Catheter Ablation for Atrial Fibrillation--论文代写范文精选

随着时间的推移，手术已被改进,一贯的方法包括隔离肺静脉(PV)。目前,韧性手持导管被用来创建线性损伤与传统射频能量,而不是手术切口。下面的essay代写范文进行详述。

ABSTRACT 
Atrial fibrillation (AF), the most common arrhythmia in adults, affects 1 in 25 people over the age of 60 years and 1 in 10 over the age of 80 years.1  There is considerable morbidity, mortality and economic burden associated with AF, all of which will increase with the expanding elderly population. Until recently, pharmacologic therapy with AV nodal blocking agents, antiarrhythmics and anticoagulation were the mainstay of therapy. Although electrical cardioversion is associated with a high immediate success rate, most patients have recurrences of AF with only 23% remaining in sinus rhythm one year after cardioversion.2  Antiarrhythmic agents have been shown to improve sinus maintenance, but these medications have variable success and are associated with many potentially serious side effects. In addition, the recently published AFFIRM trial suggests that a pharmacological rhythm control strategy has no benefit in terms of mortality or morbidity over a rate control and anticoagulation strategy.3 Over the last few years, there has been a great deal of enthusiasm regarding catheter based ablation strategies aimed at curing AF.

Percutaneous Maze Procedure
In 1959 Moe theorized that AF resulted from multiple wavelets of reentry. With this in mind, Cox performed the surgical Maze procedure in 1991.4 The Maze procedure alters the arrhythmogenic substrate by interrupting the macroreenterant circuits and reducing the critical mass of atria needed to sustain AF. This surgical approach currently is preformed in association with coronary bypass surgery and/or mitral valve repair and is successful in curing AF in 75-90% of cases.5  Although the surgical procedure has been modified over time, the approach has consistently included isolation of the pulmonary veins (PV). Currently, malleable hand-held catheters are being used to create linear lesions with conventional radiofrequency energy, rather than surgical incisions.

The surgical experience over the past decade has provided evidence that the left atrium plays a significant role in the maintenance of AF and that a reduction in the left atrial mass prevents maintenance of AF. Because an open chest procedure is associated with significant morbidity, attempts have been made to replicate the Maze procedure using a percutaneous, catheter-based approach. The MECA (Multiple Electrode Catheter Ablation) trial, sponsored by Boston Scientific/EP technologies, was designed to determine the safety and feasibility of specially designed catheters with multiple large electrodes used to create circular, biatrial linear endocardial lesions to treat atrial fibrillation (Figures 1-3). The concept and catheter design were based on encouraging animal data.6 However, the MECA study was terminated prematurely due to a relatively high complication rate and low efficacy rate.7  Additional limitations of a percutaneous Maze procedure include a long, technically difficult procedure associated with long fluoroscopy times and the risk of proarrhythmia in the form of atrial tachycardias,  which likely occur due to conduction gaps in the ablation lines. The role of linear atrial lesions in the treatment of atrial fibrillation remains unresolved.

Importance of the Pulmonary Veins 
Embryologically, the PVs form as a bud that grows from the heart towards the lungs. As a result, the PVs have a sleeve of muscle fibers that surround them. In the late 1990’s, Haissaguerre made a critical observation that the muscle fibers associated with the PVs are an important source of ectopic beats capable of triggering AF.8  

This discovery led to a revolution in interventional electrophysiology. Pulmonary vein muscle tissue has unique electrophysiologic properties and appears to be able to maintain reentry within a relatively small amount of atrial muscle mass. This is likely due to the spatial complexity and a short refractory period. This area is also a common source of rapid focal discharges. Figure 4 shows an example of a rapid, irregular atrial tachycardia arising from a right upper PV that is associated with conduction block to the left atrium. The case highlights the unusual electrophysiologic observations that are made in the PVs. Although the arrhythmia is an atrial tachycardia in this case rather than AF, it is easy to see how a rapidly discharging focus in a PV could lead to AF by causing fibrillatory conduction or by initiating reentry in the atrium.

Electroanatomic Left Atrial Ablation 
An alternative endocardial ablation technique to treat AF has been described by Pappone. This technique uses a circumferential electroanatomic approach.19  The procedure described by Pappone involves a 3-D electroanatomic mapping system (CARTO, Biosense Webster Inc.) to map the atria and PVs. Circumferential RF lesions are then created at 5 mm from the PV ostia.  This anatomic approach eliminates the need for mapping spontaneous or induced arrhythmias.  The end point for ablation is a bipolar amplitude less than 0.1 mV inside the lesion and a delay of greater than 30ms across the ablation line (figure 13). 

The one-year success rates, defined as freedom from AF, obtained by the Pappone group in 251 patients (paroxysmal AF=179, permanent AF=72) treated was 80% overall, with 86% for paroxysmal AF and 68% for permanent AF.20  Only 75% of the circumferential lesions surrounding the individual PVs met criteria for complete, defined as a bipolar amplitude < 0.1 mV. Interestingly, they found no relation between lesion completeness and clinical outcome. This finding led the Pappone and his group to use the term “electroanatomic remodeling” to describe the alteration in the atria substrate that occurs during this ablation technique which prevents atrial fibrillation. This approach of PV isolation plus substrate modification may explain the higher success rates obtained compared to exclusively isolating the PV and eliminating the “trigger” of AF in the segmental ostial ablation approach.  Which of these two approaches is superior is a source of debate. A randomized control trial comparing segmental ostial ablation to circumferential ablation in patients with paroxysmal AF is currently underway.

Conclusions
The last few years have marked the beginning of an exciting new era in the treatment for AF.  For patients with paroxysmal AF, both segmental and circumferential ablations appear to have comparable long term success rates and low rates of complications. However, for patients with persistent or permanent AF, the circumferential ablation approach using 3D-elcctroanatomic mapping appears to be more successful. Patients with AF who are suitable candidates for catheter ablation are those with symptomatic AF despite reasonable pharmacologic efforts and minimal structural heart disease. The future of ablation therapy for AF will likely be an approach which both eliminates the trigger of AF and alters the substrate which permits maintenance of the arrhythmia. New catheter designs and alternative energy sources are currently under investigation to improve the safety, efficiency, and success rate of catheter ablation for AF.

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