Catheter 3-Dimensional Mapping Systems

Introduction

Three-dimensional mapping has been in use for more than 20 years. There are several 3-dimensional mapping systems available. Three of these mapping systems are in widespread clinical use: (1) CARTO mapping system (Biosense Webster, Inc, Diamond Bar, CA); (2) EnSite NavX system (Abbott/St. Jude Medical, Inc, St. Paul, Minn); and (3) Rhythmia system (Boston Scientific, Inc, Table 7.1 ). These mapping systems can be used to aid catheter mapping and ablation of both simple and complex tachyarrhythmias but are most useful for mapping and ablation of complex atrial and ventricular arrhythmias that cannot easily be treated with traditional electrophysiological approaches.

Prior to the availability of 3-dimensional mapping systems, X-ray images (single or biplane fluoroscopy) were used to note the position of various electrograms or responses to pacing within a cardiac silhouette, which the operator had to remember. The information retained using this approach was limited, restricting the usefulness of this approach to relatively simple arrhythmia mechanisms, such as atrioventricular (AV) reentrant tachycardia (accessory pathways), AV nodal reentrant tachycardia, sustained focal atrial and ventricular tachycardia (VT), premature ventricular complexes (PVCs), and typical right atrial flutter. It is challenging to remember the entire circuit using conventional mapping techniques, with the result that macroreentrant tachycardias, other than typical right atrial flutter, could only be ablated by experienced operators.

Entrainment pacing techniques have been used to identify the general macroreentrant circuit location, but these techniques do not easily identify the arrhythmogenic channels within the circuit, representing the best target for ablation of the tachycardia. Furthermore, there are problems inherent in using entrainment pacing techniques, most importantly the risk of pacing terminating the tachycardia or changing it to a different tachycardia. Occasionally the results of entrainment pacing are also misleading, with relatively long postpacing intervals caused by decremental conduction properties within the circuit even at pacing cycle lengths only slightly shorter than tachycardia cycle length. This makes the entrainment pacing site mistakenly appear to be away from the circuit. To overcome these limitations, 3-dimensional mapping systems have been developed and are now widely used to map more complex arrhythmias in their entirety.

3-Dimensional Electroanatomic Mapping

Three-dimensional electroanatomic mapping systems can record and display the activation sequence of an entire chamber (and even multiple chambers) using color-coded algorithms. The systems record multiple data types from intracardiac electrocardiograms at each mapped site with their precise 3-dimensional location, including activation timing, unipolar and bipolar voltage, and tags to represent complex electrograms such as double or fractionated potentials. The data is then represented on a 3-dimensional reconstructed image with color coding. The operator is then able to review the entire arrhythmia circuit to identify a critical arrhythmogenic channel or precise focus of origin of the arrhythmia to target for ablation. The site of ablation can be stored and displayed on the map, so that one can return to the same site for additional ablation or can use ablation tags to ensure continuity of linear ablation lesions.

3-Dimensional Catheter Localization

The location of the mapping/ablation catheter electrode in 3-dimensional space is identified by the mapping system using magnetic-based or impedance-based technology, or a combination of the two (see Table 7.1 ).

The CARTO system primarily uses an ultralow-intensity magnetic field to localize the position of the mapping catheter in 3-dimensional space. Magnetic fields are emitted from nine separate coils in a location pad positioned underneath the table. Three magnetic sensors, arranged orthogonally on the catheter tip (NaviStar, Biosense Webster), measure the magnetic field strength to calculate the distance between each coil and the catheter tip. The position of the catheter in 3-dimensional space is then calculated by integrating the field strength detected by the sensors and comparing with reference patches on the patient’s chest and back. The catheter position is recorded as X, Y, Z coordinates within the cardiac chamber with excellent accuracy (0.54 ± 0.05 mm). This location mechanism requires use of proprietary catheters (Navistar, LassoNav, PentaRay, DecaNav, Biosense Webster) with the appropriate sensors. However, the most recent platform (CARTO 3) additionally uses current-based 3-dimensional impedance catheter localization algorithms, allowing localization of any diagnostic catheter (without magnetic location sensors) to be displayed on the map, provided impedance data from that location has already been collected by a proprietary catheter.

The EnSite NavX system (Abbott/St. Jude Medical) uses impedance-based technology for catheter localization. An alternating current (8.136 kHz) is applied sequentially between three pairs of orthogonal (X, Y, Z) surface electrode patches. The magnitude of this current recorded by electrodes on any catheter within the field will be proportional to the distance of the electrode from the surface electrode patches. The location of the catheter tip is therefore located in 3-dimensional space by triangulation. This is less accurate than magnetic location because of the nonhomogeneous impedance characteristics across the chest, and introduction of additional fluid (e.g., saline irrigation) can cause dynamic changes in impedance during the procedure. However, accuracy can be improved using magnetic field scaling (EnSite Precision) to correct for the heterogeneous and changing distribution of impedance. Any commercially available catheter can be localized with this system and used for mapping. In the most recent platform of the EnSite NavX system (EnSite Velocity), up to 128 electrodes and an unlimited number of catheters can be displayed simultaneously.

The Rhythmia mapping system (Boston Scientific) uses a proprietary 64 electrode mini-basket catheter (18-mm diameter), with all 64 electrodes located in a 3-dimensional image using a combination of magnetic and impedance-based technology to improve accuracy ( Fig. 7.1 ).

Creation of 3-Dimensional Geometry

Surface geometry of the chamber being mapped can be created based on the outer boundary of the mapping electrode locations. Internal points can be eliminated (manually or automatically) with points accepted within a range of distances (defined by the user) from the outer surface of the map. Points should be taken during one phase of respiration (usually expiration) to avoid significant distortion of geometry caused by shifts in the location of the heart produced by respiration. This can be achieved manually (by taking points only during expiration) or with automatic respiratory gating (programmed into the mapping system). The system will interpolate data between acquired points over a distance up to a maximum allowed by the user (fill-threshold/interpolation threshold). This can lead to merging of structures, such that it is often preferred to create separate maps of these structures (e.g., pulmonary veins, coronary sinus). A smaller interpolation distance (fill-threshold) requires a greater number of points to be mapped to complete chamber geometry, but provides more accurate chamber geometry.

The recent introduction of contact force sensors into mapping/ablation catheters has improved the accuracy of geometry by (1) recognizing internal points by lack of contact force and (2) reducing the deformation (tenting) of thin myocardium by excessive force.