CARTO Electroanatomic Mapping System

The CARTO mapping system 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, a data processing unit, and a graphic display unit to generate the electroanatomic model of the chamber being mapped.

The CARTO electroanatomic 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 (5 × 10 ⁻⁶ to 5 × 10 ⁻⁵ T) that is a very small fraction of the magnetic field intensity inside a magnetic resonance imaging (MRI) machine ( Fig. 6.1 ).

The CARTO Electroanatomic Mapping System.

(A) 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. (B) The magnetic field emitter, mounted under the operating table, is shown in relation to six electrode patches positioned at the patient’s back and chest monitor (to create an electrical field and allow current-based visualization of catheters not equipped by magnetic sensors). The electroanatomic map is shown in the middle. (C) Color-coded activation map superimposed on the virtual anatomical geometry of the right atrium (left anterior oblique view) during mapping and ablation of typical atrial flutter using the CARTO-3 electroanatomic mapping system. Three standard diagnostic catheters are visualized on the electroanatomic map: Halo catheter positioned around the tricuspid annulus (TA) ; decapolar catheter positioned in the coronary sinus (CS) ; and multielectrode PentaRay catheter positioned at the right atrial posterior wall. The PentaRay and decapolar catheters are equipped with magnetic sensors and can be used to collect electroanatomic data. Current-based data are used to enable visualization of the Halo catheter. IVC , Inferior vena cava.

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 the 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 exact position and orientation of the tip of the catheter, in relation to a reference sensor on the skin. The accuracy of the 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 electroanatomic 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.

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.

CARTO-3.

The CARTO-3 system is the third-generation platform from Biosense Webster that offers two additional features: Advanced Catheter Location Technology and Fast Anatomical Mapping (FAM). 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 electroanatomic map. It allows the visualization of up to five EP catheters (with and without the magnetic sensors) simultaneously with clear distinction of all electrodes. In addition to the previously noted magnetic field, CARTO-3 uses an electrical field created by two sets of patches (three on the patient’s back, three on the chest). The system sends a low-intensity current at a unique frequency that is emitted by various catheter electrodes, and the strength of the current emitted by each electrode is measured at each patch; this creates a current ratio unique to each electrode’s location. The magnetic technology calibrates the current-based technology and thereby minimizes distortions at the periphery of the electrical field. Visualization of catheters is confined into a 3-D virtual area called the “matrix,” which can be built only by using a magnetic sensor-equipped manufacturer-specific catheter ( Figs. 6.1 and 6.2 ).

CARTO-3 Color-Coded Activation Map of Focal Atrial Tachycardia Originating From the Left Atrial Appendage (LAA) .

The ablation catheter is positioned at the roof of the LAA (black arrow) . A shadow of the position of the ablation catheter at the successful ablation site is marked (white arrow) . A multipolar Halo catheter is positioned in the right atrium around the tricuspid annulus and a decapolar catheter is positioned in the coronary sinus (CS) . Shadows of the original position of each of the catheters are marked for reference. Both Halo and CS catheters are not equipped with magnetic sensors and cannot be used to collect electroanatomic data. Visualization of these catheters is enabled by current-based data and is confined into a 3-D virtual area called the “matrix,” which can be built only by using a magnetic sensor-equipped manufacturer-specific catheter. Red dots indicate radiofrequency ablation lesions. LAO , Left anterior oblique view; LIPV , left inferior pulmonary veins; LSPV , left superior pulmonary veins; MA , mitral annulus; RAO , right anterior oblique view; RIPV , right inferior pulmonary veins; RSPV , right superior pulmonary veins.

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 acquired 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 other catheters and their locations. Since each electrode emits a unique frequency, individual electrode locations are distinct, even when they are close to each other. FAM 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 electroanatomic mapping, volume data can be collected with FAM (see Fig. 6.1 ).

CARTO-Merge.

The CARTO-Merge Module allows for images from preacquired computed tomography (CT) angiogram or MRI volume data sets to be integrated on the electroanatomic image of the cardiac chamber created with the CARTO system and simultaneously display them within the same coordinate system ( Fig. 6.3 ). This can be very valuable in guiding real-time catheter ablation using the detailed cardiac chamber anatomy acquired from the CT/MRI.

The CARTO-Merge Image Integration Module.

(A) Posterior view of preacquired segmented three-dimensional (3-D) CT images of the left atrium (LA) registered on the CARTO-3 electroanatomic system (CARTO-Merge Module). The ring catheter is visualized in the left inferior pulmonary vein (PV). (B) Integration of the preacquired CT images (shown in A) with 3-D reconstruction of the LA (posterior view) using the CARTO-3 (fast anatomical mapping [FAM]). (C) Cardioscopic view of the integrated CT and surface reconstructions of the LA. LAA , Left atrial appendage; LSPV , left superior PV; RIPV , right inferior PV; RSPV , right superior PV.

CARTO-Sound.

The CARTO-Sound Image Integration Module incorporates the electroanatomic map to a map derived from intracardiac echocardiography (ICE), and allows for 3-D reconstruction of the cardiac chambers using real-time ICE. ICE is performed using a phased-array transducer catheter incorporating a navigation sensor (SoundStar, Biosense Webster) 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. 6.4 ). Three-second segments of 2-D ICE images are acquired during ECG gating to the P wave during sinus rhythm and to the R wave during AF. Since ICE images are not automatically gated to respiration by the system, images used in the analysis are acquired in the late-expiration to the midexpiration phase. 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 CARTO-Sound software. The contour lines for the chamber of interest are drawn below the border to prevent image bloating. The software then resolves each contour into a series of discrete spatial points, with an interpoint spacing of up to 3 mm (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. CARTO-Sound allows for detailed real-time visualization of the cardiac chamber and of its adjacent structures, and elimination of the chamber deformity that often happens with contact mapping.

The CARTO-Sound Image Integration Module.

(A) Intracardiac echocardiographic image showing the left atrium (LA) and left inferior pulmonary vein (PV) obtained using a 10 Fr phased-array transducer catheter. The endocardial surfaces of the LA and PV 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 left inferior PV. (D) Integration of the CARTO-Sound volume and the electroanatomic maps of the LA. LSPV , Left superior PV; RIPV , right inferior PV; RSPV , right superior PV.

