Ismail Hamam, MD, and John D. Hummel, MD
A 61-year-old woman was brought to the electrophysiology (EP) laboratory for catheter-based atrial fibrillation (AF) ablation. She was diagnosed with AF 3 years ago and has history of stroke and hypertension. She failed AF ablation 6 months ago, and despite treatment with drondedarone and sotalol, she continues to have recurrent symptomatic episodes of AF. An implantable loop recorder was placed for AF monitoring.
After discontinuation of the sotalol for 3 days and exclusion of left atrial (LA) thrombus by tranesophageal echocardiography, the patient was brought to the EP lab, and 3 venous sheaths were introduced into the right femoral vein and 2 venous sheaths into the left femoral vein. The patient presented in normal sinus rhythm, and AF was induced by isoproterenol infusion and rapid atrial pacing.
Initially, the circular mapping catheter was advanced to the right atrium (RA) and the SVC, and an electroanatomic RA shell was created using a 3-D mapping system. Under intracardiac echo guidance, the first transseptal puncture was performed with advancement of the circular mapping catheter into the LA. The circular mapping catheter was used to create an electroanatomic LA shell including the pulmonary veins using a 3-D electroanatomic mapping system. Through an 8.5-F steerable sheath, a 64-pole basket catheter was carefully advanced to the RA body and then to the LA body, and the position of each basket spline was correlated to the electroanatomic shell (Figures 39-1 to 39-3).
FIGURE 39-1 The 64-pole Constellation basket mapping catheter is shown. It consists of eight splies, and each spline has eight electrodes.
FIGURE 39-2 The 64-pole Constellation basket mapping catheter deployed in the right atrium. An implantable loop recorder, 20-pole steerable catheter, and circular mapping catheter are also seen in the fluoroscopic image.
FIGURE 39-3 (A) An LAO image of a 3-dimensional shell of the right atrium derived from an intracardiac roving mapping catheter with the 64-pole basket catheter overlaid. The basket catheter clearly lies outside the shell of the atrium revealing a lack of complete reconstruction of the atrium with the roving catheter. (B) An RAO image of a 3-dimensional shell of the right atrium derived from an intracardiac roving mapping catheter with the 64-pole basket catheter overlaid.
In each atria, after basket insertion and careful positioning, meticulous computer-based AF signal analysis was performed using specialized software (Topera, San Diego, CA, United States). After localizing these rotors in relation to the basket electrodes, radiofrequency (RF) ablation of these sites was applied in both atria (Figures 39-4 to 39-7). After matching the signal location in relation to the basket electrodes and confirming the signals with repeat recordings, several RF applications were applied to the site, which resulted in slowing and organization of the AF (see Figure 39-6). In the LA there was another rotor signal recorded on the posterior wall. Applying several RF applications resulted in acute termination of the AF to normal sinus rhythm (see Figure 39-7). The rotor ablation was complemented by conventional pulmonary vein isolation. On follow-up after 3 months, the patient was free of symptoms, and loop recorder interrogation showed no evidence of AF.
FIGURE 39-4 (A) The image is a still frame of a movie depicting the rotation of the rotor around the central core depicted by the red spot. This movie was created from the electrograms recorded from the basket electrode during AF. The signals are then processed to identify a rotor. (B) The right image is a map of the wavefront of activation of the rotor around its core.
FIGURE 39-5 This is an electroanatomic map displaying the 64-pole basket catheter deployed in the left atrium. The green shell represents the left atrium anatomy. The sites of delivery of RF current are marked by the red dots and are delivered at the region of rotor activation. The ablation catheter tip is highlighted in green.
FIGURE 39-6 More rapid atrial fibrillation cycle length (on left side of electrogram tracing) slows (longer cycle length and more organized AF tracing on right side of electrogram tracing) during RF ablation of a left atrial rotor.
FIGURE 39-7 Termination of atrial fibrillation during application of radiofrequency energy at a rotor site identified by processing of AF electrograms recorded from the 64-basket electrode.
EPIDEMIOLOGY AND PATHOPHYSIOLOGY
AF is the most common human arrhythmia and is characterized by chaotic contraction of the atrium. Mechanisms of AF can be divided into the mechanisms of initiation and mechanisms of perpetuation of arrhythmia. There are three widely held theories on the mechanisms of AF perpetuation: multiple random propagating wavelets, focal electrical discharges, and localized reentrant activity with fibrillatory conduction.
Multiple Wavelet Hypothesis
In 1959, Moe and Abildskov proposed the multiple wavelet hypothesis.1,2 This theory was experimentally confirmed in 1985 by the work of Allessie and colleagues.3 According to this hypothesis, AF results from the presence of multiple independent wavelets occurring simultaneously and propagating randomly throughout the LA and RA. This model suggests that the number of wavelets at any point in time depends on the atrial conduction velocity, refractory period, and excitable mass. The perpetuation of AF appears to require at least four to six coexisting wavelets and is favored by slowed conduction, shortened refractory periods, and increased atrial mass. One of the practical implications of this hypothesis was the fact that chronic AF could be cured in some patients by the placement of multiple surgical lesions (maze procedure) to compartmentalize the atria into regions presumably unable to sustain multiple wavelet reentry.5
Despite the fact that this theory was widely accepted, this hypothesis failed to answer many questions, such as why AF exhibits consistent spatial nonuniformities in rate and activation vectors, how ablation may terminate AF relatively early, in some cases before compartmentalization of meandering wavelets, or why extensive ablation often has little acute impact.6-8
In clinical practice, the concept of focal triggers as a cause of AF was verified to the greatest extent by Haissaguerre and colleagues who found that ectopic beats from the pulmonary veins initiated AF and that a localized catheter ablation procedure to eliminate these triggers could cure AF in some patients.9-11 This important finding made the pulmonary veins and the posterior wall of the LA the primary targets to cure AF.
