Rotors during AF: drivers or bystanders?

Computational Analysis of Mapping Catheter Geometry and Contact Quality Effects on Rotor Detection in Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia and catheter mapping has been proved to be an effective approach for detecting AF drivers to be targeted by ablation. Among drivers, the so-called rotors have gained the most attention: their identification and spatial location could help to understand which patient-specific mechanisms are acting, and thus to guide the ablation execution. Since rotor detection by multi-electrode catheters may be influenced by several structural parameters including inter-electrode spacing, catheter coverage, and endocardium-catheter distance, in this study we proposed a tool for testing the ability of different catheter shapes to detect rotors in different conditions. An approach based on the solution of the monodomain equations coupled with a modified Courtemanche ionic atrial model, that considers an electrical remodeling, was applied to simulate spiral wave dynamics on a 2D model for 7.75 s. The developed framework allowed the acquisition of unipolar signals at 2 KHz. Two high-density multipolar catheters were simulated (Advisor™ HD Grid and PentaRay^®) and placed in a 2D region in which the simulated spiral wave persists longer. The configuration of the catheters was then modified by changing the number of electrodes, inter-electrodes distance, position, and atrial-wall distance for assessing how they would affect the rotor detection. In contact with the wall and at 1 mm distance from it, all the configurations detected the rotor correctly, irrespective of geometry, coverage, and inter-electrode distance. In the HDGrid-like geometry, the increase of the inter-electrode distance from 3 to 6 mm caused rotor detection failure at 2 mm distance from the LA wall. In the PentaRay-like configuration, regardless of inter-electrode distance, rotor detection failed at 3 mm endocardium-catheter distance. The asymmetry of this catheter resulted in rotation-dependent rotor detection. To conclude, the computational framework we developed is based on realistic catheter shapes designed with parameter configurations which resemble clinical settings. Results showed it is well suited to investigate how mapping catheter geometry and location affect AF driver detection, therefore it is a reliable tool to design and test new mapping catheters.

Real-Time Rotational Activity Detection in Atrial Fibrillation

Rotational activations, or spiral waves, are one of the proposed mechanisms for atrial fibrillation (AF) maintenance. We present a system for assessing the presence of rotational activity from intracardiac electrograms (EGMs). Our system is able to operate in real-time with multi-electrode catheters of different topologies in contact with the atrial wall, and it is based on new local activation time (LAT) estimation and rotational activity detection methods. The EGM LAT estimation method is based on the identification of the highest sustained negative slope of unipolar signals. The method is implemented as a linear filter whose output is interpolated on a regular grid to match any catheter topology. Its operation is illustrated on selected signals and compared to the classical Hilbert-Transform-based phase analysis. After the estimation of the LAT on the regular grid, the detection of rotational activity in the atrium is done by a novel method based on the optical flow of the wavefront dynamics, and a rotation pattern match. The methods have been validated using in silico and real AF signals.

Spatial Resolution Requirements for Accurate Identification of Drivers of Atrial Fibrillation

Supplemental Digital Content is available in the text.

Background—

Recent studies have demonstrated conflicting mechanisms underlying atrial fibrillation (AF), with the spatial resolution of data often cited as a potential reason for the disagreement. The purpose of this study was to investigate whether the variation in spatial resolution of mapping may lead to misinterpretation of the underlying mechanism in persistent AF.

Methods and Results—

Simulations of rotors and focal sources were performed to estimate the minimum number of recording points required to correctly identify the underlying AF mechanism. The effects of different data types (action potentials and unipolar or bipolar electrograms) and rotor stability on resolution requirements were investigated. We also determined the ability of clinically used endocardial catheters to identify AF mechanisms using clinically recorded and simulated data. The spatial resolution required for correct identification of rotors and focal sources is a linear function of spatial wavelength (the distance between wavefronts) of the arrhythmia. Rotor localization errors are larger for electrogram data than for action potential data. Stationary rotors are more reliably identified compared with meandering trajectories, for any given spatial resolution. All clinical high-resolution multipolar catheters are of sufficient resolution to accurately detect and track rotors when placed over the rotor core although the low-resolution basket catheter is prone to false detections and may incorrectly identify rotors that are not present.

Conclusions—

The spatial resolution of AF data can significantly affect the interpretation of the underlying AF mechanism. Therefore, the interpretation of human AF data must be taken in the context of the spatial resolution of the recordings.

Atrial fibrillation driver mechanisms: Insight from the isolated human heart

Although there have been great technological advances in the treatment of atrial fibrillation (AF), current therapies remain limited due to a narrow understanding of AF mechanisms in the human heart. This review will highlight our recent studies on explanted human hearts where we developed and employed a novel functional-structural mapping approach by integrating high-resolution simultaneous endo-epicardial and panoramic optical mapping with 3D gadolinium-enhanced MRI to define the spatiotemporal characteristics of AF drivers and their structural substrates. The results allow us to postulate that the primary mechanism of AF maintenance in human hearts is a limited number of localized intramural microanatomic reentrant AF drivers anchored to heart-specific 3D fibrotically insulated myobundle tracks, which may remain hidden to clinical single-surface electrode mapping. We suggest that ex vivo human heart studies, by using an integrated 3D functional and structural mapping approach, will help to reveal defining features of AF drivers as well as validate and improve clinical approaches to detect and target these AF drivers in patients with cardiac diseases.