CARTO-Sound has been successfully utilized to facilitate AF catheter ablation by incorporating a real-time ICE volume map of the left atrium (LA) and pulmonary veins (PVs) with the electroanatomic map, either as a stand-alone tool to guide navigation and ablation or as a facilitator of CT/MRI image integration. In addition, studies have shown the feasibility of using CARTO-Sound to define scar boundaries in the left ventricle (LV; identified on ICE imaging by both by wall thickness and motion) to facilitate substrate mapping and ablation of ischemic VT. Of note, when AF ablation is guided by 3-D ICE-derived images, ablation points fall beyond the 3-D ICE-derived surface contour more often than when guided by FAM or merged 3-D ICE-CT volume rendering.

EnSite NavX Electroanatomic Mapping System

The EnSite NavX system consists of a set of three pairs of skin patches—a system reference patch, ECG electrodes, a display workstation, and a patient interface unit. The reference patch is placed on the patient’s abdomen and serves as the electrical reference for the system.

The EnSite NavX combines catheter location and tracking features of the LocaLisa system (Medtronic, Minneapolis, MN, United States) with the ability to create an anatomical model of the cardiac chamber using only a single conventional EP catheter and skin patches. This mapping modality is based on currents across the thorax, developed as originally applied in the LocaLisa system. In contrast to the NavX system, LocaLisa does not allow the generation of 3-D geometry of the heart cavity because catheters and desired anatomical landmarks are displayed in a Cartesian frame of reference. This technology has undergone substantial additional development in the NavX iteration.

For 3-D navigation, six electrodes (skin patches) are placed on the patient’s skin to create electrical fields along three orthogonal axes (x, y, and z). The patches are placed on both sides of the patient (x-axis), the chest and back (y-axis), and the back of the neck and inner left thigh (z-axis). Analogous to the Frank lead system, the three orthogonal electrode pairs are used to send three independent, alternating, low-power currents of 350 mA at a frequency of 5.7 kHz through the patient’s chest in three orthogonal (x, y, and z) directions, with slightly different frequencies of approximately 30 kHz used for each direction, to form a 3-D transthoracic electrical field with the heart at the center. The absolute range of voltage along each axis varies from each other, depending on the volume and type of tissue subtended between each surface-electrode pair. The voltage gradient is divided by the known applied current to determine the impedance field that has equal unit magnitudes in all three axes. Each level of impedance along each axis corresponds to a specific anatomical location within the thorax. As standard catheter electrodes are maneuvered within the chambers, each catheter electrode senses the corresponding levels of impedance, derived from the measured voltage. The mixture of the 30-kHz signals, recorded from each catheter electrode, is digitally separated to measure the amplitude of each of the three frequency components. The three electrical field strengths are calculated automatically via the difference in amplitudes measured from neighboring electrode pairs with a known interelectrode distance for three or more different spatial orientations of that dipole. Timed with the current delivery, NavX calculates the x-y-z impedance coordinates at each catheter electrode by dividing each of the three amplitudes (V) by the corresponding electrical field strength (V/cm), and expresses them in millimeters to locate the catheters graphically in real time to enable nonfluoroscopic navigation. The NavX system allows real-time visualization of the position and motion of up to 128 electrodes on both ablation and standard catheters positioned elsewhere in the heart ( Fig. 6.5 ). Importantly, the relative positions of the electrodes are calculated by assuming that changes in the recorded field potential are only caused by changes in catheter position. Therefore changes in thoracic impedance can cause the system to “drift.”

Electroanatomic Activation Mapping Using the NavX System.

(A) Color-coded activation map superimposed on the virtual anatomical geometry of the right atrium (left anterior oblique view) during mapping and ablation of typical atrial flutter using the NavX electroanatomic mapping system. Two standard diagnostic catheters (a Halo catheter positioned around the tricuspid annulus [TA] , and a decapolar catheter positioned in the coronary sinus [CS]) as visualized by the NavX system during mapping. White dots denote RF ablation points across the cavotricuspid isthmus. (B) The same color-coded activation map is viewed after changing transparency and removing diagnostic catheters. IVC , Inferior vena cava; SVC , superior vena cava.

The NavX system also allows for rapid creation of detailed models of cardiac anatomy. Sequential positioning of a catheter at multiple sites along the endocardial surface of a specific chamber establishes that chamber’s geometry. The system automatically acquires points from a nominated electrode at a rate of 96 points/s. Chamber geometry is created by several thousand points. The algorithm defines the surface by using the most distant points in any given angle from the geometry center, which can be chosen by the operator or defined by the system. In addition, the operator is able to specify fixed points that represent contact points during geometry acquisition; the algorithm that calculates the surface cannot eliminate these points. In addition to mapping at specific points, there is additional interpolation, providing a smooth surface onto which activation voltages and times can be registered (see Fig. 6.5 ). To control for variations related to the cardiac cycle, data acquisition can be gated to any electrogram. Also, electrode positions are averaged over a few seconds to minimize the effect of cardiac motion. Respiratory compensation is collected just before mapping. The algorithm records the movement that occurs with respiration and correlates it with changes in transthoracic impedance to filter low-frequency cardiac shift associated with the breathing cycle.

After creating chamber geometry, a scaling algorithm (field scaling) is applied to compensate for variations in impedance between the heart chambers and venous structures, which can otherwise result in a distortion of the x-y-z coordinates when a “roving” catheter is maneuvered among the differing regions of impedance ( Fig. 6.6 ). Field scaling is based on the measured interelectrode spacing for all locations within the geometry. Adjustments to the local strength of the navigation fields are made so that the computed catheter electrode positions match the known interelectrode spacing of the catheters used to create the geometry.

Field Scaling in the NavX Mapping System.

Three-dimensional geometry model (posterior view) of the left atrium (LA) and pulmonary veins (PVs) created by the NavX electroanatomic mapping system. Ablation and circular catheters are visualized in the left inferior PV. A decapolar catheter is visualized in the coronary sinus (CS). (A) The LA geometry model before field scaling. (B) The same geometry model is shown after applying the field scaling algorithm to compensate for variations in impedance between the LA and PVs. LAA, Left atrial appendage; LSPV , left superior PV; RIPV , right inferior PV; RSPV , right superior PV.

The NavX system works with most manufacturers’ ablation catheters, RF generators, or cryogenerators. Sites of ablation energy application can be tagged, thus facilitating the creation of lines of block with considerable accuracy by serial placement of energy applications and allowing verification of the continuity of the ablation line.

