Guidance of Radiofrequency Endocardial Ablation With Real-time Three-dimensional Magnetic Navigation System
Background Ablation therapy for certain arrhythmias requires the formation of complex lesions based on electrical and anatomic mapping. We tested the accuracy and reproducibility of a nonfluoroscopic mapping and navigation (NFM) system to guide delivery of radiofrequency (RF) energy in the right atrium (RA) of swine.
Methods and Results The NFM system uses an ultralow magnetic field to measure the real-time three-dimensional (3D) location of the tip of the locatable catheter. While in stable contact with the endocardium, between 30 and 40 consecutive tip locations were sampled and used for the 3D reconstruction of the RA geometry. The location of the catheter tip was presented in real time, superimposed over the RA geometry. We selected a point on the 3D reconstruction and delivered RF energy to that site via the tip of the locatable catheter. The catheter was then completely withdrawn and renavigated twice to the same point, at which RF energy was delivered again. At autopsy, the distance between the centers of the three ablation points (mean±SEM) was 2.3±0.5 mm (n=27). Similarly, we used the NFM system to guide the generation of linear lesions. The measured length of the linear lesions on the NFM 3D view was close to the actual lesion length measured at autopsy (correlation coefficient, .96; P=.002; n=6). Furthermore, the location, shape, and continuity of the linear lesions corresponded to the autopsy findings.
Conclusions We conclude that the NFM system can guide the application of RF energy without the use of fluoroscopy in a highly accurate and reproducible manner.
Radiofrequency catheter ablation is an established procedure for treating patients with supraventricular and ventricular arrhythmias.1 2 3 In recent years, the number of RF catheter ablations has increased remarkably, with primary success rates of >90% reported in some procedures.1 4 5 6 7
RF energy is delivered to target sites on the endocardial surface. Selection of target sites is based on a combination of anatomic and electrical criteria. These criteria are different for each type of arrhythmia. Common to all cardiac RF ablation is the use of fluoroscopy to guide the navigation of the ablation catheter to the target site. The use of fluoroscopy for this purpose is problematic for the following reasons. (1) The patient and medical team are exposed to ionizing radiation. (2) The endocardial surface is invisible on the radiograph; therefore, the ablation targets can only be approximated by the relationship between the ablation catheter tip image and that of nearby structures (such as ribs or blood vessels). (3) Sites to which RF energy was previously delivered cannot be recognized later by fluoroscopy; therefore, operators can repeatedly apply RF energy to the same site without noticing. (4) Fluoroscopy generates a two-dimensional representation of cardiac anatomy; therefore, the navigation of the ablation catheter has to rely on repeated multiple-plane views to assess the correct 3D position of the catheter.
Recently, RF ablation has been applied to treatment of atrial flutter and fibrillation.8 9 However, the ablation lesions in these cases are usually more complex than the regular point ablations for other arrhythmias. Linear ablations matching the maze surgical procedure have already been used with some success in eliminating atrial fibrillation in humans10 and dogs.11 12 The difficulties in generating complex lesions stress even more strongly the shortcomings of the current method for guiding RF energy application.
In the present study, we tested the value of a new magnetic 3D NFM system for cardiac ablation. The system uses ultralow magnetic fields to locate a sensor positioned near the tip of a regular mapping and ablation catheter. Recorded data of the catheter location and intracardiac electrogram are reconstructed in real time and presented as a 3D geometric map color-coded with the electrophysiological information. We evaluated the accuracy and reproducibility of the NFM system in guiding both point and elongated lesion generation.
The study was performed on healthy male swine weighing 30 to 40 kg. All animals were premedicated with ketamine HCl (20 mg/kg IM) and xylazine (2 mg/kg IM), anesthetized with pentobarbital (30 mg/kg), intubated, and placed on a Harvard large-animal mechanical respirator. Vascular access was obtained by vascular cutdown of the jugular veins, carotid arteries, and femoral vein and artery as needed. A 5000-U IV bolus of heparin was given shortly after the vascular cutdown. The experimental protocol was approved by the Animal Study Committee of the Technion Faculty of Medicine.
