Fundamental Concepts

The basket catheter mapping system consists of a basket catheter (Constellation, EPT, Inkster, Mich.), a conventional ablation catheter, the Astronomer (Boston Scientific, Natick, Mass.), and a mapping system.

The basket catheter consists of an open-lumen catheter shaft with a collapsible, basket-shaped, distal end. Currently, baskets are composed of 64 platinum-iridium ring electrodes mounted on eight equidistant, flexible, self-expanding nitinol splines (metallic arms; see Fig. 4-4). Each spline contains eight 1.5-mm electrodes equally spaced at 4 or 5 mm apart, depending on the size of the basket catheter used. Each spline is identified by a letter (from A to H) and each electrode by a number (distal 1 to proximal 8). The basket catheter is constructed of a superelastic material to allow passive deployment of the array catheter and optimize endocardial contact. The size of the basket catheter used depends on the dimensions of the chamber to be mapped, and it requires antecedent evaluation (usually by echocardiogram) to ensure proper size selection.

The Astronomer is used for navigation with the ablation-mapping catheter inside the basket catheter. This system consists of a switching-locating device and a laptop computer with proprietary software. The device and the laptop communicate on a standard RS-232 interface. The device delivers AC current (32 kHz, 320 mA) between the ablation catheter tip electrode and a reference electrode (skin patch), and the resulting electrical potentials are sensed at each basket catheter electrode. On the basis of the sensed voltages at each of the basket catheter electrodes, the Astronomer device determines whether the roving electrode is in close proximity to a basket catheter electrode and lights the corresponding electrodes on a representation of the basket catheter displayed on the laptop.

The mapping system consists of an acquisition module connected to a computer, which is capable of simultaneously processing bipolar electrograms from the basket catheter, 16 bipolar-unipolar electrogram signals, a 12-lead ECG, and a pressure signal. Color-coded activation maps are reconstructed on-line. The color-coded animation images simplify the analysis of multielectrode recordings and help establish the relation between activation patterns and anatomical structures (Fig. 6-1). The electrograms and activation maps are displayed on a computer monitor, and the acquired signals can be stored on optical disk for off-line analysis. Activation marks are generated automatically with a peak or slope (dV/dt) algorithm, and activation times are then edited manually as needed.

FIGURE 6-1 Basket catheter mapping during focal atrial tachycardia (AT). A, Simultaneous recordings of the surface ECG leads I and aVF and 56 bipolar electrograms from the basket catheter in a patient with focal AT. The first beat is a sinus beat. The next three beats are tachycardia beats. His bundle (H) potential is recorded in electrode pairs F2/3 and F3/4. The earliest spot of activation during sinus rhythm (SR) and AT is shown (asterisks). Spline A was located in the anterolateral right atrium (RA), splines B and C in the lateral region, splines D and E in the posterior region, and splines G and H across the tricuspid valve. The activation times are marked with red bars. B, Upper panel, Animated map of the SR beats. Lower panel, Animated map of the AT beats. Planar and three-dimensional options are shown. During SR, the impulse emerged in the high lateral area (spline B1-2) and propagated rapidly down the lateral wall. The complete activation of the RA took 85 milliseconds. During focal AT, the earliest activity emerged in the midposterior wall (spline E4-5). The activation sequence of the RA was entirely different from that of SR. The complete activation of the RA took 95 milliseconds. CL = cycle length.

(From Zrenner B, Ndrepepa G, Schneider M, et al: Computer-assisted animation of atrial tachyarrhythmias recorded with a 64-electrode basket catheter. J Am Coll Cardiol 34:2051,1999.)

Mapping Procedure

The size of the cardiac chamber of interest is initially evaluated, usually with echocardiography, to help select the appropriate size of the basket catheter. The collapsed basket catheter is advanced under fluoroscopic guidance through a long sheath into the chamber of interest; the catheter is then expanded (Fig. 6-2). Electrical-anatomical relations are determined by fluoroscopically identifiable markers (spline A has one marker and spline B has two markers located near the shaft of the basket catheter) and by the electrical signals recorded from certain electrodes (e.g., ventricular, atrial, or His bundle electrograms), which can help identify the location of those particular splines.

FIGURE 6-2 Fluoroscopic appearance of the basket catheter in the right atrium (right anterior oblique view). Note the radiopaque electrodes on the splines of the basket catheter. CS = coronary sinus.

From the 64 electrodes, 64 unipolar signals and 32 to 56 bipolar signals can be recorded (by combining electrodes 1-2, 3-4, 5-6, 7-8, or 1-2, 2-3 until 7-8 electrodes are on each spline). Color-coded activation maps can be reconstructed (see Fig. 6-1). The concepts of activation mapping discussed earlier are then used to determine the site of origin of the tachycardia.

The Astronomer navigation system permits precise and reproducible guidance of the ablation catheter tip electrode to targets identified by the basket catheter. Without the use of this navigation system, it is often difficult to identify the alphabetical order of the splines by fluoroscopic guidance.

The electrograms recorded from the basket catheter can be used to monitor changes in the activation sequence in real time and thereby indicate the effects of ablation as lesions are created. The capacity of pacing from most basket electrodes allows the evaluation of activation patterns, pace mapping, and entrainment mapping.

Clinical Implications

The multielectrode endocardial mapping system allows simultaneous recording of electrical activation from multiple sites and fast reconstruction of endocardial activation maps. This can limit the time endured in tachycardia compared with single point mapping techniques without the insertion of multiple electrodes. It also facilitates endocardial mapping of hemodynamically unstable or nonsustained tachycardias. Importantly, the recording of only a single beat can be sufficient to enable analysis of the arrhythmogenic substrate.