The EnSite Fusion iteration has the capability to integrate images from a preacquired CT/MRI scan on the real-time electroanatomic image the cardiac chamber created with the NavX system to facilitate anatomically based ablation procedures. To allow local adjustment of the EnSite System model, the registration module comes with Dynamic Registration. The system has the ability to mold the created geometry dynamically into the CT/MRI image (see later discussion).

The EnSite Precision iteration adds magnetic navigation capability. Magnetic points are collected with several new magnet-enabled catheters. Magnetic-field stability reduces the effects of “impedance drift,” corrects impedance distortion, and helps optimize catheter navigation and creation of a precise, accurate geometry model.

Rhythmia Electroanatomic Mapping System

Rhythmia is a novel 3-D electroanatomic mapping platform that is paired to a mini-basket array catheter with 64 mini-electrodes (Orion, Boston Scientific) and is capable of generating ultra–high-density electroanatomic maps. This system uses a hybrid location technology that combines impedance and magnetic location. The magnetic field is generated by a localization generator positioned under the patient’s table and is capable of locating catheters with magnetic sensors. The impedance location technology is used to track catheters that are not equipped with a magnetic sensor. The system then maps the impedance field measurements to the magnetic location coordinates and creates an impedance field map. This map is used to enhance the accuracy of the impedance location.

Orion is an 8.5 Fr bidirectional deflectable catheter with a mini-basket electrode array containing eight splines, each spline containing eight small electrodes ( Fig. 6.7 ). The surface area of each electrode is 0.4 mm ² , and the interelectrode spacing is 2.5 mm (measured from center to center). The basket can be deployed into a spherical configuration through mechanical flexion of the splines to varying diameter (minimum 3 mm, nominal 18 mm, maximum 22 mm, when measured at its equator). The location of each electrode is determined using a combination of magnetic sensor located at the tip of the catheter and impedance sensing at each of the electrodes. A flushing mechanism is present at the catheter tip to prevent thrombus formation. Other catheters are tracked by an impedance-based system.

The Orion Mini-Basket Catheter.

The 8-F bidirectional mini-basket catheter is 27 mm long from the catheter shaft to the distal tip of the basket. It consists of 8 splines, each with 8 electrodes. The nominal diameter of the basket is 18 mm when measured at its equator.

(From Anter E, Tschabrunn CM, Contreras-Valdes FM, Li J, Josephson ME. Pulmonary vein isolation using the Rhythmia mapping system: verification of intracardiac signals using the Orion mini-basket catheter. Heart Rhythm . 2015;12:1927–1934.)

Two data acquisition modes are available with the Orion catheter: continuous and manual. In the continuous data acquisition mode, operator-defined criteria for accepting cardiac beats are applied for automated map construction during uninterrupted movement of the catheter, without immediate input from the operator. The electrograms from collected beats are automatically annotated by the system. In the manual data acquisition mode, the operator collects data in an “area-by-area” manner, and manually accepts and annotates selected points.

During continuous, automated data acquisition, points are accepted only if they meet user predefined acceptance criteria such as cycle length (CL) stability, stable timing difference between two reference electrodes, respiration gating (the respiratory cycle is tracked by measuring impedance change across the chest), stable catheter location, stability of catheter signal compared with adjacent points, and morphology matching. The automated algorithms filter out points with discrepancy in comparison to those of close proximity. Far-field components are reduced by combining unipolar and bipolar electrograms.

The setup for the mapping window is automatic. The system calculates the mean CL of the rhythm over 10 seconds and consequently sets 100% of the CL equally before and after the timing reference electrode (usually one of the coronary sinus [CS] electrograms, or the QRS interval of one of the surface ECG leads for ventricular rhythms). For annotation of the local activation time of each acquired electrogram, the system combines unipolar (maximum negative dV/dt) and bipolar (maximum amplitude) electrogram. For electrograms with multiple potentials, the system selects the potential that best matches the timing of electrograms in the surrounding area. The very small size of the electrodes on the Orion catheter minimizes far-field signals and background noise, and allows accurate detection of very small amplitude signals.

The anatomical shell is gradually constructed with every accepted beat based on the location of the outermost electrodes of the basket catheter. Inclusion or exclusion of electrograms into the group of surface electrograms is based on the distance from the surface geometry (1 to 5 mm), which can be set by the operator. The system will automatically delete inner electrogram points as more signals are recorded from a spatially outside location.

All electrograms are stored for later review. Maps can be edited by the operator after data acquisition is complete. The software allows for visualization of the electrogram associated with each anatomic point, facilitating re-annotation or removal of inaccurate data. By selecting individual electrograms with a virtual roving probe, it is possible to determine and mark a region of interest (e.g., His bundle [HB] region). In addition, a cutout of anatomical structures such as the tricuspid annulus can be performed based on the corresponding electrograms in review mode ( Fig. 6.8 ).

Rhythmia Color-Coded Activation Map of Typical Counterclockwise Atrial Flutter (AFL).

Three-dimensional electroanatomic activation map of the right atrium in the right anterior oblique (left) and left anterior oblique (right) views constructed during counterclockwise typical AFL. 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. The mini-basket (Orion) catheter is visualized on the map. CS , Coronary sinus; IVC , inferior vena cava; SVC , superior vena cava.

Anatomical Reference

Once the mapping catheter (or any electroanatomically tracked electrode) is placed inside the heart, its location can be determined in relation to a fixed anatomical reference. This reference catheter is positioned inside the heart or on the body surface, and its location must remain stable throughout the procedure to prevent distortion of the electroanatomic map. The movement of the mapping catheter is then tracked relative to the position of this reference. An intracardiac reference catheter has the advantage of moving with the patient’s body and with the heart during the phases of respiration. However, the intracardiac reference catheter can change its position during the course of the procedure, especially during manipulation of the other catheters.

In the CARTO mapping system, locations of magnetically enabled catheters are displayed in relation to the fixed magnetic field sensors placed under the patient. The CARTO system continuously calculates the position of the mapping catheter in relation to this array of sensors, thus solving the problem of any possible motion artifacts. The movement of the ablation catheter is then tracked relative to the position of this reference. Slight movement of the patient relative to the location reference pad may distort the map; significant patient movement or dislocation of the location pad can lead to uncorrectable map shifts.

In the NavX system, the 3-D localization of all EP catheters is based on an impedance gradient-calculation system in relation to a reference electrode placed on the patient’s body or inside the heart (e.g., CS catheter). Since the reference electrodes and catheters are placed either on the patient’s skin or in the patient’s cardiac chambers, they move simultaneously with the patient, preventing map shifts and rendering the NavX largely insensitive to potential patient movements. The Rhythmia system also uses an anatomical sensor attached to patient’s back.

