Purkinje activation precedes myocardial activation following defibrillation after long-duration ventricular fibrillation

Mapping and ablation of ventricular fibrillation substrate

Ventricular fibrillation (VF) is a life-threatening arrhythmia and a common cause of sudden cardiac death (SCD). A basic understanding of its mechanistic underpinning is crucial for enhancing our knowledge to develop innovative mapping and ablation techniques for this lethal rhythm. Significant advances in our understanding of VF have been made especially in the basic science and pre-clinical experimental realms. However, these studies have not yet translated into a robust clinical approach to identify and successfully ablate both the structural and functional substrate of VF. In this review, we aim to (1) provide a conceptual framework of VF and an overview of the data supporting the spatiotemporal dynamics of VF, (2) review experimental approaches to mapping VF to elucidate drivers and substrate for maintenance with a focus on the His-Purkinje system, (3) discuss current approaches using catheter ablation to treat VF, and (4) highlight current unknowns and gaps in the field where future work is necessary to transform the clinical landscape.

Catheter Ablation of Ventricular Fibrillation

Ventricular fibrillation (VF) is a common cause of sudden cardiac death (SCD) and is unfortunately without a cure. Current therapies focus on prevention of SCD, such as implantable cardioverter-defibrillator (ICD) implantation and anti-arrhythmic agents. Significant progress has been made in improving our understanding and ability to target the triggers of VF, via advanced mapping and ablation techniques, as well as with autonomic modulation. However, the critical substrate for VF maintenance remains incompletely defined. In this review, we discuss the evidence behind the basic mechanisms of VF and review the current role of catheter ablation in patients with VF.

Role of Purkinje-Muscle Junction in Early Ventricular Fibrillation in a Porcine Model: Beyond the Trigger Concept

BACKGROUND

The role of the Purkinje network in triggering ventricular fibrillation (VF) has been studied, however, its involvement after onset and in early maintenance of VF is controversial.

AIM

We studied the role of the Purkinje-muscle junctions (PMJ) on epicardial-endocardial activation gradients during early VF.

METHODS

In a healthy, porcine, beating-heart Langendorff model [control, n = 5; ablation, n = 5], simultaneous epicardial-endocardial dominant frequent mapping was used (224 unipolar electrograms) to calculate activation rate gradients during the onset and early phase of VF. Selective Purkinje ablation was performed using Lugol's solution, followed by VF re-induction and mapping and finally, histological evaluation.

RESULTS

Epicardial activation rates were faster than endocardial rates for both onset and early VF. After PMJ ablation, activation rates decreased epicardially and endocardially for both onset and early VF [Epi: 9.7±0.2 to 8.3±0.2 Hz (P<0.0001) and 10.9±0.4 to 8.8±0.3 Hz (P<0.0001), respectively; Endo: 8.2 ± 0.3 Hz to 7.4 ± 0.2 Hz (P<0.0001) and 7.0 ± 0.4 Hz to 6.6 ± 0.3 Hz (P = 0.0002), respectively]. In controls, epicardial-endocardial activation rate gradients during onset and early VF were 1.7±0.3 Hz and 4.5±0.4 Hz (P<0.001), respectively. After endocardial ablation of PMJs, these gradients were reduced to 0.9±0.3 Hz (onset VF, P<0.001) and to 2.2±0.3 Hz (early VF, P<0.001). Endocardial-epicardial Purkinje fibre arborization and selective Purkinje fibre extinction after only endocardial ablation (not with epicardial ablation) was confirmed on histological analysis.

CONCLUSIONS

Beyond the trigger paradigm, PMJs determine activation rate gradients during onset and during early maintenance of VF. This article is protected by copyright. All rights reserved.

Effect of flunarizine on defibrillation outcomes and early refibrillation in a canine model of prolonged ventricular fibrillation

New Findings

What is the central question of this study?

Can successful electrical shock in combination with a delayed after‐depolarization (DAD) blocker suppress early refibrillation episodes following long duration ventricular fibrillation (LDVF)?

What is the main finding and its importance?

