Use of Magnetic Catheter Navigation for Ablation of Focal Tachycardias

Focal atrial and ventricular tachycardias are common diagnoses encountered in the electrophysiology laboratory. Arrhythmia burden varies from rare or paroxysmal premature beats to sustained or incessant monomorphic tachycardia. Location of the arrhythmia foci can vary, and careful contact mapping is often required to precisely locate the focus. Unfortunately, catheter ablation may fail, even when the focus is endocardial. The spatial resolution of activation mapping or pace mapping may be compromised by limitations in the steerability or reach of conventional unidirectional or bidirectional ablation catheters. The tip of a conventional steerable ablation catheter held by the operator may swing or move away from the desired ablation point with respiration and normal cardiac motion. The pressure that can be applied to ablation catheters in order to improve contact is often limited by the risk of cardiac perforation. Larger tip ablation catheters (8 mm) may be used to compensate for some of these limitations, but the accuracy of contact activation mapping and pace mapping is compromised. Magnetic robotic catheter navigation may help overcome many of the limitations of conventional catheter ablation. The Niobe ® system (Stereotaxis, Inc., St. Louis, Missouri) uses an externally applied magnetic field to direct the orientation of a catheter. The magnet system consists of two contralateral focused-field permanent neodynium-iron-boron magnets in a permanent housing that are rotated into the active position on the left and right of the patient's torso.1 In this navigational position, the magnets create a small but uniform magnetic field of 15 cm overlying the patient's heart. The field strength is 0.08 Tesla, which is several orders of magnitude less powerful than cardiac MRI magnets (typically 1.0-2.0 Tesla). In our laboratory (Figure 1), the Niobe navigation system is integrated with a monoplane flat panel system (Philips Corporation). The mapping and ablation catheter is equipped with a series of small permanent magnets that allow it to be controlled in the magnetic field.2 The catheter tip can be deflected by changing the orientation of the external magnetic field. Preset magnetic vectors based on standard cardiac anatomy can be used to direct the catheter to the desired location. In addition, custom vectors can be stored and reapplied for automatic navigation. A computer-controlled motor drive unit (Cardiodrive) is used to remotely advance and retract the catheter. The system allows precise navigation with a spatial resolution of 1 ° of omni-directional deflection and 1 mm for catheter advancement and retraction. The Navigant computer control system, located in the shielded control room, allows operators to control all aspects of catheter navigation, mapping, pacing, and ablation. The operator may control the catheter using any desired combination of the joystick control, keyboard, or mouse. The Stereotaxis system is integrated with the Carto mapping system (Biosense Webster, Inc., a Johnson & Johnson company, Diamond Bar, California), and navigation can be performed directly from the electroanatomic map or pre-procedure cardiac CT or MRI. We have found that use of the Stereotaxis Niobe system in combination with three-dimensional electroanatomic mapping is an efficient and effective approach for mapping and ablation of focal arrhythmias. Although much of the initial excitement for the Niobe system is based on its ability to simplify atrial fibrillation ablation, we believe that it may also have a major impact on the success of focal arrhythmia ablation. Here we present two cases to demonstrate the feasibility and utility of magnetic navigation for these types of arrhythmias. Case Report #1 A 38-year-old man presented with frequent palpitations. Ambulatory ECG monitoring revealed frequent atrial premature complex (APC) and supraventricular tachycardia. Invasive electrophysiology testing with programmed stimulation demonstrated features consistent with a focal atrial tachycardia (cycle length 410 msec), including triggered bursts of atrial tachycardia, overdrive suppression and a V-A-A-V response with ventricular overdrive pacing, and failure to initiate or terminate with extrastimuli. Activation mapping was performed using a 4 mm NaviStar ® RMT catheter (Biosense Webster, Inc.). After confirming registration of the Stereotaxis vectors to the fluoroscopic system, the automated magnetic navigation feature was used to create a three-dimensional anatomic shell of the right atrium, vena cava, and tricuspid annulus. Contact mapping was used to map the earliest local activation of APCs with respect to the atrial electrogram from electrodes 5 and 6 on the coronary sinus. Activation mapping was performed using the Carto mapping system. The earliest activation was 82 msec earlier than the surface P-wave, and was located along the mid portion of the crista terminalis in the posterolateral right atrium. Atrial pacing failed to demonstrate entrainment, and the activation map was consistent with a focal source. High output pacing revealed no evidence of phrenic nerve stimulation. Application of a single radiofrequency (RF) lesion with 50 watts of power and a temperature limit of 60 degrees for 60 seconds to this area resulted in acceleration of the tachycardia