sotalol and acute ventricular dilatation on action potential duration and dispersion of repolarization after defibrillation shocks

Ventricular dilatation shortens action potential duration and increases the defibrillation threshold, whereas sotalol prolongs action potential duration and may decrease the defibrillation threshold. Whether these action potential changes remain after defibrillation shocks, and how they relate to defibrillation success, is not known. In this study, eight monophasic action potentials were recorded simultaneously during electrical defibrillation (shock strength: 20%-200% of the defibrillation threshold) in 16 normal and acutely dilated isolated rabbit hearts at baseline and after addition of sotalol (2 x 10-5 M). Post-shock action potential duration (PS-APD) and dispersion of PS-APD [Disp(PS-APD)] of monophasic action potentials were analyzed after 322 defibrillation shocks at different repolarization levels and related to defibrillation success. Acute ventricular dilatation shortened PS-APD, whereas sotalol prolonged PS-APD. Successful defibrillation was associated with lower Disp(PS-APD) at all repolarization levels in the normal and dilated heart at baseline and with sotalol (mean difference: 33%-46%, all P < 0.005). Minimal PS-APD was longer (mean difference: 5%-11%), while maximal PS-APD was shorter (mean difference: 2%-16%) after successful defibrillation shocks than after failing defibrillation shocks. Therefore, sotalol prolongs action potential duration after defibrillation shocks. Synchronization of repolarization, caused by both prolongation of short PS-APD and shortening of long PS-APD, is associated with successful defibrillation in the normal, acutely dilated, and sotalol-treated heart.

New concepts in transthoracic defibrillation

The transition of biphasic waveforms from ICDs to external defibrillators constitutes a significant technological advances for transthoracic defibrillation. Impedance compensation has enabled the delivery of defibrillating current adapted to each patient and each shock in the same patient. Optimally designed biphasic waveforms have been shown clinically to have greater efficacy in the termination of VF when compared with monophasic waveforms, and because peak current delivery is less, these waveforms are likely to be less injurious to myocardial function. Advances in the understanding of the mechanisms of fibrillation and defibrillation have identified the electrophysiologic events that initiate and sustain VF and the effects of defibrillation shocks on those events. Definition of the role of VEP and postshock excitation has clarified the mechanisms by which shocks can either fail or succeed. The ability of the second phase of optimal biphasic waveform shocks to exploit recruited sodium channels in negatively polarized areas and thus induce rapid propagation of postshock excitation assures uniform depolarization and prevention of re-entry. This appears to be the major mechanism of greater efficacy of biphasic waveforms. It seems certain that continuing investigation of virtual electrodes will enhance our understanding of defibrillation and optimal waveforms. At the same time, much more needs to be known regarding translation of these experimental observations to mechanisms of defibrillation in human hearts with long-standing underlying structural heart disease, which often arises of multiple factors. This represents a major challenge in defibrillation research.

Spatial heterogeneity of transmembrane potential responses of single guinea-pig cardiac cells during electric field stimulation

Changes in transmembrane voltage (V(m)) of cardiac cells during electric field stimulation have a complex spatial- and time-dependent behaviour that differs significantly from electrical stimulation of space-clamped membranes by current pulses. A multisite optical mapping system was used to obtain 17 or 25 microm resolution maps of V(m) along the long axis of guinea-pig ventricular cells (n = 57) stained with voltage-sensitive dye (di-8-ANEPPS) and stimulated longitudinally with uniform electric field (2, 5 or 10 ms, 3-62 V cm(-1)) pulses (n = 201). The initial polarizations of V(m) responses (V(mr)) varied linearly along the cell length and reversed symmetrically upon field reversal. The remainder of the V(m) responses had parallel time courses among the recording sites, revealing a common time-varying signal component (V(ms)). V(ms) was depolarizing for pulses during rest and hyperpolarizing for pulses during the early plateau phase. V(ms) varied in amplitude and time course with increasing pulse amplitude. Four types of plateau response were observed, with transition points between the different responses occurring when the maximum polarization at the ends of the cell reached values estimated as 60, 110 and 220 mV. Among the cells that had a polarization change of > 200 mV at their ends (for fields > 45 V cm(-1)), some (n = 17/25) had non-parallel time courses among V(m) recordings of the various sites. This implied development of an intracellular field (E(i)) that was found to increase exponentially with time (tau = 7.2 +/- 3.2 ms). Theoretical considerations suggest that V(ms) represents the intracellular potential (phi(i)) as well as the average polarization of the cell, and that V(mr) is the manifestation of the extracellular potential gradient resulting from the field stimulus. For cells undergoing field stimulation, phi(i) acts as the cellular physiological state variable and substitutes for V(m), which is the customary variable for space-clamped membranes.