Flunarizine significantly reduced the activation of LDVF and early ventricular fibrillation (VF) recurrence following LDVF, suggesting that DADs potentially contribute to refibrillation in prolonged VF. Thus, DAD inhibition can be used as an adjunctive therapy for electrical defibrillation to treat prolonged VF and suppress refibrillation following LDVF.

Abstract

This study attempts to detect changes in the defibrillation threshold (DFT) at different stages of ventricular fibrillation (VF) (short duration VF, SDVF; long duration VF, LDVF) and during early refibrillation following successful defibrillation of LDVF by giving flunarizine, a blocker of delayed after‐depolarizations (DADs). Twelve beagles were divided into two groups (the control group, n = 6; and the flunarizine group, n = 6). Two 64‐electrode basket catheters were deployed into the left and the right ventricles for global endocardium mapping. The DFTs of SDVF and LDVF were determined at 20 s and 7 min, respectively, after VF induction in each group. Any refibrillation episodes were recorded within 15 min after the first successful defibrillation of LDVF. In the flunarizine group, the SDVF‐DFT values before and after the drug were not significantly different. The 7 min LDVF‐DFTs were markedly reduced by 26% (P < 0.05, the control group) and 38% (P < 0.01, the flunarizine group) compared to the 20 s SDVF‐DFTs within each group. The difference between SDVF‐DFT and LDVF‐DFT after flunarizine was larger than that in the control group (213 ± 65 vs. 120 ± 84 V, P < 0.05). The number of refibrillation episodes per animal (1.3 ± 1.0) following successful defibrillation of LDVF after flunarizine was 48% of that in controls (2.7 ± 2.0, P < 0.05). The effect of flunarizine on SDVF‐DFT and LDVF‐DFT indicates that the role of DADs in the defibrillation mechanism may differ as VF continues. Flunarizine significantly reduced early VF recurrence following LDVF, suggesting that DADs potentially contribute to refibrillation in a canine model of prolonged VF.

Spatiotemporal Progression of Early Human Ventricular Fibrillation

Objectives

The objective of this study was to evaluate the spatio-temporal organization and progression of human ventricular fibrillation (VF) in the left (LV) and right (RV) ventricles.

Background

Studies suggest that localized sources contribute to VF maintenance, but the evolution of VF episodes has not been quantified.

Methods

Synchrony between electrograms recorded from 25 patients with induced VF is computed and used to define the Asynchronous Index (ASI), indicating regions which are out-of-step with surrounding tissue. Computer simulations show that ASI can identify the location of VF-maintaining sources, where larger values of ASI_max correlate with more stable sources.

Results

Automated synchrony analysis shows elevated values of ASI in a majority of self-terminating episodes (LV: 8/9, RV: 7/8) and sustained episodes (LV: 11/11, RV: 12/12). The locations of ASI_max in sustained episodes co-localize with rotor cores when rotational activity is simultaneously present in phase maps (LV: 8/8, RV: 5/7, p<.05). The distribution of ASI_max differentiates self-terminating from sustained episodes (mean ASI_max = 0.60±0.14 and 0.70±0.16, respectively; p=0.01). Across sustained episodes the LV exhibits an increase in ASI_max with time.

Conclusions

Quantitative analysis identifies localized asynchronous regions that correlate with sources in VF, with sustained episodes evolving to exhibit more stable activation in the LV. This successive increase in stability indicates a stabilizing agent may be responsible for perpetuating fibrillation in a "migrate-and-capture" mechanism in the LV.

Larger rate dependence of late sodium current in cardiac Purkinje cells: A potential link to arrhythmogenesis

BACKGROUND

Purkinje cells (PCs) have a steeper rate dependence of repolarization and are more susceptible to arrhythmic activity than do ventricular myocytes (VMs). Late sodium current (I_NaL) is rate dependent and contributes to rate dependence of repolarization.

OBJECTIVE

This study sought to test our hypothesis that PCs have a larger rate dependence of I_NaL, contributing to their steeper rate dependence of repolarization and higher susceptibility to arrhythmic activity, than do VMs.

METHODS

I_NaL was recorded in isolated rabbit PCs and VMs with the whole-cell patch clamp technique. Action potential was examined using the microelectrode technique.