Endocardial mapping with a multielectrode basket catheter has been shown to be feasible and safe for various arrhythmias, including AT, AFL, VT, and pulmonary vein (PV) isolation. However, in view of more advanced mapping systems, and because of significant limitations of the current basket catheters, its use has been limited.

Limitations

Because of its poor spatial resolution, the basket catheter in its current iteration has demonstrated only limited clinical usefulness for guiding ablation of reentrant atrial or ventricular arrhythmias. The relatively large interelectrode spacing in available catheters prevents high-resolution reconstruction of the tachycardia and is generally not sufficient for a catheter-based ablation procedure, given the small size and precise localization associated with RF lesions. In addition, the quality of recordings is critically dependent on proper selection of the basket size. Resolution is limited to the proportion of electrodes in contact with the endocardium and by unequal deployment and spacing of the splines. Unfortunately, the electrode array does not expand to provide adequate contact with the entire cardiac chamber; therefore, good electrode contact at all sites on the endocardium is difficult to ensure because of irregularities in the cardiac chamber surface, so that areas crucial to arrhythmia circuit or focus may not be recorded. Moreover, regions such as the right atrial (RA) and left atrial (LA) appendages and cavotricuspid isthmus are incompletely covered by the basket catheter. As a result, arrhythmia substrates involving these structures are not recorded by the basket catheter.

Voltage, duration, and late potential maps are not provided by this mapping approach. Additionally, basket catheter mapping does not permit immediate correlation of activation times to precise anatomical sites, and a second mapping-ablation catheter is still required to be manipulated to the site identified for more precise mapping and localization of the target for ablation, as well as for RF energy delivery. Basket catheters also have limited torque capabilities and limited maneuverability, which hamper correct placement, and they can abrade the endocardium.

Carbonizations occasionally observed after ablation on the splines of the basket catheter can potentially cause embolism. Carbonizations, which appear as dark material attached to the basket catheter electrodes or splines, are thought to be caused by the concentration of RF energy on the thin splines that results in very high local temperatures that induce denaturization of proteins. Carbonization can be greatly diminished with the use of an irrigated tip catheter, as opposed to conventional ablation catheters.

High-Density Mapping Catheter

One study evaluated a mapping approach for AT that uses a novel high-density multielectrode mapping catheter (PentaRay, Biosense Webster, Inc., Diamond Bar, Calif.).¹ This 7 Fr steerable catheter (180 degrees of unidirectional flexion) has 20 electrodes distributed over 5 soft, radiating spines (1-mm electrodes separated by 4-4-4 or 2-6-2 mm interelectrode spacing), thus allowing splaying of the catheter to cover a surface diameter of 3.5 cm. The spines have been given alphabetical nomenclature (A to E), and spines A and B are recognized by radiopaque markers (Fig. 6-3).¹

FIGURE 6-3 The high-density mapping catheter. A, Picture and a fluoroscopic image of the high-density mapping catheter. Below each figure is a schematic representation indicating the orientation of the catheter spines. Note that the marker band on spine A is between electrodes 1 and 2, and on spine B it is between electrodes 2 and 3. B, Vector mapping using the high-density mapping catheter to identify the earliest site of activation of focal atrial tachycardia. Shown is the catheter in four distinct locations within the left atrium (LA). Mapping is commenced along the inferior LA, where the earliest activation is 54 milliseconds ahead of the coronary sinus (CS). The catheter is moved in the direction of the spine demonstrating the earliest activation (spine D). In the midposterior LA, activation precedes the coronary sinus (CS) by 62 milliseconds. Again, the catheter is moved in the earliest direction (spine D) to a more cranial location on the LA roof. At this site, activation precedes the CS by 75 milliseconds. At this site, spine B, which is slightly anterior, is the earliest. Moving the catheter in the direction of spine B to the anterosuperior LA demonstrates the site with the earliest endocardial activation (90 milliseconds ahead of the CS, which was 40 milliseconds ahead of the P wave).

(From Sanders P, Hocini M, Jais P, et al: Characterization of focal atrial tachycardia using high-density mapping, J Am Coll Cardiol. 46:2088, 2005.)

Localization of the atrial focus can be performed during tachycardia or atrial ectopy.¹ Guided by the ECG appearance, the catheter is sequentially applied to the endocardial surface in various atrial regions to allow rapid activation mapping. By identifying the earliest site of activation around the circumference of the high-density catheter, vector mapping is performed, moving the catheter and applying it to the endocardium in the direction of earliest activation (outer bipoles) to identify the tachycardia origin and bracket activation (i.e., demonstrating later activation in all surrounding regions).

The high-density mapping catheter can offer several potential advantages.¹ In contrast to the basket catheter, which provides a global density of mapping but limited localized resolution, the high-density mapping catheter allows splaying of the spines against the endocardial surface to achieve high-density contact mapping and better localized resolution.¹

Fundamental Concepts

The noncontact mapping system (EnSite 3000, St. Jude Medical, Inc., St. Paul, Minn.) consists of a catheter-mounted multielectrode array (MEA), which serves as the probe, a custom-designed amplifier system, a computer workstation used to display 3-D maps of cardiac electrical activity, and a conventional ablation catheter.^2,3

The MEA catheter consists of a 7.5-mL ellipsoid balloon mounted on a 9 Fr catheter around which is woven a braid of 64 insulated 0.003-mm-diameter wires (Fig. 6-4). Each wire has a 0.025-mm break in insulation that serves as a noncontact unipolar electrode.³ The system acquires more than 3000 noncontact unipolar electrograms from all points in the chamber simultaneously. The unipolar signals are recorded using a ring electrode located on the shaft of the array catheter as a reference. The raw far-field EP data acquired by the array catheter are fed into a multichannel recorder and amplifier system that also has 16 channels for conventional contact catheters, 12 channels for the surface ECG, and pressure channels. Using data from the 64-electrode array catheter suspended in the heart chamber, the computer uses sophisticated algorithms to compute an inverse solution to determine the activation sequence on the endocardial surface.