Mechanism of ventricular defibrillation for near-defibrillation threshold shocks: a whole-heart optical mapping study in swine

BACKGROUND

To study the mechanism by which shocks succeed (SDF) or fail (FDF) to defibrillate, global cardiac activation and recovery and their relationship to defibrillation outcome were investigated for shock strengths with approximately equal SDF and FDF outcomes (DFT(50)).

METHODS AND RESULTS

In 6 isolated pig hearts, dual-camera video imaging was used to record optically from approximately 8000 sites on the anterior and posterior ventricular surfaces before and after 10 DFT(50) biphasic shocks. The interval between the shock and the last ventricular fibrillation activation preceding the shock (coupling interval, CI) and the time from shock onset to 90% repolarization of the immediate postshock action potential (RT(90)) were determined at all sites. Of 60 shocks, 31 were SDF. The CI (59+/-7 versus 52+/-6 ms) and RT(90) (108+/-19 versus 88+/-8 ms) were significantly longer for SDF than FDF episodes. Spatial dispersions of CI (36+/-5 versus 34+/-3 ms) and RT(90) (40+/-16 versus 40+/-8 ms) were not significantly different for SDF versus FDF episodes. The first global activation cycle appeared focally on the left ventricular apical epicardium 78+/-32 ms after the shock.

CONCLUSIONS

For near-threshold shocks, defibrillation outcome correlates with the electrical state of the heart at the time of the shock and on RT. Global dispersion of RT was similar in both SDF and FDF episodes, suggesting that it is not crucial in determining defibrillation outcome after DFT(50) shocks.

Optical mapping of ventricular defibrillation in isolated swine right ventricles: demonstration of a postshock isoelectric window after near-threshold defibrillation shocks

BACKGROUND

Investigators who studied ventricular defibrillation by use of optical mapping techniques failed to observe an initial defibrillation event (isoelectric window or quiescent period) shown by electrode mapping studies. This discrepancy has important implications for the mechanisms of defibrillation. The purpose of the present study was to demonstrate an optical equivalent of an isoelectric window after a near-threshold defibrillation shock. Methods and Results-- We studied 10 isolated, perfused swine right ventricles. Upper limit of vulnerability was determined by shocks on T waves. A 50% probability of successful defibrillation (DFT50) was determined with an up-down algorithm. Immediately after unsuccessful defibrillation shock, new wavefronts were generated. When the shock strength was low, immediate reinitiation of reentry and ventricular fibrillation might occur without a postshock isoelectric window. However, if the shock strength was within 50 V of DFT50 (near-threshold), a synchronized activation occurred, followed by organized repolarization that ended 64+/-18 ms after shock. After a period of quiescence (18+/-24 ms), activation recurred 83+/-33 ms after shock and reinitiated ventricular fibrillation. Similar patterns of activation, including a quiescent period, were observed after shock was applied on the T wave of the paced beat that induced ventricular fibrillation. Upper limit of vulnerability correlated well with DFT50.

CONCLUSIONS

In isolated swine right ventricles, an optical equivalent of an isoelectric window exists after near-threshold defibrillation shocks. These findings support the idea that a near-threshold defibrillation shock terminates all activation wavefronts but fails to halt ventricular fibrillation because the same shock reinitiates ventricular fibrillation after an isoelectric window.

Defibrillation and the geometry of the heart: a novel measurement with implications for defibrillation mechanisms

We present a novel measurement for studying defibrillation mechanisms: the time course of changes in the size of the left ventricular (LV) cavity within 500 ms following defibrillation. Mechanical changes can be linked to electrical mechanisms via an understanding of excitation-contraction coupling. Eight mongrel dogs were internally defibrillated 5-50 seconds (including backup shocks) after the onset of 20 ventricular fibrillation (VF) episodes per animal. Two dimensional, short axis, LV cavity, ultrasound images were recorded at 30 frames per second just prior to inducing VF, during defibrillation and following the shock. Each frame was individually analysed to yield the LV cavity area as a function of time. Defibrillation shocks were followed by a highly reproducible phenomenon: (1) a dramatic and rapid increase in LV area, (2) a more or less prominent LV area plateau and (3) a decrease in the LV area. The peak fractional area increase ranged from 1.65 to 4.64 times larger than the baseline (LV area just prior to defibrillation), averaging 2.18 +/- 0.686. Successful shocks took significantly longer (p < 0.01) to return to 1.3 times the baseline (407 +/- 209 ms) than unsuccessful shocks (296 +/- 130 ms). Extrapolating to electrical mechanisms, our novel measurement demonstrates that defibrillation causes immediate relaxation and therefore suggests a significant role for deexcitation in defibrillation.

