Abstract
Three-dimensional (3-D) mapping systems have been developed and used for more than 20 years to map more complex arrhythmias that cannot easily be treated with traditional electrophysiological approaches. These systems allow 3-D reconstruction of cardiac chamber anatomy, with recording and display of intracardiac electrograms at each point on the chamber surface. Activation maps display propagation of wavefronts to identify focal pattern (earliest site with centrifugal activation) and macroreentrant pattern (continuous activation with early meets late and total activation time equal to tachycardia cycle length). Voltage maps demonstrate regions of healthy (high voltage) and diseased/scarred myocardium (low voltage) and anatomical boundaries. Image integration with computed tomography scan, magnetic resonance and intracardiac echocardiography into the mapping systems facilitates reconstruction of cardiac chamber anatomy. The recent introduction of contact force sensors into mapping/ablation catheters has also improved the accuracy of reconstruction of chamber geometry.
A fundamental limitation of most currently available 3-D mapping systems is the requirement of manual correction of annotation of activation time, especially for complex electrograms (i.e. low amplitude potentials, multiple potentials or fractionated electrograms). In order to overcome this limitation, two new systems have been developed: (1) ultra, high-resolution mapping system; and (2) ripple mapping system, allowing automatic identification of the mechanism of complex arrhythmias (both focal and macroreentrant) and localizing arrhythmogenic foci and entire reentrant circuits with arrhythmogenic channels. The ability to acquire activation and voltage map information from multiple electrodes simultaneously with accurate localization of each mapped point in 3-D space allows rapid data acquisition, facilitating catheter mapping and ablation in patients with complex arrhythmias.
Keywords
Atrial fibrillation, Atrial tachycardia, Catheter ablation, Catheter mapping, Radiofrequency current, Ventricular tachycardia
Key Points
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Several 3-dimensional mapping systems are currently available, including three systems in widespread clinical use.
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Location of mapping electrodes is identified by the mapping system using magnetic-based or impedance-based technology or a combination of these.
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These systems allow 3-dimensional reconstruction of cardiac chamber anatomy, with recording and display of electrogram data at each point on the chamber surface.
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Activation maps can display propagation of wave fronts to identify focal (earliest site with centrifugal activation) versus macroreentrant (continuous activation with early meets late and total activation time equal to tachycardia cycle length) activation patterns.
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Voltage maps can show regions of healthy (high voltage) and diseased/scarred myocardium (low voltage) and anatomic boundaries and may help to identify arrhythmogenic channels through which activation may propagate, providing optimal targets for ablation.
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Recent development of sophisticated algorithms for automatic selection of activation times in complex electrograms has made successful ablation of complex arrhythmias more widely available.
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.
| CARTO | Ensite NavX | Rhythmia | EnSite Array | |
|---|---|---|---|---|
| Contact vs. noncontact | Contact mapping | Contact mapping | Contact mapping | Noncontact mapping |
| 3-dimensional catheter localization | Magnetic | Impedance-based | Magnetic/Impedance-based | Impedance-based |
| CT/MRI image integration | Yes | Yes | Yes | No |
| Real-time ultrasound integration | Yes | No | No | No |
| Mapping accuracy | +++ | ++ | ++++ | + |
| Need for sustained stable arrhythmia | Yes | Yes | Yes | No |
| Contact force sensing catheter available | Yes | Yes | No | No |
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.