FIGURE 6-4 The multielectrode array for noncontact endocardial mapping.

The EnSite 3000 mapping system is based on the premise that endocardial activation creates a chamber voltage field that obeys the Laplace’s equation. Therefore, when one 3-D surface of known geometry is placed within another of known geometry, if the electrical potential on one surface is known, the potential on the other can be calculated.³ Because the geometry of the balloon catheter is known, and the geometry of the cardiac chamber can be reconstructed during the procedure (see later), endocardial surface potential can then be determined once the potentials over the balloon catheter are recorded. Using this concept, the raw far-field EP data acquired by the array catheter, which are generally lower in amplitude and frequency than the source potential of the endocardium itself and therefore have limited usefulness, are mathematically enhanced and resolved. The solution to the inverse Laplace’s equation using the boundary element method predicts how a remotely detected signal by the MEA would have appeared at its source, the endocardial surface, so that electrograms are reconstructed at endocardial sites in the absence of physical electrode contact at those locations (virtual electrograms). Once the potential field has been established, more than 3000 activation points can be displayed as computed electrograms or as isopotential maps. The activation time at each endocardial site is determined by taking the time instant with maximum negative time derivative (−dV/dt) on the electrogram.

The system can locate any conventional mapping-ablation catheter in space with respect to the array catheter (and thus with respect to the cardiac chamber being mapped).³ A low-current (5.68 kHz) locator signal is passed between the contact catheter electrode being located and reference electrodes on the noncontact MEA. This creates a potential gradient across the array electrodes, which is then used to position the source. This locator system is also used to construct the 3-D computer model of the endocardium (virtual endocardium) that is required for the reconstruction of endocardial electrograms and isopotential maps. This model is acquired by moving a conventional contact catheter around the endocardial surface of the cardiac chamber; the system collects the location information, thus building up a series of coordinates for the endocardium and generating a patient-specific, anatomically contoured model of its geometry. During geometry creation, only the most distant points visited by the roving catheter are recorded to ignore those detected when the catheter is not in contact with the endocardial wall.

Using mathematical techniques to process potentials recorded from the array, the system is able to reconstruct more than 3000 unipolar electrograms simultaneously and superimpose them onto the virtual endocardium, thus producing isopotential maps with a color range representing voltage amplitudes. Additionally, the locator signal can be used to display and track the position of any catheter on the endocardial model (virtual endocardium) and allows marking of anatomical locations identified using fluoroscopy and electrographic characteristics. During catheter ablation procedures, the locator system is used in real time to navigate the ablation catheter to sites of interest identified from the isopotential color maps, catalog the position of RF energy applications on the virtual endocardium, and facilitate revisitation of sites of interest by the ablation catheter.²

In addition, the most recent version of the EnSite software provides the capability of point-to-point contact mapping, to allow the creation of activation and voltage maps by acquiring serial contact electrograms and displaying them on the virtual endocardium (see later). This is useful for adding detail, familiarity, and validation of the information obtained by the noncontact method.⁴

Mapping Procedure

The EnSite 3000 system requires placing a 9 Fr MEA and a mapping-ablation catheter.³ To create a map, the balloon catheter is advanced over a 0.035-inch guidewire under fluoroscopic guidance into the cardiac chamber of interest. For RA arrhythmias, the guidewire is advanced into the SVC and the balloon is deployed in the upper third of the RA for tachycardia originating in the SVC, in the middle third for ectopic AT, and in the lower third for typical AFL. For LA arrhythmias, the guidewire is advanced into the left superior PV, and the balloon is deployed in the middle of the LA. For right ventricular (RV) arrhythmias, the guidewire is advanced into the pulmonary artery, and the balloon is deployed close to the RV outflow tract (RVOT) or in the middle of the RV. The balloon can be filled with contrast dye, thus permitting it to be visualized fluoroscopically (Fig. 6-5). The balloon is positioned in the center of the cardiac chamber of interest and does not come in contact with the walls of the chamber being mapped. In addition, the position of the array in the chamber must be secured to avoid significant movement that would invalidate the electrical and anatomical information. The array must be positioned as closely as possible (and in direct line of sight through the blood pool) to the endocardial surface being mapped, because the accuracy of the map is sensitive to the distance between the center of the balloon and the endocardium being mapped.^2,4,5

FIGURE 6-5 Fluoroscopic views of noncontact mapping catheter (EnSite, St. Jude Medical, Inc., St. Paul, Minn.) situated in the left ventricle (LV). CS = coronary sinus; ICD = implantable cardioverter-defibrillator; LAO = left anterior oblique; RAO = right anterior oblique; RVA = right ventricular apex.

During the use of this mapping modality, systemic anticoagulation is critical to avoid thromboembolic complications. Intravenous heparin is usually given to maintain the activated clotting time longer than 250 seconds for right-sided and longer than 300 seconds for left-sided mapping.

A conventional (roving) deflectable mapping catheter is also positioned in the chamber being mapped and used to collect geometry information. The mapping catheter is initially moved to known anatomical locations, which are tagged. A detailed geometry of the chamber is then reconstructed by moving the mapping catheter and tracing the contour of the endocardium (using the locator technology). To create detailed geometry, attempts must be made to make contact with as much endocardium as possible. This requires maneuvering on all sides of the array, which can be challenging and can require decreasing the profile of the balloon by withdrawing a few milliliters of fluid. This results in the rapid formation of a relatively accurate 3-D geometric model of the cardiac chamber. Creation of chamber geometry can be performed during sinus rhythm or tachycardia.^2,4

Once the chamber geometry has been delineated, tachycardia is induced and mapping is started. The data acquisition process is performed automatically by the system, and all data for the entire chamber are acquired simultaneously. Following this, the segment must be analyzed by the operator to find the early activation during the tachycardia (Fig. 6-6).

