Long-duration ventricular fibrillation exhibits 2 distinct organized states

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.

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.

Effects of combination of sotalol and verapamil on initiation, maintenance, and termination of ventricular fibrillation in swine hearts

INTRODUCTION

Sotalol and verapamil alone reduce reentry incidence during ventricular fibrillation (VF). We tested whether the combination of these two drugs had a synergistic effect on initiation, maintenance, and termination of VF.

METHODS

Six open-chest pigs received intravenous sotalol (1.5 mg/kg) followed by verapamil (0.136 mg/kg). VF threshold (VFT) was determined by a burst pacing protocol. Two 20 seconds episodes of VF were recorded from a 21 × 24 unipolar electrode plaque on the lateral posterior left ventricular epicardium before and after each drug. VF activation patterns were quantified. The duration of long duration VF (LDVF) maintenance was compared to our previously published data.

RESULTS

Sotalol alone and combined with verapamil significantly increased the VFT from 12.3 ± 4.1 to 20.3 ± 7.1 and 26.7 ± 8.6 mA compared with baseline (P < .05). Sotalol decreased the number of wavefronts by 20%, VF activation rate by 17% and conduction velocity 11%, while the addition of verapamil neutralized these effects. Addition of verapamil to sotalol further decreased the fractionation incidence from 14% to 29% and multiplicity from 24% to 31% compared with baseline. The combination of the two drugs increased the VF cycle length, decreased synchronicity, increased regularity index and shortened the duration of LDVF maintenance compared with our previous data of verapamil alone or no drug. Synchronicity index was lower and regularity index was higher in animals in which VF spontaneously terminated earlier than 10 minutes than in animals in which VF terminated longer than 10 minutes.

CONCLUSION

The combination of sotalol and verapamil increased VFT but accelerated LDVF termination.

Endocardial Activation Drives Activation Patterns During Long-Duration Ventricular Fibrillation and Defibrillation

BACKGROUND

Understanding the mechanisms that drive ventricular fibrillation is essential for developing improved defibrillation techniques to terminate ventricular fibrillation (VF). Distinct organization patterns of chaotic, regular, and synchronized activity were previously demonstrated in VF that persisted over 1 to 2 minutes (long-duration VF [LDVF]). We hypothesized that activity on the endocardium may be driving these activation patterns in LDVF and that unsuccessful defibrillation shocks may alter activation patterns.

METHODS AND RESULTS

The study was performed using a 64-electrode basket catheter on the left ventricle endocardium and 54 6-electrode plunge needles inserted into the left ventricles of 6 dogs. VF was induced electrically, and after short-duration VF (10 seconds) and LDVF (7 minutes), shocks of increasing strengths were delivered every 10 seconds until VF was terminated. Endocardial activation patterns were classified as chaotic (varying cycle lengths and nonsynchronous activations), regular (highly repeatable cycle lengths), and synchronized (activation that spreads rapidly over the endocardium with diastolic periods between activations).

CONCLUSIONS

The results showed that the chaotic pattern was predominant in early VF, but the regular pattern emerges as VF progressed. The synchronized pattern only emerged occasionally during late VF. Failed defibrillation shocks changed chaotic and regular activation patterns to synchronized patterns in LDVF but not in short-duration VF. The regular and synchronized patterns of activation were driven by rapid activations on the endocardial surface that blocked and broke up transmurally, leading to an endocardial to epicardial activation rate gradient as LDVF progressed.

Restitution characteristics of His bundle and working myocardium in isolated rabbit hearts

The Purkinje system (PS) and the His bundle have been recently implicated as an important driver of the rapid activation rate after 1-2 minutes of ventricular fibrillation (VF). It is unknown whether activations during VF propagate through the His-Purkinje system to other portions of the the working myocardium (WM). Little is known about restitution characteristic differences between the His bundle and working myocardium at short cycle lengths. In this study, rabbit hearts (n = 9) were isolated, Langendorff-perfused, and electromechanically uncoupled with blebbistatin (10 μM). Pacing pulses were delivered directly to the His bundle. By using standard glass microelectrodes, action potentials duration (APD) from the His bundle and WM were obtained simultaneously over a wide range of stimulation cycle lengths (CL). The global F-test indicated that the two restitution curves of the His bundle and the WM are statistically significantly different (P<0.05). Also, the APD of the His bundle was significantly shorter than that of WM throughout the whole pacing course (P<0.001). The CL at which alternans developed in the His bundle vs. the WM were shorter for the His bundle (134.2±13.1ms vs. 148.3±13.3ms, P<0.01) and 2:1 block developed at a shorter CL in the His bundle than in WM (130.0±10.0 vs. 145.6±14.2ms, P<0.01). The His bundle APD was significantly shorter than that of WM under both slow and rapid pacing rates, which suggest that there may be an excitable gap during VF and that the His bundle may conduct wavefronts from one bundle branch to the other at short cycle lengths and during VF.

Conversion to Purkinje-Related Monomorphic Ventricular Tachycardia After Ablation of Ventricular Fibrillation in Ischemic Heart Disease

Background—

Catheter ablation is an effective therapy for ventricular fibrillation (VF) arising from the Purkinje system in ischemic heart disease. However, some patients experience newly emergent monomorphic ventricular tachycardia (VT) after the ablation of VF. We evaluated the prevalence and mechanism of monomorphic VT after VF ablation.

Methods and Results—

Twenty-one consecutive patients with primary VF because of ischemic heart disease who underwent catheter ablation were retrospectively analyzed. Twenty of 21 patients were in electrical storm. Ventricular premature contractions triggering VF arose from the left Purkinje system and were targeted for ablation. Before the ablation, 14 of 21 patients had only VF, and the other 7 had VF and concomitant monomorphic VT. Four of the 14 patients with only VF (29%) exhibited newly emergent monomorphic VT after VF ablation. Three of these patients had Purkinje-related VTs, which were successfully eliminated by the ablation of a Purkinje network located in the same low-voltage area as the site of prior successful VF ablation. During a median follow-up of 28 months (interquartile range, 16–68 months), VF recurred in 6 of 21 patients (29%); however, there were neither electrical storms nor monomorphic VT, and all recurring arrhythmias were controlled by medical therapy alone.

Conclusions—

Over one fifth of patients with primary ischemic VF experienced newly emergent Purkinje-related monomorphic VT after VF ablation. The circuit of the monomorphic VT associated with the Purkinje network was located in the same low-voltage area as the Purkinje tissue that triggered VF and could be suppressed by additional ablation.