Based on animal models, some authors have proposed that in the presence of an appropriately heterogeneous AF substrate a focal trigger can result in sustained high-frequency reentrant AF drivers (rotors).12-15 The waves that emerge from the rotors undergo spatially distributed fragmentation and give rise to fibrillatory conduction. In the early 1990s, Schuessler and colleagues demonstrated in an isolated canine RA that with increasing concentrations of acetylcholine, activation patterns characterized by multiple reentrant circuits converted to a single, relatively stable, high-frequency reentrant circuit that resulted in fibrillatory conduction.15
In humans, multiple studies have used spectral analysis and dominant frequency mapping as a surrogate for localized AF sources, with ablation at these sites resulting in prolongation of the AF cycle length and termination of paroxysmal AF, indicating their role in the maintenance of AF.16,17 Recent human studies have directly demonstrated, for the first time, that a majority of AF patients exhibit rotors and focal impulses where targeted ablation alone can acutely eliminate AF.18,19
Identifying and Ablating AF Rotors
During the EP study, a 64-pole basket catheter (see Figure 39-1) is introduced through an 8.5-F steerable sheath to the RA and then to the LA after transseptal puncture. The basket catheter has a unique, flexible basket design that conforms to atrial anatomy to aid in accurate placement and to save repositioning time. The 64 electrodes provide comprehensive, real-time 3-D information in a single beat. The average interelectrode spacing of 4 to 8 mm is sufficient to detect small reentry circuits in human atria. Care should be taken to optimize the contact between the atrial wall and the catheter’s electrodes by using fluoroscopy, electrogram analysis, and intracardiac echocardiography. An ACT of more than 300 seconds should be maintained throughout the procedure to prevent thromboembolic complications. After appropriate positioning of the basket catheter and placement of a unipolar catheter as a reference, AF is induced in patients who present in normal sinus rhythm by decremental atrial pacing or with a high dose of isoproterenol.
Computational AF maps are generated intraprocedurally using a novel software system, where electrograms from the basket electrodes are acquired and then analyzed. This software preprocesses electrograms to remove QRS signals and improve signal-to-noise ratio, and then it analyzes AF cycles at each electrode using reported human atrial tissue electrophysiology from previous studies to reconstruct propagation movies of electrogram amplitude at each electrode over successive time points, projected onto a grid.20-22
These maps often reveal electrical rotors defined as sequential clockwise or counterclockwise activation contours (isochrones) around a center of rotation emanating outward to control local AF activation, or focal impulses defined by centrifugal activation contours (isochrones) from an origin (see Figure 39-4). Rotors and focal impulses should be considered AF sources only if consistent in multiple recordings over a period of time (usually 10 minutes). The location of these AF sources can be defined in relation to the basket electrodes and then correlated to the electroanatomical map in order to apply RF ablation at these anatomic sites.
In a study of 80 consecutive AF patients, it was concluded that rotors and focal sources are common in AF.23 It was found that rotors exist in 96% of the patients. Study patients often demonstrated more than one coexisting electrical rotor or repetitive focal beat, for an average of 1.8 ± 0.9 for both atria. In the initial study, the authors showed that rotational centers and focal beat origins in fibrillation were unexpectedly stable and can last for at least several hours.
In the Conventional Ablation for Atrial Fibrillation With or Without Focal Impulse and Rotor Modulation (CONFIRM) trial, electrical rotors and focal impulses were present in 98 of 101 cases with sustained AF (97%). Each subject demonstrated 2.1 ± 1.0 sources of which 70% were rotors and 30% focal impulses.19 Also, the authors concluded that when sources were present, their number was higher for persistent than for paroxysmal AF and for spontaneous versus induced AF, and was unrelated to age, historical duration of AF, or whether subjects were undergoing first ablation or had had prior conventional ablation.
In another series of 14 patients, localized sources were demonstrated in all mapped patients for an average of 1.9 ± 0.8 sources per patient. Of 23 AF sources in this series (21 rotors, 2 focal sources), 18 were found in LA and 5 in the RA. All sources were stable for at least 20 to 30 minutes during mapping and remapping.24
Localizing the critical sites in human AF yields a promising opportunity to eliminate AF by targeting these sources with radiofrequency energy. The CONFIRM trial was the first prospective study to test the hypothesis that by eliminating the rotors or focal impulses, the outcome after AF ablation will improve.19 The authors in this trial concluded that the acute endpoint (AF termination or consistent slowing) was achieved in 86% of sites targeted ablation group versus in 20% of the conventional ablation therapy cohort, and resulted in higher freedom from AF during a median 273 days (interquartile range: 132 to 681 days) after a single procedure (82.4% versus 44.9%; P = 0.001). These conclusions were similar in other studies.23,24
The mechanism by which localized ablation terminates rotors is unclear but likely involves the elimination or alteration of functional or anatomical heterogeneities such as fiber anisotropy, fibrosis, scar, or other factors central to maintaining AF, leading to its termination.23
Despite the high rate of acute AF termination and improved success rate over the long term, there are many limitations and complications that have been reported. The technical connectivity, basket availability, and limitation of basket size in patients with large atria are the most important limitations. The distance between the electrodes in the basket catheter may produce suboptimal resolution for a small focal origin or rotor core, so there is a possibility that many rotors that originate between the electrodes will not be detected. There is also the possibility that additional sources will not be mapped (septal LA) due to the suboptimal contact between the atrial surface and the basket splines.
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