The navigation and mapping system is composed of a miniature passive magnetic field sensor, an external ultralow magnetic field emitter (location pad), and a processing unit (CARTO, Biosense).
The locatable catheter (NAVI-STAR, Cordis-Webster) is similar to a regular electrophysiological 8F deflectable catheter. The tip of the catheter is mounted on the distal end of the shaft and includes the tip and several more proximal electrodes that allow recording of unipolar or bipolar signals. Just proximal to the tip electrode lies the magnetic sensor, totally embedded within the catheter. Magnetic fields emitted from the location pad are received by the sensor and are transmitted along the catheter shaft to the main processing unit.
The locator pad is located beneath the operating table and generates ultralow magnetic fields (5×10−6 to 5×10−5 T) that code the mapping space around the animal’s chest with both temporal and spatial distinguishing characteristics. These fields contain the information necessary to resolve the location and orientation of the sensor.
Catheter Mapping and Ablation
Under fluoroscopic guidance, two locatable 8F catheters were introduced through the appropriate vascular access and positioned inside the heart chambers. The first catheter was introduced into the coronary sinus or the right ventricular apex and served as a reference catheter. A mapping catheter was introduced into the mapped chamber. The mapping system determined the location and orientation of both the mapping and reference catheters. The location of the mapping catheter was gated to a fiducial point in the cardiac cycle and recorded relative to the location of the fixed reference catheter at that time, thus compensating for both animal and cardiac motion. While moving the catheter inside the heart, the system analyzed its location and presented it to the user, thus allowing navigation without the use of fluoroscopy. The mapping catheter was dragged over the endocardium, sequentially acquiring the location of its tip together with its electrogram while in stable contact with the endocardium.
The LAT at each site was determined from the unipolar intracardiac electrogram. We collected unipolar recordings filtered at 0.5 to 240 Hz. The LAT at each site was calculated as the interval between a fiducial point on the body-surface ECG or intracardiac electrogram and the steepest negative intrinsic deflection from the unipolar recording. The stability of the catheter-wall contact was evaluated at each site by examination of the end-diastolic stability (the distance in millimeters between two successive end-diastolic locations) and the LAT stability (measured as the difference in milliseconds between two successive LATs). A point was added to the map only if the end-diastolic location stability was <2 mm and the LAT stability was <2 ms.13 The electrophysiological information was color-coded and superimposed on the 3D chamber geometry (Fig 1⇓). The reconstruction was updated in real time with acquisition of a new site. The map was presented as a 3D object in which interpolation of activation between adjacent sites was allowed only for nearby sites. Whenever the distance between adjacent sites was higher than a predetermined threshold (triangle-fill threshold), no interpolation was made and the sites were connected by a “wire” frame (Fig 1⇓). We used a threshold of 25 mm for the figures displayed in this article.
RF energy was delivered from the distal electrode of the locatable mapping catheter (4.0 mm), whereas the anode was a larger patch electrode placed on the animal’s back. RF ablation was performed with a 500-kHz RF generator (RFG-3C; Radionics) in a temperature-controlled mode of 80°C for 60 seconds. These corresponded to energy of 20 to 35 W. Impedance measurements served to determine constant contact with the endocardium and coagulum formation on the catheter tip (impedance rise >150 Ω).
Protocol 1. The aim of this set of experiments was to perform three distinct point ablations at a single anatomic site. Subsequent to executing the electroanatomic map of the RA, we selected an arbitrary site for the point ablations to be performed. We delivered RF energy to that site three times, and each ablation point was tagged (Fig 2⇓). For each RF application, we navigated the catheter to the target from a site outside the RA. Nine swine were used in this protocol. The heart was excised, and the distance between the centers of the ablation sites, as well as the total area of the combined lesion, was measured.