FIGURE 6-6 Noncontact mapping of idiopathic right ventricular outflow tract (RVOT) tachycardia. A, Right anterior oblique and right lateral views of color-coded isopotential map of RVOT activation during a single premature ventricular complex (PVC). B, Surface electrocardiographic (white) and intracardiac contact (blue) and virtual noncontact (yellow, V_1-1 through V_1-4) electrograms are shown during the PVC. Inset, Virtual electrograms at the site of earliest activation (note the QS pattern).

The noncontact mapping system is capable of simultaneously reconstructing more than 3360 unipolar electrograms over the virtual endocardium. From these electrograms, isopotential or isochronal maps can be reconstructed (see Fig. 6-6). Because of the high density of data, color-coded isopotential maps are used to depict graphically regions that are depolarized, and wavefront propagation is displayed as a user-controlled 3-D “movie” (Videos 8 and 9 ). The color range represents voltage or timing of onset. The highest chamber voltage is at the site of origin of the electrical impulse. Although the electrode closest to the origin of the impulse is influenced the most, all the electrodes on the array catheter are influenced, the degree of influence diminishing with the distance between the electrode and each endocardial point.

In addition, the system can simultaneously display as many as 32 electrograms as waveforms (see Fig. 6-6). Unipolar or bipolar electrograms (virtual electrograms) can be selected at any given interval of the tachycardia cycle by using the mouse from any part of the created geometry and displayed as waveforms as if from point, array, or plaque electrodes. The reconstructed electrograms are subject to the same electrical principles as contact catheter electrograms, because they contain far-field electrical information from the surrounding endocardium, as well as the underlying myocardium signal vector, and distance from measurement may affect the contribution to the electrogram. These selected unipolar waveforms are used to augment information obtained from the 3-D map by demonstrating the slope of depolarization, the presence of double potentials or fractionation, and differentiation of far-field signals from more relevant endocardial activation.⁴

In the unipolar electrogram, signals associated with high conduction velocity (e.g., the His-Purkinje system) possess a greater slope (–dV/dt), and thus are characterized by high-frequency spectral components (more than 32 Hz). Electrograms recorded in normally conducting atrial or ventricular myocardium possess spectral components in the midrange from 4 Hz to 16 Hz, whereas electrograms in regions of slow conduction are composed of lower frequency spectral components from 1 Hz to 4 Hz. Thus, the high-pass filter must be adjusted between 1 Hz and 32 Hz, helping to modulate the extent to which low-frequency signals are visible on the 3-D display. Identification of true local activation and its differentiation from the far-field signals are essential to successful utilization of noncontact mapping. The true local activation always shows advancement of isopotential lines, but far-field signals would show retraction of isopotential lines on the 3-D display when the whole electrograms are traced.⁵

Substrate mapping based on scar or diseased tissue has been introduced to the noncontact mapping technology. High-density voltage mapping of the atrial substrate is performed using the peak negative voltage of the reconstructed unipolar electrograms. Areas with slow conduction are identified along the reentrant circuit of the atypical AFL. An atrial substrate characterized by an abnormally low peak negative voltage can potentially predict areas with slow conduction during macroreentrant tachycardias, which would provide a substrate for reentry.⁶

Clinical Implications

The MEA has been successfully deployed in all four cardiac chambers by using a transvenous, transseptal, or retrograde transaortic approach to map atrial and ventricular tachyarrhythmias.

The biggest advantage of noncontact endocardial mapping is its ability to recreate the endocardial activation sequence from simultaneously acquired multiple data points over a few (theoretically one) tachycardia beats, without requiring sequential point-to-point acquisitions, thus obviating the need for prolonged tachycardia episodes that the patient might tolerate poorly. This technique can be used to map nonsustained arrhythmias, premature atrial complexes (PACs), premature ventricular complexes (PVCs), and irregular rhythms such as AF or polymorphic VT, and rhythms that are not hemodynamically stable, such as very rapid VT (see Fig. 6-6). The system generates isopotential maps of the endocardial surface at successive cross sections of time, and when these are animated, the spread of the depolarization wave can be visualized. These maps are particularly useful for identifying rapid breakthrough points and slowly conducting macroreentrant pathways, such as the critical slow pathways in ischemic VT or reentrant AT in patients with surgically corrected congenital heart disease. In macroreentrant tachycardias such as typical AFL or VT, the reentry circuit can be fully identifiable, along with other aspects, such as the slowing, narrowing, and splitting of activation wavefronts in the isthmus. The system can also map multiple cardiac cycles in real time, a method that discloses changes in the activation sequence from one beat to the next. Because mapping data are acquired without direct contact of conventional electrode catheters with the endocardium, the use of noncontact mapping can help avoid the mechanical induction of ectopic activity that is frequently seen during conventional mapping. An additional advantage of this system is that any catheter from any manufacturer can be used in conjunction with this mapping platform. Other useful features include radiation-free catheter navigation, revisitation of points of interest, and cataloging ablation points on the 3-D model.²

In complex substrate-related arrhythmias, the use of activation mapping alone may not be sufficient for rhythm analysis or identifying ablation targets. Substrate mapping based on scar or diseased tissue is of value in these cases. Although substrate mapping used to be relatively limited with the noncontact mapping technology (very low-amplitude signals may not be detected, particularly if the distance between the center of the balloon catheter and endocardial surface exceeds 40 mm), dynamic substrate mapping, which has been introduced, allows the creation of voltage maps from a single cardiac cycle (in contrast to the contact mapping system, in which the mapping catheter is moved point to point over the endocardial surface). Dynamic substrate mapping also provides the capability of identifying low-voltage areas, as well as fixed and functional block, on the virtual endocardium through noncontact methods, provided that points more than 40 mm from the electrode array are excluded from analysis. Combining substrate mapping with the ability of the noncontact system to assess activation over a broad area from a single beat may facilitate ablation of hemodynamically unstable or nonsustained macroreentrant tachycardias.^4,6,7

