Relating spatial heterogeneities to rotor formation in studying human ventricular fibrillation

Computer based method for identification of fibrotic scars from electrograms and local activation times on the epi- and endocardial surfaces of the ventricles

Cardiac fibrosis stands as one of the most critical conditions leading to lethal cardiac arrhythmias. Identifying the precise location of cardiac fibrosis is crucial for planning clinical interventions in patients with various forms of ventricular and atrial arrhythmias. As fibrosis impedes and alters the path of electrical waves, detecting fibrosis in the heart can be achieved through analyzing electrical signals recorded from its surface. In current clinical practices, it has become feasible to record electrical activity from both the endocardial and epicardial surfaces of the heart. This paper presents a computational method for reconstructing 3D fibrosis using unipolar electrograms obtained from both surfaces of the ventricles. The proposed method calculates the percentage of fibrosis in various ventricular segments by analyzing the local activation times and peak-to-peak amplitudes of the electrograms. Initially, the method was tested using simulated data representing idealized fibrosis in a heart segment; subsequently, it was validated in the left ventricle with fibrosis obtained from a patient with nonischemic cardiomyopathy. The method successfully determined the location and extent of fibrosis in 204 segments of the left ventricle model with an average error of 0.0±4.3% (N = 204). Moreover, the method effectively detected fibrotic scars in the mid-myocardial region, a region known to present challenges in accurate detection using electrogram amplitude as the primary criterion.

Regional Diversities in Fibrogenesis Weighed as a Key Determinant for Atrial Arrhythmogenesis

Atrial fibrosis plays a key role in atrial myopathy, resulting in the genesis of atrial fibrillation (AF). The abnormal distribution of fibrotic tissue, electrical coupling, paracrine interactions, and biomechanical–electrical interactions have all been suggested as causes of fibrosis-related arrhythmogenesis. Moreover, the regional difference in fibrogenesis, specifically the left atrium (LA) exhibiting a higher arrhythmogenesis and level of fibrosis than the right atrium (RA) in AF, is a key contributor to atrial arrhythmogenesis. LA fibroblasts have greater profibrotic cellular activities than RA fibroblasts, but knowledge about the regional diversity of atrial regional fibrogenesis remains limited. This article provides a comprehensive review of research findings on the association between fibrogenesis and arrhythmogenesis from laboratory to clinical evidence and updates the current understanding of the potential mechanism underlying the difference in fibrogenesis between the LA and RA.

Left atrial voltage mapping: defining and targeting the atrial fibrillation substrate

Low atrial endocardial bipolar voltage, measured during catheter ablation for atrial fibrillation (AF), is a commonly used surrogate marker for the presence of atrial fibrosis. Low voltage shows many useful associations with clinical outcomes, comorbidities and has links to trigger sites for AF. Several contemporary trials have shown promise in targeting low voltage areas as the substrate for AF ablation; however, the results have been mixed. In order to understand these results, a thorough understanding of voltage mapping techniques, the relationship between low voltage and the pathophysiology of AF, as well as the inherent limitations in voltage measurement are needed. Two key questions must be answered in order to optimally apply voltage mapping as the road map for ablation. First, are the inherent limitations of voltage mapping small enough as to be ignored when targeting specific tissue based on voltage? Second, can conventional criteria, using a binary threshold for voltage amplitude, truly define the extent of the atrial fibrotic substrate? Here, we review the latest clinical evidence with regard to voltage-based ablation procedures before analysing the utility and limitations of voltage mapping. Finally, we discuss omnipole mapping and dynamic voltage attenuation as two possible approaches to resolving these issues.