The Rhythmia mapping system uses a hybrid location technology that combines impedance location with magnetic location technology and uses two location references, one for each localization technology. The magnetic technology uses a location reference attached to the patient’s back, while the impedance technology uses a stationary intracardiac electrode (e.g., a CS electrode) selected by the user.

Electrical Reference

The electrical reference is the fiducial marker on which the entire mapping procedure is based. The timing of the fiducial point is used to determine the activation timing in the mapping catheter in relation to the acquired points and to ensure collection of data during the same part of the cardiac cycle. It is therefore vital to the performance of the system. All the local activation timing information recorded by the mapping catheter at different anatomical locations during mapping (displayed on the completed 3-D map) is relative to this fiducial point, with the acquisition gated so that each point is acquired during the same part of the cardiac electrical signal. It is important that the rhythm being mapped is monomorphic and the fiducial point is reproducible at each sampled site.

The fiducial point is defined by the user by assigning a reference channel and an annotation criterion. The system has a great deal of flexibility in terms of choosing the reference electrogram and gating locations. Any surface ECG lead or intracardiac electrogram in bipolar or unipolar mode can serve as a reference electrogram. For the purpose of stability when intracardiac electrograms are selected, CS electrograms are usually chosen for mapping supraventricular rhythms, and a right ventricle (RV) electrode or a surface ECG lead is commonly chosen as the electrical reference during mapping ventricular rhythms. Care must be taken to ensure the reference electrogram is distinct and stable, and that automatic sensing of the reference is reproducible and is not subject to oversensing in the case of annular electrograms (e.g., oversensing of a ventricular electrogram on the CS reference electrode during mapping an atrial rhythm). Any component of the reference electrogram may be chosen for a timing reference, including maximum (peak positive) deflection, minimum (peak negative) deflection, maximum upslope (dV/dt), or maximum downslope.

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. 6.9 ). Within this window, activation is considered early or late relative to the reference. Timing and voltage of electrograms falling outside this window are excluded from the map and cannot be tagged without altering the window. The total length of the window of interest should not exceed the tachycardia cycle length (TCL; generally 10 or 20 milliseconds less than the TCL). 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 focal tachycardias, the window of interest is usually selected so that it starts about 50 milliseconds before the onset of the tachycardia complex on the surface ECG (P wave or QRS), regardless of the timing of the electrical reference. For macroreentrant tachycardias, the sensing window should approximate the TCL, and designating activation times in a circuit as early or late is arbitrary. If the activation window spans more than one tachycardia cycle, the resulting map can be ambiguous, lack coherency, and give rise to a spurious pattern of adjacent regions of early and late activation ( Figs. 6.9 and 6.10 ). In theory, a shift in the window or electrical reference would not change a macroreentrant circuit but only result in a phase shift of the map ( Fig. 6.11 ).

Influence of Window of Interest on Activation Mapping.

CARTO-3 activation map of a macroreentrant left atrial tachycardia is shown (anterior-cranial view). In (A) the correct window of interest width was chosen encompassing one complete cardiac cycle. A mid-diastolic potential (red arrow) is recorded from the top of the left atrium, with its timing at the left portion of the window (darker vertical panel) , giving rise to an activation map at left with direction of propagation as indicated by the white arrow. In (B) the window of interest is too wide, including portions of two cardiac cycles. It is possible for the activation at this same site to be assigned as indicated by the red arrow, yielding a completely different activation map at left, seeming to show splitting of wavefronts going around the right pulmonary veins (PVs). In (C) the window is too narrow, and while no actual electrogram occurs within the window, the system is obligated to take an activation time (noise, at red arrow ). This again yields a very different activation map as shown at left. LSPV , Left superior PV; LIPV , left inferior PV; RSPV , right superior PV; RIPV , right inferior PV.

Window of Interest and Activation Mapping.

Rhythmia activation map of the right atrium constructed during typical counterclockwise atrial flutter (cycle length [CL], 260 milliseconds) shown in right anterior oblique (left panels) and left anterior oblique (right panels) views. This system offers the ability to change the mapping window in retrospect. (A) The window of interest is correctly set at 255 milliseconds (just shorter than the tachycardia CL), and the color activation map clearly reveals continuous progression of colors (from red to purple ) with close proximity of earliest and latest local activation ( red meeting purple ), consistent with peritricuspid reentry. (B) The window of interest is retrospectively changed to 200 milliseconds (significantly shorter than one tachycardia cycle), yielding an ambiguous map. (C) The window of interest is retrospectively changed to 510 milliseconds (spanning about two tachycardia cycles), yielding an ambiguous map with spurious pattern of activation times. CS, Coronary sinus; IVC , inferior vena cava; SVC , superior vena cava; TA , tricuspid annulus.

Influence of the Electrical Reference on Activation Mapping.

CARTO-3 activation map of the right atrium (left anterior oblique view) constructed during typical atrial flutter (cycle length [CL], 310 milliseconds) with “counterclockwise” rotation around the tricuspid annulus (TA) . (A) The distal bipole of a Halo catheter (positioned at the lateral aspect of the TA, indicated by yellow star ) was selected as the electrical reference (fiducial point) of the activation map. Note that the early-meets-late zone is located at the septal aspect of the right atrium. The window of interest was set at 300 milliseconds (150 milliseconds before to 150 milliseconds after the timing of the electrical reference). (B) The proximal bipole of the coronary sinus (CS) catheter (positioned at the CS ostium, indicated by yellow star ) was selected as the electrical reference for activation mapping of the same tachycardia. Again, the window of interest was set at 150 milliseconds before to 150 milliseconds after the timing of the electrical reference. Now, the early-meets-late zone is located at the lateral aspect of the RA. In both maps, there is continuous progression of colors (from red to purple ) with close proximity of earliest and latest local activation ( red meeting purple ), consistent with counterclockwise peritricuspid reentry. The change in the electrical reference did not cause a change a macroreentrant circuit but only resulted in a phase shift of the map. Note that the early-meets-late zone in both maps does not localize the ablation target (which is the cavotricuspid isthmus in these cases); rather, it is merely a function of where the offset and onset of the window of interest are defined relative to the timing of the selected electrical reference. IVC , Inferior vena cava; SVC , superior vena cava.