Limitations

Because the geometry of the cardiac chamber is contoured at the beginning of the study during sinus rhythm, changes of the chamber size and contraction pattern during tachycardia or administration of medications (e.g., isoproterenol) can adversely affect the accuracy of the location of the endocardial electrograms. Moreover, because isopotential maps are predominantly used, ventricular repolarization must be distinguished from atrial depolarization and diastolic activity. Early diastole can be challenging to map during VT.⁸

The overall accuracy of the reconstructed electrograms decreases with the distance of the area mapped from the array catheter, thus creating problems in mapping large cardiac chambers. Virtual electrogram quality deteriorates at a distance of more than 4 cm from the array catheter and at polar regions. Therefore, the array must be positioned as closely as possible to the endocardial area of interest, and at times it can be necessary to reposition the array catheter to acquire adequate isopotential maps.

Only data segments up to a maximum of 10 seconds in length can be stored retroactively by the EnSite system once the record button has been pushed. Thus, continuous recording and storage of all segments of an arrhythmia are not possible at the time of evaluation of the arrhythmia maps. Therefore, some isolated PACs or PVCs that can be of interest during evaluation and mapping could be missed. In addition, the acquired geometry with the current version of software is somewhat distorted, requiring multiple set points to establish the origin and shape of complicated structures such as the LA appendage or PVs clearly. Otherwise, these structures can be lost in the interpolation among several neighboring points. In addition, synchronized mapping of multiple chambers requires multiple systems. Importantly, maps are highly sensitive to changes in filtering frequencies used in postprocessing analysis.

Sometimes it is difficult to manipulate the ablation catheter around the outside of the balloon, especially during mapping in the LA. Special attention and care also are necessary during placement of the large balloon electrode in a relatively small cardiac chamber. Another disadvantage is that the balloon catheter cannot be moved after completion of geometry creation because it will change the activation localization and result in distortion of isopotential maps.⁵

Although the risk of complications is low, aggressive anticoagulation measures because of balloon deployment in the cardiac chamber expose patients to potential bleeding complications.

Fundamental Concepts

The CARTO mapping system (Biosense Webster, Inc.) consists of an ultralow magnetic field emitter, a magnetic field generator locator pad (placed beneath the operating table), an external reference patch (fixed on the patient’s back), a deflectable 7 Fr quadripolar mapping-ablation catheter with a 4- or 8-mm tip and proximal 2-mm ring electrodes, location sensors inside the mapping-ablation catheter tip (the three location sensors are located orthogonally to each other and lie just proximal to the tip electrode, totally embedded within the catheter), a reference catheter (placed intracardially), a data processing unit, and a graphic display unit to generate the electroanatomical model of the chamber being mapped.

CARTO is a nonfluoroscopic mapping system that uses a special catheter to generate 3-D electroanatomical maps of the heart chambers. This system uses magnetic technology to determine the location and orientation of the mapping-ablation catheter accurately while simultaneously recording local electrograms from the catheter tip. By sampling electrical and spatial information from different endocardial sites, the 3-D geometry of the mapped chamber is reconstructed in real time and analyzed to assess the mechanism of arrhythmia and the appropriate site for ablation.²

Electroanatomical mapping is based on the premise that a metal coil generates an electrical current when it is placed in a magnetic field. The magnitude of the current depends on the strength of the magnetic field and the orientation of the coil in it. The CARTO mapping system uses a triangulation algorithm similar to that used by a global positioning system (GPS). The magnetic field emitter, mounted under the operating table, consists of three coils that generate a low-intensity magnetic field, approximately 0.05 to 0.2 gauss, which is a very small fraction of the magnetic field intensity inside a magnetic resonance imaging (MRI) machine (Fig. 6-7).

FIGURE 6-7 CARTO (Biosense Webster, Inc., Diamond Bar, Calif.) electroanatomical map setup. The three hemispheres represent fields from three different electromagnets situated beneath the patient. The catheter tip contains an element that is sensed by these fields, and this triangulating information is used to monitor the location and orientation of the catheter tip in the heart.

The sensor embedded proximal to the tip of a specialized mapping catheter detects the intensity of the magnetic field generated by each coil and allows for determination of its distance from each coil. These distances determine the area of theoretical spheres around each coil, and the intersection of these three spheres determines the location of the tip of the catheter. The accuracy of determination of the location is highest in the center of the magnetic field; therefore, it is important to position the location pad under the patient’s chest. In addition to the x, y, and z coordinates of the catheter tip, the CARTO system can determine three orientation determinants—roll, yaw, and pitch—for the electrode at the catheter tip. The position and orientation of the catheter tip can be seen on the screen and monitored in real time as the catheter moves within the electroanatomical model of the chamber mapped. The catheter icon has four color bars (green, red, yellow, and blue), enabling the operator to view the catheter as it turns clockwise or counterclockwise. In addition, because the catheter always deflects in the same direction, each catheter will always deflect toward a single color. Hence, to deflect the catheter to a specific wall, the operator should first turn the catheter so that this color faces the desired wall.²

The unipolar and bipolar electrograms recorded by the mapping catheter at each endocardial site are archived within that positional context. Using this approach, local tissue activation at each successive recording site produces activation maps within the framework of the acquired surrogate geometry.

When mapping the heart, the system can deal with four types of motion artifacts: cardiac motion (the heart is in constant motion; thus the location of the mapping catheter changes throughout the cardiac cycle), respiratory motion (intrathoracic change in the position of the heart during the respiratory cycle), patient motion, and system motion. Several steps are taken by the CARTO mapping system to compensate for these possible motion artifacts and to ensure that the initial map coordinates are appropriate, including using a reference electrogram and an anatomical reference.