RESULTS

Compared with VMs, PCs exhibited a significantly larger rate dependence of I_NaL with a larger I_NaL to basic cycle length (BCL) slope. Moreover, PCs had a larger rate dependence of I_NaL decay and slower recovery kinetics. Interestingly, the larger rate dependence of I_NaL matched to a steeper rate dependence of action potential duration (APD) in PCs. The I_NaL blocker tetrodotoxin significantly blunted, while the I_NaL enhancer anemone toxin (ATX-II) significantly increased, the rate dependence of I_NaL and APD in PCs and VMs. In the presence of ATX-II, the rate dependence of I_NaL in PCs was markedly larger than that in VMs, causing a much steeper rate dependence of APD in PCs. Accordingly, PCs exhibited greater rate-dependent electrical instability and were more prone to ATX-II-induced early afterdepolarizations, which were completely inhibited by the I_NaL inhibitor ranolazine.

CONCLUSION

PCs have a significantly larger rate dependence of I_NaL than do VMs because of distinctive I_NaL decay and recovery kinetics, which contributes to their larger rate adaptation, and simultaneously predisposes them to a higher risk of arrhythmogenesis.

Endocardial focal activation originating from Purkinje fibers plays a role in the maintenance of long duration ventricular fibrillation

Aim

To determine the role of repetitive endocardial focal activations and Purkinje fibers in the maintenance of long duration ventricular fibrillation (LDVF, VF>1 minute) in canine hearts in vivo.

Methods

The study was conducted in electrophysiological laboratory of Shanghai Ruijin hospital from July 2010 to August 2012. A 64-electrode basket was introduced through a carotid artery into the left ventricle (LV) of 11 beagle dogs for global endocardial electrical mapping. In the Lugol’s solution group (n = 5), the subendocardium was ablated by washing with Lugol’s solution. In the control group, (n = 6) saline was used for ablation. Before and after saline or Lugol ablation, we determined QRS duration and QT/QTc interval in sinus rhythm (SR). We also measured the activation rates in the first 2 seconds of each minute during 7 minutes of VF for each group. If VF terminated spontaneously in less than 7 minutes, the VF segments used in activation rate analysis were reduced accordingly.

Results

At the beginning of VF there was no difference between the groups in the activation rate. However, after 1 minute of LDVF the Lugol’s solution group had significantly slower activation rate than the control group. In the control group, all episodes of LDVF (6/6) were successfully sustained for 7 minutes, while in the Lugol’s solution group 4/5 episodes of LDVF spontaneously terminated before 7 minutes (4.8 ± 1.4 minutes) (P = 0.015). In the control group, at 5.1 ± 1.3 minutes of LDVF, a successive, highly organized focal LV endocardial activation pattern was observed. During this period, activations partly arose in PF and spread to the working ventricular myocardium. Mapping analysis showed that these events were consistent with repetitive endocardial focal activations. No evidence of similar focal activations was observed in the Lugol’s solution group.

Conclusions

Repetitive endocardial focal activations in the LV endocardium may be associated with activation of subendocardial PFs. This mechanism may play an important role in the maintenance of LDVF.

Rotor stability separates sustained ventricular fibrillation from self-terminating episodes in humans

OBJECTIVES

This study mapped human ventricular fibrillation (VF) to define mechanistic differences between episodes requiring defibrillation versus those that spontaneously terminate.

BACKGROUND

VF is a leading cause of mortality; yet, episodes may also self-terminate. We hypothesized that the initial maintenance of human VF is dependent upon the formation and stability of VF rotors.

METHODS

We enrolled 26 consecutive patients (age 64 ± 10 years, n = 13 with left ventricular dysfunction) during ablation procedures for ventricular arrhythmias, using 64-electrode basket catheters in both ventricles to map VF prior to prompt defibrillation per the institutional review board-approved protocol. A total of 52 inductions were attempted, and 36 VF episodes were observed. Phase analysis was applied to identify biventricular rotors in the first 10 s or until VF terminated, whichever came first (11.4 ± 2.9 s to defibrillator charging).