Protocol 2. The objective of this set of experiments was to perform a line of ablation in the RA. We tested our capability to form a continuous line consisting of 6 to 12 separate ablation points guided by the NFM system. Subsequent to executing the electroanatomic map of the RA, we chose the path on which we planned to deliver RF energy. Each ablation point was tagged and added to the map as described in the previous protocol. The catheter was removed after each point ablation and navigated back to a nearby site on the path, where another burst of RF energy was applied (Fig 2B⇑). Six swine were used in this protocol. We compared the length of the lesion as measured on the NFM system with that measured at autopsy, as well as the shape and continuity.
Animals were killed and hearts were removed. Special care was taken to avoid damaging the ablated regions. Distances and areas were measured on preserved specimens. Tissues were preserved in 4% formaldehyde for further analysis and micrograph preparations.
Results are reported as mean±SEM. Correlation coefficients were calculated with the Pearson test. A value of P<.05 was considered statistically significant.
Mapping and Ablation Procedure
Mapping of the RA was performed for each ablation procedure. In this study, the average number of points for a single map was 42.3±4.9 (n=12 maps). The mapping procedure lasted from 7.40 minutes (30 points) to 29.12 minutes (55 points), with one map of 66 points carried out in 12.43 minutes. The average single-point ablation procedure duration was <10 minutes for the three successive point ablations and was between 12 and 37 minutes for the linear ablations consisting of 6 to 12 individual ablation points. We used fluoroscopy during deployment of the mapping and reference catheters in the heart chambers and during acquisition of the initial three boundary locations of the mapped chamber. Total fluoroscopy time was always <2 minutes, and no fluoroscopy was used during RF energy application.
Single Ablation Points
The distances between the centers of the three ablation points and the area of the combined ablation lesion were measured from the tissue specimens. Fig 3⇓ presents the result of a representative point ablation experiment, the left panel showing the electroanatomic map with the tagged ablation sites and the right panel the corresponding sites on the heart tissue. A single RF energy application generated a circular lesion with an average diameter of 5.0 mm and an area of 19.6 mm2. Table 1⇓ summarizes the results of nine experiments.
We compared the length of a longitudinal lesion performed on the heart with the length of the lesion as viewed on the NFM map. Fig 4⇓ presents a typical trial of linear lesion, the left panel showing the electroanatomic map with a line composed of several tagged ablation points and the right panel the same ablation line on the heart tissue. Table 2⇓ summarizes the comparison between the linear ablations performed with the NFM system on the reconstructed maps and the actual lines performed on the heart tissue. The correlation coefficient between the length of intended and actually performed lines was .96, with P=.002 (n=6).
In the present study, we tested the accuracy of ablation guidance using a new 3D NFM system. Specifically, there were two reasons for the present study: (1) to test the in vivo accuracy of the NFM system using a methodology different from prior in vivo and in vitro accuracy studies13 and (2) to test the ability of the NFM system to guide the delivery of complex RF lesions to the heart. The results indicate that RF energy application can be guided accurately and reproducibly to form point or elongated lesions without the need for fluoroscopy. We discuss below some aspects of the NFM system used for mapping and ablation, as well as possible clinical implications.
In previous studies, we evaluated the accuracy of mapping using the NFM system.13 In those studies, we showed that the location system used in the NFM system has a static in vitro accuracy of 0.2 mm and an in vivo accuracy of 0.9 mm. Similar studies have been performed in humans, with similar results.14 The reconstruction procedure used by the NFM system to calculate the heart chamber volumes has been shown to be accurate.15
The mapping procedure was relatively short and created the electroanatomic “road map” necessary to guide the ablation procedure described in this article. Nevertheless, this mapping procedure was sequential and required that the heart beat with the same rate and rhythm throughout the mapping procedure.
We evaluated the reproducibility of navigating the ablation catheter to a site on the endocardium using the NFM system. We found that a site identified by the electroanatomic map of this system corresponded to a site on the heart, as indicated by the relatively small distances between the centers of the repeated ablation points. The system also assisted in the guidance of RF energy delivery for longitudinal lesions to the RA. We showed that we could design a line of ablation on the viewing station and deliver the RF energy accordingly to form the desired lesion on the endocardium. The more experienced we became, the shorter the procedure became, together with an overall improvement of accuracy. No fluoroscopy was used to guide the mapping or ablation procedure except for that required for acquiring the first three boundary points.