Window of Interest

Defining an electrical window of interest is a crucial aspect in ensuring the accuracy of the initial map coordinates. The window of interest is defined as the time interval relative to the fiducial point during which the local activation time is determined (Fig. 6-8). Within this window, activation is considered early or late relative to the reference. The total length of the window of interest should not exceed the tachycardia cycle length (CL; usually 90% of the tachycardia CL; Videos 11 and 12 ). The boundaries are set relative to the reference electrogram. Thus, the window is defined by two intervals, one extending before the reference electrogram and the other after it. For macroreentrant circuits, the sensing window should approximate the tachycardia CL, and designating activation times in a circuit as early or late is arbitrary. In theory, a change in the window or reference would not change a macroreentrant circuit but only result in a phase shift of the map. If the activation window spans two adjacent beats of an arrhythmia, the resulting map can be ambiguous, lack coherency, and give rise to a spurious pattern of adjacent regions of early and late activation (see Fig. 6-8).^9,10

FIGURE 6-8 CARTO (Biosense Webster, Inc., Diamond Bar, Calif.) window of interest and influence on activation map. Top, Activation maps of macroreentrant typical atrial flutter (cycle length [CL], 268 milliseconds, with “clockwise” rotation around the tricuspid annulus). Bottom, So-called window of interest for the atrial flutter. Figures on left half show results of having the window too wide (spanning more than one tachycardia cycle); the computer picks an inappropriately “early” activation time (−365 milliseconds, more than the flutter CL), thus yielding a map (above) that makes little sense. When the same site has correct activation time (−122 milliseconds) assigned by narrowing the window (lower right), the color activation map clearly reveals peritricuspid reentry.

Local Activation Time

Another important concept in CARTO mapping is determination of the local activation time. Once the reference electrogram, anatomical reference, and window of interest have been chosen, the mapping catheter is moved from point to point along the endocardial surface of the cardiac chamber being mapped (Fig. 6-9; see Video 10 ). These points can be acquired in a unipolar or bipolar configuration. These electrograms are analyzed using the principles of activation mapping discussed in Chapter 5. The local activation time at each sampled site is calculated as the time interval between the fiducial point on the reference electrogram and the corresponding local activation determined from the unipolar or bipolar local electrogram recorded from that site.²

FIGURE 6-9 Screen shot of the CARTO (Biosense Webster, Inc., Diamond Bar, Calif.) electroanatomical mapping system. A and B, Left anterior oblique and right anterior oblique views of the activation maps of a left atrial tachycardia (AT). C, List of saved mapping points selected by the operator. D, Selection of surface ECG leads (V1), reference electrogram (R1-R2), and local activation (M1-M2) recorded at the tip of the ablation catheter within the window of interest. E, Local electrogram amplitude and local activation time relative to the reference electrogram. Other panels on screen are various controls and indicators for the CARTO system.

CARTOMerge

The CARTOMerge Module (Biosense Webster, Inc.) allows for images from a preacquired computed tomography (CT) angiogram or MRI scan to be integrated on the electroanatomical image of the cardiac chamber created with the CARTO system (Fig. 6-10).¹¹ CARTOMerge helps to guide real-time catheter ablation by using the detailed cardiac chamber anatomy acquired from the CT/MRI (see later discussion). These images have also been used in AF catheter ablation procedures to identify correctly the anatomy and location of the PVs, as well as other structures such as the esophagus (Video 13) .

FIGURE 6-10 Integration of CT and electroanatomical mapping data. Right anterior oblique (RAO) and left anterior oblique (LAO) views of the electroanatomical contour acquired with catheter manipulation in the left atrium (LA) during atrial fibrillation ablation are shown overlaid on a CT image of the LA acquired several days earlier. Small red circles are tagged sites at which radiofrequency energy was applied to isolate the pulmonary vein (PV) antra. MA = mitral annulus.

CARTOSound

The CARTOSound Image Integration Module (Biosense Webster, Inc.) incorporates the electroanatomical map to a map derived from intracardiac echocardiography (ICE) and allows for 3-D reconstruction of the cardiac chambers using real-time ICE. Echocardiographic imaging is performed using a 10 Fr phased-array transducer catheter incorporating a navigation sensor (SoundStar, Biosense Webster, Inc.) that records individual 90-degree sector image planes of the cardiac chamber of interest, including their location and orientation, to the CARTO workspace. A 3-D volume-rendered image is created by obtaining ECG-gated ICE images of the endocardial surface of the cardiac chamber of interest (Fig. 6-11). Following optimizing each image by adjusting frequency (5 to 10 MHz) and contrast, the chamber endocardial surfaces are identified (based on differences in the echo intensity of blood and tissue), and their contours are traced automatically, and overwritten by hand as necessary, using the CARTOSound software. The software then resolves each contour into a series of discrete spatial points, with an interpoint spacing of up to 3 millimeters (closer spacing on curved contours or at angulations). The CARTO software interpolates these points to create models of the chamber endocardial surface in the CARTO workspace. CARTOSound allows for detailed real-time visualization of the cardiac chamber and of its adjacent structures and elimination of chamber deformity, which often happens with contact mapping (Video 14 ). The CARTOSound volume map of the cardiac chamber may be used as a stand-alone tool to guide navigation and ablation or as a facilitator of CT/MRI image integration (see later).^12,13

FIGURE 6-11 The CARTOSound Image Integration Module (Biosense Webster, Inc., Diamond Bar, Calif.). A, Intracardiac echocardiographic (ICE) image showing the left atrium (LA) and left inferior pulmonary vein (LIPV) obtained using a 10 Fr phased-array transducer catheter. The endocardial surfaces of the LA and LIPV are traced. B, Three-dimensional geometry of the LA is reconstructed by interpolation of points on the traced endocardial surface from multiple ICE images. C, CARTO 3 anatomical reconstruction of the LA (posterior view). Note that the circular and ablation catheters are visualized in the LIPV. D, Integration of the CARTOSound volume and the electroanatomical maps of the LA.