Image Integration with Computed Tomography Scan, Magnetic Resonance, and Intracardiac Echocardiography
Preacquired computed tomography (CT) or magnetic resonance (MR) images of cardiac chambers can be loaded into most 3-dimensional mapping systems with their images superimposed on the map being created by the system. The images are usually aligned in space after a number of anatomically distant and recognizable locations (e.g., each pulmonary vein ostium, coronary sinus, or aorta) have been acquired. This can be achieved manually or with use of automatic best-fit algorithms . The CT/MR image positions can be adjusted if needed as more anatomic mapping points are collected. CT and MR images are also useful for showing the presence and location of extracardiac structures such as the esophagus and phrenic nerves as well as small structures such as coronary arteries, coronary sinus branches, pulmonary vein branches, and accessory pulmonary veins. It is important to note that the preacquired anatomic images may have been taken during different volume loading status or in different rhythms (sinus rhythm vs. atrial fibrillation), which may result in different chamber geometry ( Fig. 7.2 ). For this reason, real-time intracardiac echocardiography (ICE) may provide a more accurate background geometry even though images may be less anatomically detailed. One mapping system (CARTO, Biosense Webster) is able to take multiple cross-sectional 2-dimensional ultrasound images and reconstruct 3-dimensional geometry. Using an intracardiac ultrasound catheter with location sensor (CARTOSOUND, Biosense Webster), the ultrasound image created can be superimposed on the electroanatomic map without need for alignment algorithms ( Figs. 7.3–7.5 ). Intracardiac echocardiography can show not only the chamber outline but also the details of endocardial structures such as the papillary muscles (see Figs. 7.3,7.4 ) , the moderator band (see Fig. 7.5 ), and ventricular trabeculation with real-time display of a mapping/ablation catheter (see Fig. 7.4 ). Intracardiac echo is also useful to demonstrate the presence of a pouch in the sub-Eustachian isthmus, which can affect ablation of typical right atrial (tricuspid annular) flutter. This system also provides an advantage when mapping left and right ventricular outflow tract ectopic beats or VT by the ability to demonstrate the anatomy and relative position of the pulmonic valve and aortic coronary cusps, including the locations of the coronary artery ostia (see Fig. 7.5 ), where the relationship of the outflow tracts to each other is quite complex. Once earliest activation of outflow tract ectopy has been defined in the right ventricular outflow tract, knowledge of the juxtaposition of left ventricular outflow tract (and aortic cusp) structures can indicate a need to map the left ventricular outflow tract before selecting an ablation site, particularly if there is a far-field component to the right ventricular electrogram at the earliest site.
Activation Mapping
Activation maps require collection of timing data at precise locations with reference to a fixed, stable reference electrogram. Annotation timing from the reference electrogram chosen must not vary from beat to beat or for the duration of creating a single entire map. For an atrial tachycardia, a stable reference electrogram is often selected from available coronary sinus electrograms, choosing one with stable electrogram morphology and atrial electrogram substantially larger than ventricular electrogram. If no suitable coronary sinus electrograms are available (because of instability of the catheter or congenital absence of the coronary sinus), another catheter with stable location and timing such as one in the right atrial appendage or the superior vena cava can be used. If no stable reference electrogram can be found using conventional catheters, a screw-in temporary pacing lead may be used. Ideally the reference electrogram is in the same chamber as the anticipated tachycardia but not in an area where extensive catheter manipulation is required (to avoid dislodgement of the reference electrode during catheter manipulation). Appropriate cardiac beats for inclusion in the map are selected based on a number of criteria, either manually or automatically: (1) cycle length stability; (2) consistent timing of local electrograms on consecutive beats relative to the reference electrogram timing; (3) electrode location stability; and (4) appropriate timing within the respiratory cycle (respiratory gating). Despite using these criteria, occasionally premature atrial beats are mistakenly included in the map. To more reliably exclude premature atrial beats and also to automatically recognize a change in tachycardia to another of similar cycle length, the Rhythmia mapping system also uses the relative timing of two distant reference electrograms, with data points collected only when their relative timing matches that of the target arrhythmia.
For PVCs, or VT, a surface electrocardiogram is often used as a timing reference, with selection of a surface lead where QRS morphology is sufficiently different from that in sinus rhythm to allow data collection only during the ventricular arrhythmia. Alternatively, an intracardiac ventricular electrogram can be used, optimally from the ventricle not being mapped or from a catheter within a ventricular branch of the coronary sinus such as the anterior interventricular vein or the middle cardiac vein. More recently, some 3-dimensional mapping systems have developed the ability to use a template derived from the surface 12-lead electrocardiograms to automatically select only beats from the target PVC or VT for data collection (template matching function).
