RADAR: Effect of Structural Remodeling on Scar Formation as Assessed by DE-MRI of the Left Atrium Following Open-Irrigation Cooled-Tip Radiofrequency Catheter Ablation of Paroxysmal Atrial Fibrillation

Atrial fibrillation (AF) is an electrophysiological condition characterized by disorganized electrical activity that involves the entire atria. AF manifests initially as paroxysms of arrhythmia that later progress into persistent and permanent forms. It is a progressive disease associated with structural, electrical and contractile changes in the left atrium (LA). A major determinant of the progression of AF is structural remodeling or fibrosis that occurs in the left atrium. A more extensively remodeled atrium presents the substrate needed for the arrhythmia to persist. Structural remodeling is also a major determinant for success of rhythm control strategies in AF. While catheter ablation has reproducibly been shown to be superior to anti-arrhythmic drug (AAD) therapy for rhythm control in AF, the success of this procedure is significantly affected by the extent of structural remodeling present at the time of catheter ablation.

Delayed-Enhancement Magnetic Resonance Imaging (DE-MRI) has been demonstrated to be a very effective modality in identifying fibrotic and scarred cardiac tissue with excellent correlation to electro-anatomical mapping. This is related to the characteristics of Gadolinium, an extracellular contrast agent that is very effective in identifying regions of fibrotic non-viable myocardium. DE-MRI technology can be a very powerful, non-invasive method, of identifying the extent and the distribution of structural remodeling or fibrosis associated with AF. Specific image acquisition sequences have allowed for reproducible identification of high pixel intensity regions within the 2-dimensional images of the atrial wall. 3-Dimensional reconstruction of the entire left atrium then provides a quantification of the overall volume occupied by these hyper-enhanced regions relative to the entire left atrial wall volume. Used prior to catheter ablation, DE-MRI can therefore identify regions of significant structural remodeling or fibrosis. The same technology has also been shown to be very useful in examining the amount and distribution of ablation-induced scarring.

Catheter ablation has emerged as an effective interventional therapeutic modality for AF patients. Current ablation techniques, including pulmonary vein antrum isolation (PVAI), use radiofrequency (RF) energy to induce thermal damage to the LA substrate in an attempt to electrically disconnect and isolate arrhythmogenic foci originating from the pulmonary veins (PVs). Multiple catheter technologies for RF have been developed. Catheters may be either cooled or not. Cooling allows for higher and deeper energy delivery with larger lesion formation. Open irrigation cooled-tip RF ablation of AF is associated with less impedance rise, microbubble formation, and esophageal injury when compared with traditional non-cooled ablation catheters. It is also associated with shorter RF delivery time, fluoroscopy, and procedural time.

Understanding the temporal sequence of events of LA structural remodeling (preceding AF ablation) to scar formation (following AF ablation) can prove valuable in determining whether certain patients are more susceptible to recovery of electrical conduction and recurrence of AF. DE-MRI recently has been introduced as a means to visualize this RF-induced scarring post-ablation. These scans can be reconstructed into three-dimensional (3D) recreations of the LA allowing for further analysis and quantification of the LA scar formation following RF ablation.

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