Mapping Atrial Fibrillation: 2015 Update

Atrial fibrillation requires a trigger that initiates the arrhythmia and substrate that favors perpetuation. Cardiac mapping is necessary to locate triggers and substrate so that an ablation strategy can be optimized. The most commonly used cardiac mapping approach is isochronal or activation mapping, which aims to create a spatial model of electrical wavefront propagation. Historically, activation mapping has been successful for mapping point source and single or double wave reentrant arrhythmias, while mapping multiple wavelets or driving sources that underlie most episodes of atrial fibrillation remains challenging. In the multiple wavelet model of AF there is no particular area critical to sustain atrial fibrillation, and a "critical mass" of atrium is required to maintain AF. Recent studies suggest endocardial and epicardial dissociation may play an important role. Investigation of driving sources that sustain AF has focused on the presence of rotors. Rotors in human AF have now been observed using multiple imaging modalities, however ablation strategies targeting rotors remain of unproven benefit. In addition, substrate mapping of AF is now feasible. Increasing degrees of atrial fibrosis on delayed enhancement magnetic resonance imaging (DE-MRI) has been shown to correlate with poor procedural outcomes for AF ablation, which suggests the increased burden of scar promotes more complex and extensive arrhythmia substrate. Atrial fibrosis is also identifiable using electrogram voltage tagging in an electro-anatomic mapping system. Patient-specific ablation strategies targeting areas of fibrosis are currently under investigation. Recent technological advances have facilitated greater understanding of the potential role for AF mapping and has allowed initiation of clinical studies to evaluate the effectiveness of mapping-based intervention. Multi-modality mapping is likely to play an increasingly important role in AF ablation, but is currently limited by the inability to simultaneously record and interpret electrical signals from both atria and from both the epicardium and endocardium.

Rotor mapping and ablation to treat atrial fibrillation

PURPOSE OF REVIEW

Rotors have long been postulated to drive atrial fibrillation, but evidence has been limited to animal models. This changed recently with the demonstration using focal impulse and rotor modulation (FIRM) mapping that rotors act as human atrial fibrillation sources. This mechanistic approach to diagnosing the causes of atrial fibrillation in individual patients has been supported by substantially improved outcomes from FIRM-guided ablation, resulting in increased attention to rotors as therapeutic targets.

RECENT FINDINGS

In this review, we outline the pathophysiology of rotors in animal and in-silico studies of fibrillation, and how this motivated FIRM mapping in humans. We highlight the characteristics of rotors in human atrial fibrillation, now validated by several techniques, with discussion on similar and discrepant findings between techniques. The interventional approaches to eliminate atrial fibrillation rotors are explained and the ablation results in latest studies using FIRM are discussed.

SUMMARY

We propose that mapping localized sources for human atrial fibrillation, specifically rotors, is moving the field towards a unifying hypothesis that explains several otherwise contradictory observations in atrial fibrillation management. We conclude by suggesting areas of potential research that may reveal more about these critical sites and how these may lead to better and novel treatments for atrial fibrillation.

Atrial fibrillation driven by micro-anatomic intramural re-entry revealed by simultaneous sub-epicardial and sub-endocardial optical mapping in explanted human hearts

AIMS

The complex architecture of the human atria may create physical substrates for sustained re-entry to drive atrial fibrillation (AF). The existence of sustained, anatomically defined AF drivers in humans has been challenged partly due to the lack of simultaneous endocardial-epicardial (Endo-Epi) mapping coupled with high-resolution 3D structural imaging.

METHODS AND RESULTS

Coronary-perfused human right atria from explanted diseased hearts (n = 8, 43-72 years old) were optically mapped simultaneously by three high-resolution CMOS cameras (two aligned Endo-Epi views (330 µm2 resolution) and one panoramic view). 3D gadolinium-enhanced magnetic resonance imaging (GE-MRI, 80 µm3 resolution) revealed the atrial wall structure varied in thickness (1.0 ± 0.7-6.8 ± 2.4 mm), transmural fiber angle differences, and interstitial fibrosis causing transmural activation delay from 23 ± 11 to 43 ± 22 ms at increased pacing rates. Sustained AF (>90 min) was induced by burst pacing during pinacidil (30-100 µM) perfusion. Dual-sided sub-Endo-sub-Epi optical mapping revealed that AF was driven by spatially and temporally stable intramural re-entry with 107 ± 50 ms cycle length and transmural activation delay of 67 ± 31 ms. Intramural re-entrant drivers were captured primarily by sub-Endo mapping, while sub-Epi mapping visualized re-entry or 'breakthrough' patterns. Re-entrant drivers were anchored on 3D micro-anatomic tracks (15.4 ± 2.2 × 6.0 ± 2.3 mm2, 2.9 ± 0.9 mm depth) formed by atrial musculature characterized by increased transmural fiber angle differences and interstitial fibrosis. Targeted radiofrequency ablation of the tracks verified these re-entries as drivers of AF.

CONCLUSIONS

Integrated 3D structural-functional mapping of diseased human right atria ex vivo revealed that the complex atrial microstructure caused significant differences between Endo vs. Epi activation during pacing and sustained AF driven by intramural re-entry anchored to fibrosis-insulated atrial bundles.