followed by complete cessation of the arrhythmia and normal sinus rhythm. Five additional RF lesions were empirically placed in a circular pattern around the initial lesion; none of these resulted in atrial premature beats of tachycardia. At three-month follow-up, the patient had no recurrence of his palpitations. Case Report #2 A 44-year-old man with a history of myocarditis presented with frequent monomorphic premature ventricular complexes (PVCs) and wide complex tachycardia documented by ambulatory ECG monitoring. His ejection fraction was normal. He was taken to the electrophysiology lab, where frequent monomorphic PVCs were present in the baseline state. The PVCs had left bundle branch block morphology with an inferior axis, consistent with origin from the right ventricular outflow tract. Programmed ventricular stimulation did not induce sustained VT or VF. Electroanatomic mapping was performed using a 4 mm NaviStar RMT catheter. The Stereotaxis system was used to navigate the catheter for contact mapping. After creating a three-dimensional anatomic shell of the right ventricular with the tricuspid and pulmonary valves, contact mapping was used to create an activation map in reference to the surface maximum intrinsicoid deflection of the QRS of the PVC on surface lead II. The earliest activation was 101 msec earlier than the reference deflection, and was mapped to the posterior aspect of the septal mid right ventricular outflow tract (Figure 3). Pacing at this spot resulted in a perfect morphologic match to the PVC in all 12 ECG leads. Application of a single RF lesion (50 watts and 60 degree limit) resulted in a flurry of PVCs identical to the clinical morphology, followed by abrupt termination of the patient's arrhythmia. A second lesion was empirically applied in the same region, but did not induce PVCs. At two-month follow-up, the patient had no evidence of recurrence of his PVCs or monomorphic VT. Discussion These cases demonstrate the utility of magnetically-guided robotic navigation for mapping and ablation of focal tachycardias. In the first case, Stereotaxis allowed us to map in precise 1 ° deflections and 1-mm steps, which allowed for accurate and reproducible activation and pace mapping. By creating a stored magnet orientation for the site of earliest activation, we could return to the location of interest with remarkable precision, in this case, the crista terminalis. Tissue contact is affected only by the direction of magnetic field and reach of the soft, floppy catheter. As a result, application of a magnetic vector allows the ablator tip to remain fixed to the endocardium and is largely unaffected by respiratory and cardiac motion. Finally, because the position of the soft, floppy magnetic catheter is not dependent on catheter stiffness, guiding sheath, or operator pressure, there is virtually no risk of perforation. In the second case, we were interested in mapping the right ventricular outflow tract for focal VT. With a conventional steerable catheter, the right ventricular outflow tract is difficult to navigate with a catheter that only deflects in a single plane. To compensate, the operator must rotate the system and rely on torque transmission to guide the ablation catheter. This strategy makes navigation and dense contact mapping difficult, particularly in large or hyperdynamic ventricles, where catheter reach can be compromised.3 During ablation, cardiac systolic motion may cause the ablation catheter to slide, leading to inadequate lesion formation and increased perforation risk. By using 1-mm and 1-degree magnetic navigational increments, we were able to form a dense and complete activation map. At the point of earliest activation, we reproduced a perfect surface ECG pace map. However, in our experience, even small 1-degree or 1-mm movements from the VT focus led to imperfect pace maps, which emphasize the importance of fine mapping for precise localization.4 In both cases, a single RF lesion with a 4 mm ablation tip resulted in apparent elimination of the clinical arrhythmia. In a thirteen-month period, 108 ablations were performed at our institution using the Stereotaxis system, including nine accessory pathways, ten slow pathway modifications, nine right ventricular outflow tract tachycardias, 60 atrial fibrillation ablations, and twelve atrial tachycardias (focal and macroreentrant). In six cases (two accessory pathways; three left atrial ablations; one right ventricular dysplasia), the operators switched to a manual catheter system in order to use an 8-mm electrode tip (when previously not available for use with Stereotaxis), an irrigated tip ablation catheter, or because of the concern of insufficient catheter contact. In all 108 cases, there were no perforations or Stereotaxis equipment malfunctions. In summary, the use of magnetic navigation is a feasible and effective approach for treatment of focal tachycardias. Although the technology has primarily been used for atrial fibrillation ablation, it can be successfully used for focal right atrial and right ventricular ablations. While there are no randomized trials evaluating the benefits of remote magnetic navigation for focal tachycardias, the technology may be useful when dense activation or pace mapping is required, or when handheld catheter manipulation is difficult due to the patient's anatomy or cardiorespiratory motion.