RESULTS

Rotors were present in 16 of 19 patients with VF and in all patients with sustained VF. Sustained, but not self-limiting VF, was characterized by greater rotor stability: 1) rotors were present in 68 ± 17% of cycles in sustained VF versus 11 ± 18% of cycles in self-limiting VF (p < 0.001); and 2) maximum continuous rotations were greater in sustained (17 ± 11, range 7 to 48) versus self-limiting VF (1.1 ± 1.4, range 0 to 4, p < 0.001). Additionally, biventricular rotor locations in sustained VF were conserved across multiple inductions (7 of 7 patients, p = 0.025).

CONCLUSIONS

In patients with and without structural heart disease, the formation of stable rotors identifies individuals whose VF requires defibrillation from those in whom VF spontaneously self-terminates. Future work should define the mechanisms that stabilize rotors and evaluate whether rotor modulation may reduce subsequent VF risk.

Placement of implantable cardioverter-defibrillators in paediatric and congenital heart defect patients: a pipeline for model generation and simulation prediction of optimal configurations

There is currently no reliable way of predicting the optimal implantable cardioverter-defibrillator (ICD) placement in paediatric and congenital heart defect (CHD) patients. This study aimed to: (1) develop a new image processing pipeline for constructing patient-specific heart-torso models from clinical magnetic resonance images (MRIs); (2) use the pipeline to determine the optimal ICD configuration in a paediatric tricuspid valve atresia patient; (3) establish whether the widely used criterion of shock-induced extracellular potential (Φe) gradients ≥5 V cm(-1) in ≥95% of ventricular volume predicts defibrillation success. A biophysically detailed heart-torso model was generated from patient MRIs. Because transvenous access was impossible, three subcutaneous and three epicardial lead placement sites were identified along with five ICD scan locations. Ventricular fibrillation was induced, and defibrillation shocks were applied from 11 ICD configurations to determine defibrillation thresholds (DFTs). Two configurations with epicardial leads resulted in the lowest DFTs overall and were thus considered optimal. Three configurations shared the lowest DFT among subcutaneous lead ICDs. The Φe gradient criterion was an inadequate predictor of defibrillation success, as defibrillation failed in numerous instances even when 100% of the myocardium experienced such gradients. In conclusion, we have developed a new image processing pipeline and applied it to a CHD patient to construct the first active heart-torso model from clinical MRIs.

Cardiac optogenetics

Optogenetics is an emerging technology for optical interrogation and control of biological function with high specificity and high spatiotemporal resolution. Mammalian cells and tissues can be sensitized to respond to light by a relatively simple and well-tolerated genetic modification using microbial opsins (light-gated ion channels and pumps). These can achieve fast and specific excitatory or inhibitory response, offering distinct advantages over traditional pharmacological or electrical means of perturbation. Since the first demonstrations of utility in mammalian cells (neurons) in 2005, optogenetics has spurred immense research activity and has inspired numerous applications for dissection of neural circuitry and understanding of brain function in health and disease, applications ranging from in vitro to work in behaving animals. Only recently (since 2010), the field has extended to cardiac applications with less than a dozen publications to date. In consideration of the early phase of work on cardiac optogenetics and the impact of the technique in understanding another excitable tissue, the brain, this review is largely a perspective of possibilities in the heart. It covers the basic principles of operation of light-sensitive ion channels and pumps, the available tools and ongoing efforts in optimizing them, overview of neuroscience use, as well as cardiac-specific questions of implementation and ideas for best use of this emerging technology in the heart.

The effects of acute amiodarone on short- and long-duration ventricular defibrillation threshold in canines

INTRODUCTION

Some studies have shown that the defibrillation threshold (DFT) differs between short-duration ventricular fibrillation (SDVF, <1 minute) and long-duration ventricular fibrillation (LDVF > 1 minute). The goal of this study is to evaluate the effects of acute intravenous amiodarone on SDVF-DFT and LDVF-DFT and its possible mechanism as well.