The operator needs to predefine an appropriate window of timing interest for data collection, based on the tachycardia cycle length, and suspected arrhythmia. If a macroreentrant circuit is suspected, the window must be equal to (100% of) tachycardia cycle length to identify the entire reentrant circuit. The timing of early activation relative to the reference electrogram will depend on the operator selected onset of the window of interest relative to the timing of the reference electrogram and has no inherent meaning (i.e., activation is continuous, and there is no true early or late site). If focal atrial tachycardia with 1:1 AV conduction is suspected, the offset of the window is best chosen to be just before the onset of the QRS complexes, with the onset of the window at 50 to 70 ms before the onset of the P wave. This will usually result in red coloration of the site of earliest activation (see later). If AV conduction is not 1:1, then the window should again be the entire tachycardia cycle length and points chosen from beats with no ventricular electrogram. Where the atrial tachycardia mechanism is unknown (macroreentry vs. focal), the best practice is to set the window duration equal to tachycardia cycle length (100% of cycle length).
Local activation time can be determined using a number of different properties of either the unipolar or bipolar electrograms at each mapped site. Most widely used 3-dimensional mapping systems require preselection of a single criterion for determining local activation timing. However, each single criterion has its limitations. The unipolar electrograms are recorded between a mapping electrode and a reference electrode (either a surface electrode, Wilsons central terminal, or an independent electrode located in the inferior vena cava) resulting in a large far-field component to the recorded electrogram, which may obscure a small local potential. The bipolar electrogram provides more localized information, but the accuracy of timing from these electrograms depends on the space between the bipolar electrodes and the direction of propagation of the wave front in relation to the orientation of the two electrodes. Local activation time is usually selected by using either the steepest negative slope (maximum negative depolarization velocity (dV/dt) or downslope) or electrogram peak (maximum positive or negative amplitude). These criteria may be accurate in normal myocardium with a narrow, high-amplitude electrogram, but are less accurate in diseased/scarred myocardium. The electrocardiograms recorded in scarred myocardium often exhibit complex potentials such as low-amplitude potentials, multiple potentials, or fractionated electrograms. At sites with these electrogram properties, any single predefined timing selection criterion often results in inaccurate local activation timing. These complex electrograms are particularly prevalent where the arrhythmogenic channels are found within macroreentrant circuits, limiting the accuracy of automatic timing annotation for this arrhythmogenic substrate. When the mapping electrode is located at the site of surviving muscle bundles within scar tissue, surrounded by high-voltage normal myocardium, the local electrogram will usually exhibit a low-amplitude, but sharp (near-field), isolated potential from the muscle bundle as well as a high-amplitude, far-field potential originating from surrounding healthy myocardium. This can result in incorrect selection of timing (from the large far-field potential) when maximum voltage is the preselected criterion for timing annotation. The maximum negative dV/dt of the bipolar electrograms is usually able to better determine which potential represents local activation. However, selection of timing using maximum negative dV/dt may be inconsistent when bipolar electrograms have multiple component potentials.
We (the authors of this chapter) usually use the maximum negative dV/dt of the bipolar electrograms to annotate activation timing for the bulk of the map and then manually correct the annotation at sites exhibiting complex electrograms (such as double potentials and low-amplitude fractionated electrograms) using additional information on the unipolar electrograms, and taking into consideration the timing of electrograms in the surrounding area (to identify the direction of activation), and the distribution of low-and high-voltage areas (as explained above). If one is unsure of the local activation time in a complex electrogram, a tag can be used to mark the site for later analysis when activation timing and voltage of the surrounding myocardium is known (see Fig 7.2 ).
Each acquired point on the map is assigned a local electrogram time relative to the reference electrogram with color coding from early to late progressing from red (earliest) through orange, yellow, green, light blue, dark blue, and purple (latest), used for display on a 3-dimensional reconstructed image ( Figs. 7.2, 7.6, and 7.7 ). Alternatively, 3-dimensional mapping systems can display a moving image of the wave front progressing over the map from early to late (propagation map).
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