Left Atrial Ablation for Atrial Fibrillation Guided by EnSite NavX With Digital Image Fusion

Atrial fibrillation is the most common sustained arrhythmia in humans, with a prevalence of 1% in the general population, and is associated with a significant increase in morbidity and mortality.¹ The impressive results of surgical MAZE procedures in treating atrial fibrillation have formed the basis for considerable enthusiasm for treating this arrhythmia with a similar but percutaneous catheter ablation approach. Left atrial ablation using a percutaneous radiofrequency catheter to create circumferential lesions encircling the pulmonary veins, either in isolation, or in pairs, has been shown to be effective in treating patients with paroxysmal and to a lesser extent, persistent atrial fibrillation.^2,3 A system capable of producing a three-dimensional model of the chamber in which ablation is to take place and of displaying the position of the ablation catheter on this model is necessary in order to create continuous lesions in the left atrium using point-by-point ablation.^2,3 We describe a case of left atrial ablation guided by the new EnSite NavX system (Endocardial Solutions Inc., St. Paul, Minnesota) together with its novel three-dimensional CT/MRI image importing function (Digital Image Fusion).

Clinical History

A 72-year-old man with drug refractory, highly symptomatic, lone paroxysmal atrial fibrillation for five years and which had become persistent for the last two years was referred to our unit for catheter ablation.

Ablative Procedure

After thrombus in the left atrium and its appendage had been excluded by trans-esophageal echocardiography, a single transeptal puncture was performed under fluoroscopic guidance and continuous pressure monitoring. Full anti-coagulation with heparin was initiated and the ACT maintained between 300 and 350 seconds during the procedure. A quadripolar Josephson diagnostic catheter was inserted to the coronary sinus via right subclavian vein as a reference. A 5 mm irrigated tip catheter (Biosense Webster Inc., Diamond Bar, California) was used for mapping and ablation. A reconstruction of the three-dimensional geometry of the left atrium was created by using the EnSite NavX system alongside an equivalent three-dimensional model based on data from a cardiac CT scan (Digital Image Fusion function) (Figure 1). Radiofrequency energy was delivered with power and temperature limits of 30 W and 50 °C, respectively. Ablation was performed in the left atrium well away from pulmonary vein ostia in order to avoid the risk of pulmonary vein stenosis and exclude ostial foci from the left atrium. Lesions encircling pairs of ipsilateral pulmonary veins were created. Because of the close proximity between the left pulmonary veins and the left atrial appendage, ablation was continued around the base of the appendage anteriorly on the left. In addition, two linear lesions were produced; one along the roof of the left atrium, connecting the lines around the right and left pulmonary vein pairs, and the other between the left pulmonary vein pair and mitral valve annulus. At the end of the procedure, the patient was successfully cardioverted internally under midazolam sedation using a biphasic 15J synchronized shock between the right atrium and left or right pulmonary artery (Alert® PA System, EP MedSystems Inc., New Jersey). Follow-up. The patient was followed up with monthly after the procedure in the outpatient clinics, where careful history was taken and 12-lead ECG was performed. During a follow-up period of two months after the procedure, there has been no recurrence of palpitation or documented atrial fibrillation.

EnSite NavX

The EnSite NavX system is capable of accurately locating any electrode catheter within the three-dimensional navigation field, allowing reconstruction of the geometry of any cardiac chamber and providing accurate, real-time catheter navigation to guide mapping and ablation. The system consists of three pairs of patches placed on the body surface in orthogonal axes (Figure 2). A low power 5.7 kHz electrical potential is generated across each pair of patches and the voltage gradient from each axis generates the three-dimensional navigation field. Based on this, the system measures the local voltage of any electrode that is placed within the navigation field and can thus accurately locate the position of the catheter electrodes. Up to 64 electrodes on a number of catheters can be located simultaneously at a sampling rate of 93 Hz.

Digital Image Fusion

The current version of Digital Image Fusion function of the new EnSite NavX system allows a three-dimensional CT/MRI image of a segmented left atrium to be displayed side by side and synchronously rotated with the constructed geometry. The three-dimensional segmented image of the left atrium can be obtained by processing the CT/MRI study using dedicated software (Analyze 5.0, AnalyzeDirect, Lenexa, Kansas). The segmented image is then imported into the EnSite System and can be used as a template for geometry construction. The side-by-side display of the two models of cardiac anatomy is unique to this new EnSite NavX system and is the first step in the evolution of Digital Image Fusion for electrophysiologic mapping. This case illustrates an efficient and successful left atrial ablation guided by a new three-dimensional navigation system, EnSite NavX, with the aid of a novel Digital Image Fusion function. As this technology evolves, additional integration of CT/MRI images and EnSite NavX created geometries may further facilitate the accuracy and efficacy of linear ablative procedures.

Acknowledgement. Dr. Tom Wong is supported by the Wellcome Trust; Dr. Pipin Kojodjojo is supported by the British Heart Foundation.