METHODS

Twelve open-chest dogs were randomly divided into 2 groups (control group, normal saline, 10 mL·kg·h maintenance, n = 6; amiodarone group, loading dose 10 mg/kg over 10 minutes, maintenance dose 5 mg·kg·h, n = 6). VF was electrically induced and recorded with a 12 × 12 unipolar electrode plaque (2-mm spacing) sutured on the left ventricular epicardium and a plunge needle (6 unipolar electrode) inserted in the left ventricular apex. The DFTs of SDVF and LDVF were determined 20 seconds and 3 minutes after VF induction, respectively. Restitution was estimated from activation recovery intervals during pacing from the plaque and plunge needle electrodes. The activation rate was estimated by Fast Fourier Transform analysis of VF at same electrodes. The VF cycle length was defined as the reciprocal of the activation rate. The epicardial and transmural dispersion of the maximal slope of the restitution curve and VF cycle length of SDVF and LDVF were calculated, respectively.

RESULTS

: In controls, LDVF-DFT was higher than SDVF-DFT (645 ± 61 vs. 520 ± 63 V, P < 0.01). Amiodarone did not significantly alter SDVF-DFTs (496 ± 49 vs. 552 ± 69 V, P > 0.05) but decreased LDVF-DFT by 31% (P < 0.01). Compared with control, amiodarone significantly reduced the maximum slope of the restitution curve (1.12 ± 0.35 vs. 0.81 ± 0.25, P = 0.03) and its epicardial dispersion (0.32 ± 0.07 vs. 0.24 ± 0.04,coefficient of variation, P = 0.017). Amiodarone significantly increased the SDVF-CL (92 ± 8 vs. 99 ± 10 milliseconds, P < 0.01) and epicardial dispersion 0.14 ± 0.06 vs. 0.18 ± 0.05, P < 0.01. Amiodarone did not change the LDVF-CL (228 ± 12 vs. 226 ± 10 milliseconds, P > 0.05) or epicardial dispersion (0.17 ± 0.03 vs. 0.15 ± 0.02, P > 0.05) compared with control. However, the drug significantly decreased the transmural dispersion of LDVF-CL (68 ± 28 vs. 39 ± 14 milliseconds, P < 0.01) without changing the transmural dispersion of SDVF-CL (29 ± 22 vs. 32 ± 30 milliseconds, P > 0.05).

CONCLUSIONS

Acute amiodarone significantly decreased the LDVF-DFT. The decreased transmural dispersion of LDVF-CL by amiodarone might contribute to the decreased LDVF-DFT.

Three-dimensional mechanisms of increased vulnerability to electric shocks in myocardial infarction: altered virtual electrode polarizations and conduction delay in the peri-infarct zone

Defibrillation efficacy is decreased in infarcted hearts, but the mechanisms by which infarcted hearts are more vulnerable to electric shocks than healthy hearts remain poorly understood. The goal of this study was to provide insight into the 3D mechanisms for the increased vulnerability to electric shocks in infarcted hearts. We hypothesized that changes in virtual electrode polarizations (VEPs) and propagation delay through the peri-infarct zone (PZ) were responsible. We developed a micro anatomically detailed rabbit ventricular model with chronic myocardial infarction from magnetic resonance imaging and enriched the model with data from optical mapping experiments. We further developed a control model without the infarct. The simulation protocol involved apical pacing followed by biphasic shocks. Simulation results from both models were compared.The upper limit of vulnerability(ULV) was 8 V cm(-1) in the infarction model and 4 V cm(-1) in the control model. VEPs were less pronounced in the infarction model, providing a larger excitable area for postshock propagation but smaller transmembrane potential gradients to initiate new wavefronts. Initial post-shock transmural activation occurred at a later time in the infarction model, and the PZ served to delay propagation in subsequent beats. The presence of the PZ was found to be responsible for the increased vulnerability.

Mechanisms of defibrillation

Electrical shock has been the one effective treatment for ventricular fibrillation for several decades. With the advancement of electrical and optical mapping techniques, histology, and computer modeling, the mechanisms responsible for defibrillation are now coming to light. In this review, we discuss recent work that demonstrates the various mechanisms responsible for defibrillation. On the cellular level, membrane depolarization and electroporation affect defibrillation outcome. Cell bundles and collagenous septae are secondary sources and cause virtual electrodes at sites far from shocking electrodes. On the whole-heart level, shock field gradient and critical points determine whether a shock is successful or whether reentry causes initiation and continuation of fibrillation.