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. 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.

In general, the local activation time at each site is determined from the intracardiac bipolar electrogram and is measured in relation to the fixed reference electrogram. For bipolar electrograms, the initial peak of a filtered (30 to 300 Hz) bipolar signal coincides with depolarization beneath the recording electrode and appears to correlate most consistently with local activation time, corresponding to the maximal negative dV/dt of the unipolar recording. However, in the setting of complex multicomponent or fractionated bipolar electrograms, the determination of local activation time becomes challenging, and the decision about which activation time is most appropriate needs to be made in the context of the particular rhythm being mapped. Therefore, during mapping procedures, the onset (rather than the peak or nadir) of a high frequency component of the local bipolar electrogram often is used because it is easier to determine reproducibly, especially when measuring heavily fractionated, low-amplitude local electrograms. The onset of the bipolar electrogram likely precedes the maximal −dV/dt in unipolar electrogram by 15 to 30 milliseconds.

For filtered and unfiltered unipolar electrograms, the maximum negative slope (i.e., maximum change in potential, dV/dt _max ) of the signal coincides best with the arrival of the depolarization wavefront directly beneath the electrode.

Contemporary mapping systems offer automatic data acquisition and timing annotation of accepted points. Using these automated algorithms, timing of local activation is annotated at the point of maximum amplitude of the bipolar signal or the maximum negative dV/dt of the unipolar signal. For electrograms with multiple potentials, the system selects the potential that best matches the timing of electrograms in the surrounding area. When automated timing annotation is used, it is important to utilize the same parameters for the timing of local activation when additional points are acquired manually or during the editing of automatically acquired data.

Data Acquisition

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. The CARTO and Rhythmia systems require the use of proprietary catheters with a location sensor to collect mapping data. In contrast, NavX-guided procedures are performed using the same catheter setup as conventional approaches. Any electrode can be used to gather data, create static isochronal and voltage maps, and perform ablation procedures. Standard EP catheters of choice are introduced into the heart; up to 128 electrodes can be viewed simultaneously. The NavX system can locate the position of the catheters from the moment that they are inserted in the vein. Therefore all catheters can be navigated to the heart under guidance of the EnSite NavX system, and the use of fluoroscopy can be minimized for preliminary catheter positioning. However, interrupted fluoroscopy must be used repeatedly when an obstacle to catheter advancement is encountered. Once in the heart, one intracardiac catheter is used as a reference for geometry reconstruction. A shadow (to record original position) is placed over this catheter to recognize displacement during the procedure, in which case the catheter can be returned easily to its original location under the guidance of NavX. A shadow can also be displayed on each of the other catheters to record that catheter’s exact spatial position at some particular time (e.g., where PV potentials are best seen).

The mapping catheter is initially positioned (using fluoroscopy) at known anatomical points that serve as landmarks in the chamber of interest for the electroanatomic map. For example, to map the right atrium (RA), points such as the superior vena cava (SVC), inferior vena cava (IVC), HB, tricuspid annulus, and 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.

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 the 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 super-impositioning 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. Furthermore, contact force measurement at the tip of the mapping electrode (when available) can help optimize electrode-tissue contact and improve mapping accuracy.

Contemporary mapping systems enable automated data acquisition from the designated mapping catheter. The algorithm automatically accepts and annotates activation times for points that fulfill an operator-defined set of acceptance criteria. Beats are included in the map based on CL stability, relative timing of reference electrograms, electrode location stability, and respiratory gating, among other optional criteria. These algorithms help streamline the mapping and validation process and reduce overall mapping and manual annotation time.

Because respiratory excursions can cause significant shifts in apparent catheter location, respiratory compensation is collected just before mapping to filter low-frequency cardiac shift associated with the breathing cycle. The current iteration of the CARTO mapping system allows for automatic gating to the respiratory cycle.

Each selected point is tagged on the 3-D map. Lines of block (manifest as double potentials) are tagged for easy identification because they can serve as boundaries for the subsequent design of ablation strategies. Electrically silent areas (defined as having an endocardial potential amplitude less than 0.05 mV [the baseline noise in the mapping 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 multiple endocardial sites. Catheters other than the ablation catheter, such as the multipolar Lasso or Penta-Ray, can further enhance the collection of points, increase the mapping speed, and improve map resolution. 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 indicating late-activated areas, and yellow and green indicating intermediate activation times (see Figs. 6.5 and 6.11 ). Between these points, the mapping systems assign an activation time over the area around each acquired point, and the adjoining triangles are colored with these interpolated values. The size of this area is determined by setting the triangle “fill threshold” or “interpolation threshold,” which is adjustable. If the points are spaced widely apart (beyond the fill threshold), no interpolation is done. 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.

Sampling an adequate number of homogeneously distributed points is necessary. If inadequate numbers of points are taken and the fill threshold allows interpolation over a large area, the colors assigned to the poorly mapped areas will not be representative of the actual conduction pattern and activation timing. Thus bystander sites can be mistakenly identified as part of a reentrant circuit, and lines of conduction 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. The use of multielectrode mapping catheters can improve map resolution and expedite the mapping process.

It is also important to identify areas of scar or central obstacles to conduction; failure to do so can confuse an electroanatomic 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.

The electroanatomic model, which can be viewed 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 electroanatomic maps can be presented in two or three dimensions as activation, isochronal, propagation, or voltage maps.

Activation Map

During mapping, the electrogram obtained at a given site is stored and the activation time catalogued as compared with the designated reference electrogram. The accrued points in the map are assigned to an isochronal color scale based on their respective activation times. The activation maps display the local activation time, color-coded and overlaid on the reconstructed 3-D geometry ( see Figs. 11.21 and 12.14 ). Each color shift represents a temporal fraction of the entire TCL. 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.

The electroanatomic maps of focal tachycardias demonstrate radial spreading of activation, from the earliest local activation site in all directions, and in these cases, activation time is markedly shorter than TCL ( Fig. 6.2 ). In contrast, a continuous progression of colors around the mapped chamber, with close proximity of earliest and latest local activation (“early-meets-late” zone), suggests the presence of a macroreentrant tachycardia (see Fig. 6.11 ). Importantly, the early-meets-late zone should not be used as an indicator of the location of the critical isthmus of the macroreentrant circuit (which is the usual ablation target). Rather, it is merely a function of where the offset and onset of the window of interest are defined relative to the timing of the selected reference electrogram. As noted previously, the early-meets-late zone can shift in location and timing in response to shifts in the window of interest or electrical reference (see Figs. 6.8 and 6.11 ).