Dynamical anchoring of distant arrhythmia sources by fibrotic regions via restructuring of the activation pattern

Rotors are functional reentry sources identified in clinically relevant cardiac arrhythmias, such as ventricular and atrial fibrillation. Ablation targeting rotor sites has resulted in arrhythmia termination. Recent clinical, experimental and modelling studies demonstrate that rotors are often anchored around fibrotic scars or regions with increased fibrosis. However, the mechanisms leading to abundance of rotors at these locations are not clear. The current study explores the hypothesis whether fibrotic scars just serve as anchoring sites for the rotors or whether there are other active processes which drive the rotors to these fibrotic regions. Rotors were induced at different distances from fibrotic scars of various sizes and degree of fibrosis. Simulations were performed in a 2D model of human ventricular tissue and in a patient-specific model of the left ventricle of a patient with remote myocardial infarction. In both the 2D and the patient-specific model we found that without fibrotic scars, the rotors were stable at the site of their initiation. However, in the presence of a scar, rotors were eventually dynamically anchored from large distances by the fibrotic scar via a process of dynamical reorganization of the excitation pattern. This process coalesces with a change from polymorphic to monomorphic ventricular tachycardia.

Rotors are waves of cardiac excitation like a tornado causing cardiac arrhythmia. Recent research shows that they are found in ventricular and atrial fibrillation. Burning (via ablation) the site of a rotor can result in the termination of the arrhythmia. Recent studies showed that rotors are often anchored to regions surrounding scar tissue, where part of the tissue still survived called fibrotic tissue. However, it is unclear why these rotors anchor to these locations. Therefore, in this work, we investigated why rotors are so abundant in fibrotic tissue with the help of computer simulations. We performed simulations in a 2D model of human ventricular tissue and in a patient-specific model of a patient with an infarction. We found that even when rotors are initially at large distances from the fibrotic region, they are attracted by this region, to finally end up at the fibrotic tissue. We called this process dynamical anchoring and explained how the process works.

Mapping and Ablation of Idiopathic Ventricular Fibrillation

Idiopathic ventricular fibrillation (IVF) is the main cause of unexplained sudden cardiac death, particularly in young patients under the age of 35. IVF is a diagnosis of exclusion in patients who have survived a VF episode without any identifiable structural or metabolic causes despite extensive diagnostic testing. Genetic testing allows identification of a likely causative mutation in up to 27% of unexplained sudden deaths in children and young adults. In the majority of cases, VF is triggered by PVCs that originate from the Purkinje network. Ablation of VF triggers in this setting is associated with high rates of acute success and long-term freedom from VF recurrence. Recent studies demonstrate that a significant subset of IVF defined by negative comprehensive investigations, demonstrate in fact subclinical structural alterations. These localized myocardial alterations are identified by high density electrogram mapping, are of small size and are mainly located in the epicardium. As reentrant VF drivers are often colocated with regions of abnormal electrograms, this localized substrate can be shown to be mechanistically linked with VF. Such areas may represent an important target for ablation.

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.

STRUCTURAL AND FUNCTIONAL BASES OF CARDIAC FIBRILLATION. DIFFERENCES AND SIMILARITIES BETWEEN ATRIA AND VENTRICLES

Evidence accumulated over the last 25 years suggests that, whether in the atria or ventricles, fibrillation may be explained by the self-organization of the cardiac electrical activity into rapidly spinning rotors giving way to spiral waves that break intermittently and result in fibrillatory conduction. The dynamics and frequency of such rotors depend on the ion channel composition, excitability and refractory properties of the tissues involved, as well as on the thickness and respective three-dimensional fiber structure of the atrial and ventricular chambers. Therefore, improving the understanding of fibrillation has required the use of multidisciplinary research approaches, including optical mapping, patch clamping and molecular biology, and the application of concepts derived from the theory of wave propagation in excitable media. Moreover, translation of such concepts to the clinic has recently opened new opportunities to apply novel mechanistic approaches to therapy, particularly during atrial fibrillation ablation. Here we review the current understanding of the manner in which the underlying myocardial structure and function influence rotor initiation and maintenance during cardiac fibrillation. We also examine relevant underlying differences and similarities between atrial fibrillation and ventricular fibrillation and evaluate the latest clinical mapping technologies used to identify rotors in either arrhythmia. Altogether, the data being discussed have significantly improved our understanding of the cellular and structural bases of cardiac fibrillation and pointed toward potentially exciting new avenues for more efficient and effective identification and therapy of the most complex cardiac arrhythmias.

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.