CARTO 3

The CARTO 3 system is the third-generation platform from Biosense Webster that offers three unique features: Advanced Catheter Location Technology, Fast Anatomical Mapping (FAM), and a streamlined workflow feature set referred to as Connection of Choice. Advanced Catheter Location technology is a hybrid technology that combines magnetic location technology and current-based visualization data to provide accurate visualization of multiple catheter tips and curves on the electroanatomical map. It can visualize up to five catheters (with and without the magnetic sensors) simultaneously with clear distinction of all electrodes. Three coils generate a magnetic field, and a location sensor in the catheter measures the strength of the field and the distance from each coil. The location of the sensor in the catheter is determined by the intersection of the three fields. In addition to this magnetic field, CARTO 3 uses an electrical field created by two sets of patches. The magnetic technology calibrates the current-based technology and thereby minimizes distortions at the periphery of the electrical field. The system generates a small current that is sent from the electrodes of the catheters to six patches on the patient’s thorax. Each electrode emits current at a unique frequency. The strength of the current emitted by each electrode is measured at each patch and creates a current ratio that is unique to each electrode’s location.

Mapping is performed in two steps. Initially, the magnetic mapping permits precise localization of the catheter with the sensor. This is associated with the current ratio of the electrode closest to the sensor. As the catheter with the sensor moves around a chamber, multiple locations are created and stored by the system. The system integrates the current-based points with their respective magnetic locations, resulting in a calibrated current-based field that permits accurate visualization of catheters and their locations. Each electrode emits a unique frequency that provides clear distinction of the electrodes, especially when they are close to each other. Fast Anatomical Mapping is a feature that permits rapid creation of anatomical maps by movement of a sensor-based catheter throughout the cardiac chamber. Unlike point-by-point electroanatomical mapping, volume data can be collected with Fast Anatomical Mapping (Fig. 6-12; Video 15 ). Catheters other than the ablation catheter, such as the multipolar Lasso, can further enhance the collection of points and increase the mapping speed. The CARTO 3 system provides highly accurate geometry of a cardiac chamber, which can be visualized in multiple views. Connection of Choice is enabled by a CARTO hardware configuration featuring a central connection point for all catheters and equipment while preserving the signal quality of intracardiac electrograms. Catheter connections have been redesigned for “plug-and-play” functionality and automatic catheter recognition.

FIGURE 6-12 Electroanatomical CARTO 3 and CARTO XP (Biosense Webster, Inc., Diamond Bar, Calif.) activation maps of the right atrium in the left anterior oblique view constructed during typical atrial flutter. A, CARTO 3 activation map during tachycardia. The depolarization wavefront travels counterclockwise around the tricuspid annulus (TA), as indicated by a continuous progression of colors (from red to purple) with close proximity of earliest and latest local activation (red meeting purple). Purple dots denote ablation lesions delivered across the cavotricuspid isthmus. B, CARTO XP during clockwise typical tachycardia is shown. Note the difference between the two systems in geometry reconstruction of the cardiac chamber and vascular structures. CS = coronary sinus; IVC = inferior vena cava; SVC = superior vena cava.

Mapping Procedure

Following selection of the reference electrogram, positioning of the anatomical reference, and determination of the window of interest, the mapping catheter is positioned in the cardiac chamber of interest under fluoroscopic guidance. The CARTO system requires the use of a special Biosense Webster catheter with a location sensor embedded in its distal end. The 7 Fr quadripolar catheters come with a deflectable tip in one or two directions in a single plane and various deflectable curve sizes; some of these catheters have asymmetrical bidirectional deflectable curves. The distal tip is capable of RF energy delivery. Catheters with 4- or 8-mm-tip conventional RF ablation electrodes are available, as well as 3.5-mm irrigated tip ablation catheters.

The mapping catheter is initially positioned (using fluoroscopy) at known anatomical points that serve as landmarks for the electroanatomical map. For example, to map the RA, points such as the superior vena cava (SVC), inferior vena cava (IVC), His bundle, tricuspid annulus, and CS ostium (CS os) are marked. The catheter is then advanced slowly around the chamber walls to sample multiple points along the endocardium, thus sequentially acquiring the location of its tip together with the local electrogram.²

Points are selected only when the catheter is in stable contact with the wall. The system continuously monitors the quality of catheter-tissue contact and local activation time stability to ensure validity and reproducibility of each local measurement. The stability of the catheter and contact is evaluated at every site by examining the following: (1) local activation time stability, defined as a difference between the local activation calculated from two consecutive beats of less than 2 milliseconds; (2) location stability, defined as a distance between two consecutive gated locations of less than 2 mm; (3) morphological superpositioning of the intracardiac electrogram recorded on two consecutive beats; and (4) CL stability, defined as the difference between the CL of the last beat and the median CL during the procedure. Respiratory excursions that can cause significant shifts in apparent catheter location can be addressed by visually selecting points during the same phase of the respiratory cycle.

Each selected point is tagged on the 3-D map. The local activation time at each site is determined from the intracardiac bipolar electrogram and is measured in relation to the fixed reference electrogram (see Video 10 ). Lines of block (manifest as double potentials) are tagged for easy identification because they can serve as boundaries for subsequent design of ablation strategies. Electrically silent areas (defined as having an endocardial potential amplitude less than 0.05 mV, which is the baseline noise in the CARTO system and the absence of capture at 20 mA) and surgically related scars are tagged as “scar” and therefore appear in gray on the 3-D maps and are not assigned an activation time (see Figs. 14-1 and 14-2). The map can also be used to catalog sites at which pacing maneuvers are performed during assessment of the tachycardia.