It is also important to recognize that a focal tachycardia can produce an electroanatomic activation map that mimics reentry when anatomical or functional barriers to conduction close to the site of origin of the tachycardia (such as anisotropic conduction, scars, incisions, or prior ablation lines) cause the delay of wavefront propagation to span the entire TCL and arrive late to sites close to the focus of the tachycardia. For example, a focal AT originating from the coronary sinus ostium (CS os) in a patient with prior cavotricuspid isthmus (CTI) ablation can produce an electroanatomic map mimicking that of peritricuspid macroreentry (counterclockwise typical AFL; Fig. 6.12 ). When the “early-meets-late zone” is observed in an activation map that does not span the entire TCL, inadequate mapping should be suspected and more detailed mapping should be performed before concluding macroreentry as the mechanism of the tachycardia.

Focal Versus Macroreentrant Atrial Tachycardia (AT) in a Patient With Prior Cavotricuspid Isthmus Ablation.

A right atrial electroanatomic map during AT (tachycardia cycle length [TCL] 390 milliseconds) is shown. At left, the activation pattern (arrows) suggests typical counterclockwise atrial flutter; mapped sites account for 384 milliseconds (98% of TCL). Overdrive pacing had suggested a focal automatic process; additional mapping showed a focal site at the coronary sinus (CS) ostium (right panel, asterisk) where ablation terminated AT. A revised pattern of activation shows propagation in both directions from the focus. IVC, Inferior vena cava; SVC , superior vena cava.

On the other hand, a macroreentrant tachycardia can produce an electroanatomic activation map that mimics a focal mechanism. If an insufficient number of activation points is obtained, it may be falsely concluded through the interpolation of activation times that the wavefront propagates from a focal source ( see Fig. 13.11 ). This is frequently encountered when the macroreentrant tachycardia originates from the chamber contralateral to the one being mapped. In the latter situation, the activation map will localize the site of the earliest local activation to the earliest breakthrough of conduction into the chamber being mapped. Notably this breakthrough location, although showing the earliest recorded activation timing relative to the intracardiac electrical reference, may not be presystolic (as compared with the onset of the P wave or QRS on the surface ECG) and, hence, cannot be the site of origin of a focal tachycardia. This provides an additional clue to help interpret the activation map and should prompt more detailed mapping. Similarly, a focal pattern can be observed during endocardial mapping when significant parts of the macroreentry circuits are located intramurally or epicardially.

Isochronal Map

The electroanatomic mapping system can generate isochrones of electrical activity as color-coded static maps. The isochronal map depicts all the points with an activation time within a specific range (e.g., 10 milliseconds) with the same color. Depending on conduction velocity, each color layer is of variable width; isochrones are narrow in areas of slow conduction and broad in areas of fast conduction. Displaying information as an isochronal map helps demonstrate the direction of wavefront propagation, which is perpendicular to the isochronal lines, along the vector of the color changes. Furthermore, isochronal crowding (i.e., multiple colors evident over a small distance) indicating a conduction velocity of 0.033 cm/msec (slower than 0.05 cm/msec) is considered a zone of slow conduction, whereas a collision of two wavefronts traveling in different directions separated temporally by 50 milliseconds is defined as a region of local block. Spontaneous zones of block or slow conduction (less than 0.033 cm/msec) may have a major role in the stabilization of certain arrhythmias.

Propagation Map

Activation mapping data can be displayed in a color-coded animated dynamic map of activation wavefront (propagation map; see Figs. 11.21 and 12.13 ). Propagation of electrical activation is visualized superimposed on the 3-D anatomical reconstruction of the cardiac chamber in relation to the anatomical landmarks and barriers. Analysis of the propagation map can allow estimation of the conduction velocity along the reentrant circuit and identification of areas of slow conduction.

EnSite Precision enables the visualization of propagation of the activation wavefront over voltage maps, which better illustrate activation patterns in relation to regions of electrical scar ( Fig. 6.13 ).

Propagation Map of Typical Atrial Flutter (AFL).

Activation and voltage maps of the right atrium are acquired during typical counterclockwise AFL using the EnSite Precision mapping system. Voltage maps are shown in the left anterior oblique projection. Red color corresponds to endocardial atrial bipolar potentials with an amplitude of ≤0.1 mV; purple color corresponds to electrogram amplitude of ≥1 mV. (A to D) Propagation of the activation wavefront is visualized over the voltage map as bright green dots (“sparkles”) traveling in a counterclockwise direction around the tricuspid annulus (TA) , as indicated by the white arrows. CS , Coronary sinus; IVC , inferior vena cava; SVC , superior vena cava.

Entrainment Map

A graphical representation of entrainment mapping can be constructed by plotting values of the differences between the postpacing intervals (PPIs) and the TCLs (PPI–TCL) on the electroanatomic mapping system to generate color-coded 3-D entrainment maps ( Fig. 6.14 ). This approach can potentially help accurately determine and visualize the 3-D location of the entire reentrant circuit, even though the area of slow conduction of the tachycardia is not specified. Because none of the electroanatomic mapping systems contain an algorithm for color-coding of entrainment information, the modus for activation mapping is altered manually. At each 3-D location of the catheter tip stored on the electroanatomic mapping system, entrainment stimulation is performed, and the difference between PPI and TCL is calculated and associated with that site on the electroanatomic mapping system (as if it were an “activation time”). For that, the local electrogram stored at the 3-D location is completely disregarded. The annotation marker is manually moved into a position where the numeric timing information equals the entrainment information (PPI–TCL). That timing information then is displayed in a color-coded fashion as if it were activation time, but instead it represents information on the length of the entrainment return cycle. With the color range, red represents points closest to the reentrant circuit (i.e., sites with smaller PPI–TCL differences, approaching 0, signifying their inclusion in the reentrant circuit) and purple represents points far away from the circuit (i.e., sites with the largest PPI–TCL differences).

Electroanatomic Color-Coded Entrainment Mapping.