Although not directly comparable, the average reported fluoroscopy time for RA mapping and ablation procedures is generally between 15.2 and 63 minutes.16 In the present study, we clearly showed that no significant x-ray exposure was needed either to map or to ablate the RA.
Ablation in the RA
Navigation of the catheter to the site of origin of a focal arrhythmia or to a certain anatomic location at which the RF energy should be delivered is easily accomplished with the NFM system. We found that the distances between the centers of the three individual ablation points were very small, reaching almost half the length of the ablation catheter-tip electrode. In addition, the ability to visualize the catheter tip in real time on the monitor screen (Fig 2⇑) allowed selection of the desired tip orientation during RF energy delivery. The orientation of the catheter tip is important for precise determination of the lesion size and shape. Catheter tip orientation is also important for determining temperature rise during RF delivery. In their study, McRury et al17 recorded up to 12°C divergence from the attempted temperature, provided that the catheter orientation was at an angle of ≤45°.
There are no published data on the accuracy of RF ablation procedures using fluoroscopic guidance. The only published studies of the accuracy of RF ablation used intracardiac echocardiography.18 19 However, one may judge the value of the NFM system to guide the delivery of RF energy by reviewing the reported number of RF applications per cardiac ablation procedure. Usually, between 3 and 20 RF applications are delivered during a standard RF ablation of a single supraventricular site.16 There are several reasons for repeated RF applications, including inappropriate detection of the target and insufficient energy delivery. The NFM system’s systematic “bookkeeping” of all electrograms and previous ablation sites may decrease the number of RF energy applications.
For AV nodal reentry tachycardias, AV reentry tachycardias, and atrial tachycardias, ablation therapy is the treatment of choice.20 Recent studies report a high success rate (>90%) when the current methods and devices for ablation treatment of these arrhythmias are used.6 More recently, ablation treatments for atrial flutter and fibrillation are being developed. Currently, the success rate for ablation of these indications is <90%. One of the differences between the ablation procedures for treatment of atrial flutter and fibrillation is the need to induce a continuous line of block. The NFM system can guide these catheter ablation procedures using the real-time navigation on top of an electroanatomic map of the chamber and tagging sites at which RF energy was delivered.
Limitations of the NFM System
The NFM mapping concept is based on a beat-by-beat approach. This approach, although highly accurate, is time-consuming, because the mapping catheter has to be dragged to each new site and positioned for acquiring the information of a new point. To accurately perform this process, one must confirm that the heart is beating at the same rate and rhythm before including the information from each new site. Furthermore, if the patient develops a new arrhythmia, the entire electroanatomic map should be acquired, because both the exact chamber anatomy and its activation sequence are changed. This limitation is partially overcome by using an “auto-map” mode, in which the mapping system automatically accepts information from new sites when certain stability criteria are met.
As reported in previous studies, the mapping system has an error in determining the location of the catheter tip.13 This error may result from respiratory movements not fully corrected by the reference catheter as well as from animal body movements. The overall effect of these errors on repositioning the catheter was found in the current study to be <2 mm. This error corresponds with <50% of the single-ablation lesion size.
Our studies performed in the swine RA show that the NFM system produces reconstruction of the atrium that allows highly accurate guidance for catheter ablation procedures. The short electroanatomic mapping procedure, the precise 3D navigation, and the reproducibility of catheter-tip repositioning suggest that NFM guidance for RF ablation may be better than fluoroscopy. Further clinical studies are needed to assess the value of the NFM system in treating arrhythmia patients.
Selected Abbreviations and Acronyms
|LAT||=||local activation time|
|NFM||=||nonfluoroscopic mapping and navigation|
This study was partially funded by Biosense, Inc. We wish to thank Ruth Singer for her editorial assistance.
- Received November 13, 1996.
- Revision received March 12, 1997.
- Accepted March 30, 1997.
- Copyright © 1997 by American Heart Association
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