The role of electroporation in defibrillation

Electric shock is the only effective therapy against ventricular fibrillation. However, shocks are also known to cause electroporation of cell membranes. We sought to determine the impact of electroporation on ventricular conduction and defibrillation. We optically mapped electrical activity in coronary-perfused rabbit hearts during electric shocks (50 to 500 V). Electroporation was evident from transient depolarization, reduction of action potential amplitude, and upstroke dV/dt. Electroporation was voltage dependent and significantly more pronounced at the endocardium versus the epicardium, with thresholds of 229+/-81 versus 318+/-84 V, respectively (P=0.01, n=10), both being above the defibrillation threshold of 181.3+/-45.8 V. Epicardial electroporation was localized to a small area near the electrode, whereas endocardial electroporation was observed at the bundles and trabeculas throughout the entire endocardium. Higher-resolution imaging revealed that papillary muscles (n=10) were most affected. Electroporation and conduction block thresholds in papillary muscles were 281+/-64 V and 380+/-79 V, respectively. We observed no arrhythmia in association with electroporation. Further, preconditioning with high-energy shocks prevented reinduction of fibrillation by 50-V shocks, which were otherwise proarrhythmic. Endocardial bundles are the most susceptible to electroporation and the resulting conduction impairment. Electroporation is not associated with proarrhythmic effects and is associated with a reduction of vulnerability.

Virtual sources associated with linear and curved strands of cardiac cells

Transmembrane potential (V(m)) responses in cardiac strands with different curvature were characterized during uniform electric-field stimulation with the use of modeling and experimental approaches. Linear and U-shaped strands (width 100-150 micrometer) were stained with voltage-sensitive dye. V(m) was measured by optical mapping across the width and at sites of beginning curvature. Field pulses were applied transverse to the strands during the action-potential plateau. For linear strands, V(m) contained 1) a rapid passive component (V(m)(ar)) nearly linear and symmetric across the width, 2) a slower hyperpolarizing component (V(m)(as)) greater and faster on the anodal side, and 3) at high field strengths a delayed depolarizing component (V(m)(ad)) greater on the anodal side. For U-shaped strands, V(m) at sites of beginning curvature also contained rapid and slow components (V(m)(br) and V(m)(bs), respectively) that included contributions from the linear strand response and from the fiber curvature. V(m)(ar), V(m)(br), and part of V(m)(bs) could be attributed to passive behavior that was modeled, and V(m)(as), V(m)(ad), and part of V(m)(bs) could be attributed to active membrane currents. Thus curved strands exhibit field responses separable into components with characteristic amplitude, spatial, and temporal signatures.

Marked reduction of ventricular defibrillation threshold by application of an auxiliary shock to a catheter electrode in the left posterior coronary vein of dogs

INTRODUCTION

For endocardial shocks near the defibrillation threshold (DFT), postshock activity originates from the lateral left ventricular apex, where the shock field is weak. This study tested the hypothesis that an auxiliary shock (AS) delivered between an electrode at this site and a superior vena cava (SVC) electrode before the primary endocardial shock (PS) would reduce the DFT.

METHODS AND RESULTS

In six pentobarbital-anesthetized dogs (26 to 36 kg), catheter electrodes were placed in the right ventricular (RV) apex and the SVC. To simulate transvenous introduction, a small electrode was inserted into the posterior cardiac vein using an epicardial approach. For dual shock treatments, AS (2-msec monophasic) was applied to the coronary vein electrode at different time intervals before a biphasic PS (4 msec/3 msec) to the RV-SVC electrodes. The mean DFT energy for dual shocks treatments were significantly reduced (P < 0.05) in comparison to the control treatment (no AS, 26.5+/-8.8 J). Mean DFT energy after 10 seconds of electrically induced ventricular fibrillation for dual shocks, in which AS and PS were separated by 1, 5, 10, and 20 msec, were 10.2+/-4.1 J, 10.9+/-5.5 J, 11.3+/-6.3 J, and 15.4+/-7.2 J, respectively. These values were all significantly lower than the PS alone (26.5+/-8.8 J).

CONCLUSION

Addition of an AS from the posterior cardiac vein before an endocardial PS reduces DFT energy by more than 50%. Such DFT reduction could improve therapeutic safety margin or permit reduction in volume of implantable cardioverter defibrillators.

Virtual electrodes and deexcitation: new insights into fibrillation induction and defibrillation

Previous models of fibrillation induction and defibrillation stressed the contribution of depolarization during the response of the heart to a shock. This article reviews recent evidence suggesting that comprehending the role of negative polarization (hyperpolarization) also is crucial for understanding the response to a shock. Negative polarization can "deexcite" cardiac cells, creating regions of excitable tissue through which wavefronts can propagate. These wavefronts can result in new reentrant circuits, inducing fibrillation or causing defibrillation to fail. In addition, deexcitation can lead to rapid propagation through newly excitable regions, resulting in the elimination of excitable gaps soon after the shock and causing defibrillation to succeed.