Left atrial (LA) electroanatomic color-coded entrainment map in a patient with atrial tachycardia following pulmonary vein isolation for treatment of atrial fibrillation. The LA is shown in various attitudes as indicated. The colors represent the difference between the postpacing interval (PPI) at each site following entrainment pacing and the tachycardia cycle length (TCL): the less the difference, the closer to the circuit. In the top row, red and orange areas have minimal difference between the PPI and TCL (i.e., in or near circuit), while purple areas have large differences (i.e., remote from circuit). Bottom row shows the same data with the timing scale set such that 0 to 30 milliseconds is red (i.e., in peri-right pulmonary veins circuit). Note that not all areas that are in the circuit are reasonable ablation target areas. MA , Mitral annulus; RIPV , right inferior pulmonary veins; RSPV , right superior pulmonary veins.

Color-coded 3-D entrainment mapping allows determination of the full active reentrant circuit (vs. passively activated regions of the chamber) and the obstacle around which the tachycardia is circulating, and it provides very useful information on the location of potential ablation sites (see Fig. 6.14 ). However, ablation will not terminate reentry at all these sites. (Just as, although the circuit in orthodromic supraventricular tachycardia includes the ventricle, ablation at one or two sites in that ventricle will not eliminate reentry.) The final choice is determined by the location of anatomical barriers and width of putative isthmuses, so that strategic ablation lines, mainly connecting anatomical barriers, can be applied to transect the circuit and treat the arrhythmia.

Limitations of Electroanatomic Activation Mapping

Although 3-D mapping systems with image integration have been widely adopted for ablation procedures, many of their theoretical benefits remain to be proven. Therefore these systems should remain just one type among the tools facilitating complex catheter ablation procedures and should not distract the electrophysiologist from established EP principles and endpoints.

The sequential data acquisition required for map creation remains very time-consuming because the process of creation of an electroanatomic map requires tagging many points, depending on the spatial details needed to analyze a given arrhythmia. Because the acquired data are not coherent in time, multiple beats are required, and stable, sustained, or frequently repetitive arrhythmia is usually needed for creation of the activation map. Given that these points do not provide real-time, constantly updated information, more time may be needed for making new maps to see a current endocardial activation sequence, detect a change in arrhythmia, or fully visualize multiple tachycardias. In addition, rapidly changing or transient arrhythmias are not easily recorded and may be mapped only if significant substrate abnormalities are present. For macroreentrant tachycardias, variation of the TCL by more than 10% can prevent complete understanding of a circuit, and it decreases the confidence in the electroanatomic activation map. Single premature ventricular complexes (PVCs) or premature atrial complexes (PACs) or nonsustained events may be mapped, although at the expense of an appreciable amount of time. The use of multielectrode catheters for data acquisition helps address many of these issues.

One difficulty with current methods is that incorrect assignment of activation for a few electrograms can invalidate the entire activation map, and manual adjustment is often required to achieve the optimal representation. This is the major drawback of mapping with multipolar electrode catheters; although data from a large number of sites can be acquired quickly, unless all electrograms are adjudicated to ensure correct designation of activation time by the mapping system, the map may be very misleading. In addition, data interpolation between mapped points is used to improve the quality of the display; however, areas of unmapped myocardium are then assigned simple estimates of timing and voltage information that may not be accurate.

If highly fractionated and wide potentials are present, it can be difficult to assign an activation time. In some macroreentrant circuits, much of the TCL is occupied by fractionated low-amplitude potentials. Furthermore, the assignment of a single time value to a multicomponent electrogram does not represent the quality of the electrogram and dismisses important information about the potential role of the recorded potential in the arrhythmia circuit. The subjective selection of an individual local potential within a multicomponent electrogram can drastically alter a propagation map. If these potentials are dismissed or assigned relatively late activation times, a macroreentrant tachycardia may mimic a focal arrhythmia, and it will appear as if substantially less than 90% of the TCL is mapped.

Factors Influencing Voltage Mapping Resolution

Bipolar electrogram amplitude is influenced by multiple variables that can affect the accuracy and resolution of the voltage map. These include the electrode size, interelectrode distance, conduction velocity between the bipolar electrodes, vector of activation, and the angle at which the electrode engages the tissue, and signal filtering, among others.

Electrode size.

The resolution of voltage mapping is influenced electrode size and interelectrode spacing. The spatial resolution of the standard mapping catheter is limited due to the large electrode surface area and wide interelectrode spacing. These catheters record signals produced by relatively large tissue mass and, hence, are more likely to exhibit larger bipolar electrogram amplitudes. Furthermore, low-amplitude signals produced by smaller mass of viable tissue can be lost when recorded with large electrodes. Therefore, while voltage mapping likely identifies large unexcitable areas of scar, small strands of fibrosis, which could harbor the arrhythmogenic substrate, may escape detection amidst the background of high-amplitude far-field signals. Similarly, small strands of surviving myocardium within an area of dense scar may not be detected during voltage mapping.

Smaller electrodes with closer interelectrode spacing record signals from smaller tissue mass and are subjected to less signal averaging and cancellation effects. As a result, data acquisition with smaller electrodes allows for the accurate detection of very small amplitude signals while limiting the effects of far-field signals and background noise. This can be of particular advantage in the low-voltage zones and areas of heterogeneous scar distribution, where the increased mapping resolution offered by the multielectrode catheters allows identifying surviving myocardial bundles channels, otherwise considered dense scar by standard linear catheters ( Fig. 6.15 ).

Standard Electrode Versus Small Electrode for Delineation of Late Potentials in a Patient With Cardiomyopathy.

Endocardial recordings from the inferolateral left ventricular free wall are shown from approximately the same location at almost the same time (two catheters in left ventricle). In (A) with a standard 3.5 mm distal tip, only far-field signals are recorded. In (B) with much smaller electrodes on a multipolar catheter, discrete high-frequency late potentials (red arrows) are observed (after end of QRS complex, red dotted line ). Abl, Ablation; CS , coronary sinus.

It is important to note that a minimal electrogram amplitude to identify an unexcitable scar has not yet been strictly defined. As different catheters with various electrode sizes and interelectrode spacings are becoming available, individualized validation is required, and catheter-specific thresholds are needed to improve scar characterization.

Limitations of Electroanatomic Voltage Mapping

Electrogram amplitude is annotated to the electrogram peak. In regions of scar, far-field signals are frequently of larger amplitude than local electrograms. Therefore automated voltage annotation of the larger far-field electrograms can introduce errors in the voltage map, especially in scar regions ( Fig. 6.16 ). Manual tagging of abnormal potentials or manual annotation of near-field electrogram voltage can help improve the map accuracy, but this can be challenging with high-density point acquisition.