Sampling the location of the catheter together with the local electrogram is performed from a plurality of endocardial sites. The points sampled are connected by lines to form several adjoining triangles in a global model of the chamber. Next, gated electrograms are used to create an activation map, which is superimposed on the anatomical model. The acquired local activation times are then color-coded and superimposed on the anatomical map with red indicating early-activated sites, blue and purple late-activated areas, and yellow and green intermediate activation times (see Figs. 11-17, 11-19, and 12-10). Between these points, colors are interpolated, and the adjoining triangles are colored with these interpolated values. However, if the points are spaced widely apart, no interpolation is done. The degree to which the system interpolates activation times is programmable (as the triangle fill threshold) and can be modified if necessary. As each new site is acquired, the reconstruction is updated in real time to create a 3-D chamber geometry color progressively encoded with activation time.²

Sampling an adequate number of homogeneously distributed points is necessary. If a map is incomplete, bystander sites can be mistakenly identified as part of a reentrant circuit. Regions that are poorly sampled have activation interpolated between widely separated points. This can give the appearance of conduction, but critical features such as lines of block can be missed. In addition, low-resolution mapping can obscure other interesting phenomena, such as the second loop of a dual-loop tachycardia. Some arrhythmias, such as complex reentrant circuits, require more than 80 to 100 points to obtain adequate resolution. Other tachycardias can be mapped with fewer points, including focal tachycardias and some less complex reentrant arrhythmias, such as isthmus-dependent AFL.^9,10

It is also important to identify areas of scar or central obstacles to conduction; failure to do so can confuse an electroanatomical map because interpolation of activation through areas of conduction block can give the appearance of wavefront propagation through, rather than around, those obstacles. This occurrence precludes identification of a critical isthmus in reentrant arrhythmias to target for ablation. A line of conduction block can be inferred if there are adjacent regions with wavefront propagation in opposite directions separated by a line of double potentials or dense isochrones.⁹

The electroanatomical model, which can be seen in a single view or in multiple views simultaneously and freely rotated in any direction, forms a reliable road map for navigation of the ablation catheter. Any portion of the chamber can be seen in relation to the catheter tip in real time, and points of interest can easily be revisited even without fluoroscopy. The electroanatomical maps can be presented in two or three dimensions as activation, isochronal, propagation, or voltage maps.

Activation Map

The activation maps display the local activation time color-coded overlaid on the reconstructed 3-D geometry (see Fig. 6-8; also see Figs. 11-17, 11-19, and 12-10). Activation mapping is performed to define the activation sequence. A reasonable number of points homogeneously distributed in the chamber of interest must be recorded. The selected points of local activation time are color-coded—red for the earliest electrical activation areas and orange, yellow, green, blue, and purple for progressively delayed activation areas (see Videos 10 and 11 ). The electroanatomical maps of focal tachycardias demonstrate radial spreading of activation, from the earliest local activation site (red) in all directions, and, in these cases, activation time is markedly shorter than tachycardia CL (see Figs. 11-17 and 11-19). On the other hand, a continuous progression of colors (from red to purple) around the mapped chamber, with close proximity of earliest and latest local activation, suggests the presence of a macroreentrant tachycardia (see Figs. 12-10, 14-1, and 14-2). It is important to recognize that if an insufficient number of points is obtained in this early meets late zone, it may be falsely concluded through the interpolation of activation times that the wavefront propagates in the wrong direction (Fig. 6-13).^2,9,10

FIGURE 6-13 Right atrial electroanatomical (CARTO, Biosense Webster, Inc., Diamond Bar, Calif.) activation map in a patient with prior atrial tachycardia following atrial septal defect repair years before. The view is from the aspect of the right rib margin (upward toward the lateral right atrium). A, The activation pattern suggests a focal process with centrifugal spread of activation from the central red area (169 data points taken). B, With additional detailed mapping below the red area, a return path for a reentrant circuit is evident (178 data points). Entrainment data had already diagnosed macroreentry, but the relatively detailed activation map had missed the small area that became clear on more detailed mapping. CL = cycle length.

Voltage Map

The voltage map displays the peak-to-peak amplitude of the electrogram sampled at each site. This value is color-coded and superimposed on the anatomical model, with red as the lowest amplitude and orange, yellow, green, blue, and purple indicating progressively higher amplitudes (Fig. 6-14). The gain on the 3-D color display allows the user to concentrate on a narrow or wide range of potentials. By diminishing the color scale, as may be required to see a fascicular potential or diastolic depolarization during reentry, larger amplitude signals are eliminated. To visualize the broad spectrum of potentials present during a tachycardia cycle, the scale would be opened up to include an array of colors representing a spectrum of voltages. Local electrogram voltage mapping during sinus, paced, or any other rhythm can help define anatomically correct regions of no voltage (presumed scars or electrical scars), low voltage, and normal voltage. The true range of normal is often difficult to define, however, especially with bipolar recordings, and different criteria have been used. Myocardial scars are seen as low voltage, and their delineation can help in understanding the location of the arrhythmia.

FIGURE 6-14 Electroanatomical (CARTO, Biosense Webster, Inc., Diamond Bar, Calif.) voltage map of the left ventricle in a patient with ventricular tachycardia after anteroapical myocardial infarction. An adjusted voltage scale is shown at right; all sites with voltage lower than 0.5 mV are red on the map, and those with voltage higher than 0.6 mV are purple, with interpolation of color for intermediate amplitudes. The gray area denotes no detectable signal (scar). A large anteroapical infarction is clearly evident. Red circles denote ablation sites.