Shock-induced figure-of-eight reentry in the isolated rabbit heart

The patterns of transmembrane potential on the whole heart during and immediately after fibrillation-inducing shocks are unknown. To study arrhythmia induction, we recorded transmembrane activity from the anterior and posterior epicardial surface of the isolated rabbit heart simultaneously using 2 charge-coupled device cameras (32,512 pixels, 480 frames/second). Isolated hearts were paced from the apex at a cycle length of 250 ms. Two shock coils positioned inside the right ventricle (-) and atop the left atrium (+) delivered shocks at 3 strengths (0.75, 1.5, and 2.25 A) and 6 coupling intervals (130 to 230 ms). The patterns of depolarization and repolarization were similar, as is evident in the uniformity of action potential duration at 75% repolarization (131.4¿8.3 ms). At short coupling intervals (<180 ms), shocks hyperpolarized a large portion of the ventricles and produced a pair of counterrotating waves, one on each side of the heart. The first beat after the shock was reentrant in 90% of short coupling interval episodes. At long coupling intervals (>180 ms), increasingly stronger shocks depolarized an increasingly larger portion of the heart. The first beat after the shock was reentrant in 18% of long coupling interval episodes. Arrhythmias were most often induced at short coupling intervals (98%) than at long coupling intervals (35%). The effect and outcome of the shock were related to the refractory state of the heart at the time of the shock. Hyperpolarization occurred at short coupling intervals, whereas depolarization occurred at long coupling intervals. Consistent with the "critical point" hypothesis, increasing shock strength and coupling interval moved the location where reentry formed (away from the shock electrode and pacing electrode, respectively).

Biphasic truncated exponential waveform defibrillation

This paper presents data from studies that have compared the efficacies of biphasic truncated exponential (BTE) and monophasic damped sine (MDS) waveform defibrillation in patients with out-of-hospital cardiac arrest and in in-hospital defibrillation. When a shock is delivered, rhythms evolve rapidly in a variety of directions and take different courses, even over a short time. When defibrillation is defined as termination of ventricular fibrillation at 5 seconds postshock, whether to an organized rhythm or asystole, low-energy BTE shocks appear to be more effective than high-energy MDS shocks in out-of-hospital arrest. For future research, the terms associated with defibrillation should be standardized and used uniformly by all investi-gators. In particular, there should be an agreed-upon definition of shock efficacy.

Energy levels for defibrillation: what is of real clinical importance?

Today, transthoracic and intracardiac defibrillation offer a well-accepted and widely used form of therapy for patients with life-threatening ventricular arrhythmias. Despite the wide clinical use of defibrillators, the mechanisms by which an electrical shock halts fibrillation are still not completely understood. During a shock, different amounts of current flow through the different parts of the heart and the current distribution is highly uneven. This current distribution is affected by changes in the shock potential gradient through the heart, changes in fiber orientation, and changes in myocardial conductivity caused by connective tissue barriers. It would be ideal if the potential gradient distribution throughout the ventricles could be measured directly for each individual patient during defibrillator implantation and follow-up and the shock strength could be programmed based on this measurement, but so far this is not possible. A more feasible approach is to determine, by trial and error, the magnitude of the shock strength delivered through the defibrillation electrodes for successful defibrillation. There is no distinct threshold value above which all shocks succeed and below which all shocks fail to defibrillate. Rather, increasing shock strength increases the likelihood the shock will succeed. Therefore, instead of a distinct defibrillation threshold, a probability of success curve exists. However, increasing the shock strength above an optimal range can actually decrease the success rate for defibrillation. One possible explanation is that the high voltage gradients caused by such large shocks damage cells and result in postshock arrhythmias that may reinitiate fibrillation. Another problem that can affect the probability of defibrillation success for a particular programmed energy setting is that the shock strength required for defibrillation may increase over time due to (1) the growth of fibrotic tissue around the defibrillation electrode; (2) migration of the lead; (3) acute ischemia; or (4) other changes in the underlying cardiac disease (e.g., worsening of heart failure). Such possible increases in the defibrillation shock strength requirement should be compensated for before they occur by adding a margin of safety to the shock strength needed for effective defibrillation. When programming an implantable defibrillator, it is important to keep in mind that the defibrillation shock should be (1) strong enough to defibrillate at least 98% of the time with the first shock; (2) weak enough not to cause severe post-shock arrhythmias or reinitiation of fibrillation; but (3) strong enough to compensate for changes of defibrillation energy requirements over time. This usually can be accomplished by setting the defibrillator 7-10 J higher than the defibrillation threshold determined by a standard step-down protocol.