Epicardial Late Potential Obscured by Far-Field Signal in a Patient With Dilated Cardiomyopathy.

The ablation catheter is on the epicardial surface after percutaneous pericardial access; a very large potential within the QRS complex (green arrow) will be taken by the mapping system to characterize the site as being “normal,” whereas distinct high-frequency late (red arrows) are evident well after the end of the QRS complex (dotted red line) . Abl, Ablation; CS , Coronary sinus.

Voltage mapping during NSR depends on the assumption that the arrhythmogenic substrate is limited to fixed myocardial scar and anatomical barriers. It is now well known that functional lines of block (present during tachycardia but not in NSR) play an important role in arrhythmogenesis, and these barriers cannot be detected by substrate mapping performed in NSR. Therefore conducting channels developing during arrhythmias and surrogates of channels and conduction barriers identified by substrate mapping in NSR may not correspond.

Even when conducting channels within the scar area can be identified by voltage mapping, their relationship to the arrhythmia circuit remains to be assessed by other mapping methods (e.g., entrainment mapping). Voltage mapping does not distinguish abnormal bystander areas that are not involved in a tachycardia circuit from clinically relevant channels.

It is also important to recognize that the transmural distribution of the scar may not be reliably represented by voltage mapping from either the endocardial or epicardial surface. In particular, identifying septal or mid-myocardial substrates can be challenging.

Limitations of Anatomical Mapping

Significant anatomical distortions in complex structures can occur with low-density mapping, whereby individual interpolation schemes in the region of curvature do not depict the accurate geometry, especially at areas of exvaginations (e.g., the PVs, atrial appendages). This can be mitigated by (1) the acquisition of a larger number of points (increased density) to reduce the extent of interpolation; (2) the acquisition of a set of points at the critical junctions between the different anatomical structures; and (3) creating geometric shells of these structures in separate maps and then combining them into the main chamber.

In addition, a change in rhythm during the mapping procedure can alter cardiac geometry to the extent that anatomical points acquired during one rhythm cannot be relied on after a change in rhythm ( Fig. 6.19 ). This is relevant during the mapping of PVCs (especially PVCs originating from the RV and those with short coupling intervals), because electrical and spatial information acquired during the arrhythmia can potentially be spatially separated from the same locations when they are assigned during normal rhythm (e.g., at the time of RF delivery after tachycardia termination). Therefore, after termination of the arrhythmia, revisiting the site of early activation tagged during PVCs or tachycardia may not be feasible and can potentially be misleading as a target for ablation. This can be mitigated by annotating the site of interest after tachycardia termination before moving the mapping catheter from its original location.

Alteration of Cardiac Geometry by a Change in Cardiac Rhythm.

CARTO electroanatomic activation map (color-coded) of the focus of the premature ventricular complexes (PVCs) in the right ventricular outflow tract (anterior view). The acquisition reference is set to the peak of the R wave in the surface ECG lead II. Upper panel, Electrical information (local activation time) is acquired by the CARTO system during activation mapping during the PVC. Spatial location of that point is acquired simultaneously (as indicated by the catheter location visualized on cardiac geometry, red dot ). Lower panel, When the spatial location is acquired by the CARTO system during sinus rhythm (blue dot) , the location of the ablation catheter is assigned to a spatially removed location of the virtual chamber model of the right ventricular outflow tract (RVOT; distance between the two points is 9 mm). Although the tip of the ablation catheter is maintained at the same anatomical location, the spatial information annotated on the virtual chamber model of the RVOT during sinus rhythm is removed and then tagged during the PVC. Therefore when the target of ablation is identified during PVCs, revisiting that target during sinus rhythm may not be accurate unless the acquired electroanatomical location during the PVC is also annotated (before moving the catheter from the identified ablation target) during sinus rhythm.

Furthermore, electroanatomic mapping systems do not provide real-time correlation of catheter position and heart border motion. Therefore localizing the catheter tip against the virtual anatomical shell does not establish catheter tip-tissue contact, and other methods to confirm adequate electrode-tissue contact should be utilized (e.g., pacing and recording data at the catheter tip, intermittent fluoroscopic imaging, ICE, and force contact sensors). In fact, geometric reconstructions of the cardiac chamber during the same procedure can vary depending on the method used. For example, the 3-D ICE-derived LA geometric reconstruction was found to be smaller than reconstructions derived from electroanatomic mapping and merged CT images. During procedures lasting several hours, chamber sizes may change based on the accrual of 1 or more liters of saline infused.

CARTO

Although all three electroanatomic mapping systems demonstrate a high level of intrinsic accuracy, the magnetic field localization technology of CARTO appears to have superior accuracy at point localization performance and fewer problems with interstructure delineation as compared with impedance-based systems. Another advantage of using magnetic fields for catheter localization is that the fields remain stable over time and are unaffected by biological material; hence the localization accuracy of CARTO is less subject to inhomogeneous tissue characteristics. Electrical field distortions seen with EnSite NavX do not occur with CARTO.

A limitation of the CARTO system is the requirement of a special Biosense Webster catheter with a location sensor embedded proximal to its tip. No other catheter types may be used for electroanatomical data acquisition. Furthermore, the magnetic signal necessary for the CARTO system can potentially create interference with other EP laboratory recording systems. Defibrillators and pacemakers are safe with the system, but the magnetic field can prevent device communication with its programmer, and the magnetic field may need to be disabled temporarily to allow device programming. Percutaneous LV assist devices can cause interference and distortion on the mapping system. Magnetic fields used with the Stereotaxis remote magnetic navigation system are not problematic for the CARTO system.

Although CARTO-3 allows current-based visualization of EP catheters without magnetic sensors, the visualization of catheters is confined into a 3-D virtual area (matrix) that can be built only by using a magnetic sensor-equipped manufacturer-specific catheter. Furthermore, the system still cannot process electrical or location data from the nonproprietary catheters (i.e., catheters without magnetic sensors) to build the virtual geometry or for mapping purposes.

Importantly, the coordinates of the magnetic field of CARTO are linked to the table and not the patient’s body. Therefore significant movement of the patient can cause uncorrectable shifts requiring remapping.

EnSite NavX

One of the principal advantages of this system is its open platform. EnSite NavX enables the display in real time up to 128 electrodes simultaneously on multiple EP catheters with almost every commercially available catheter, including pacemaker leads. This system works with most manufacturers’ ablation catheters, RF generators, and cryogenerators. The EnSite System can also be integrated with the Sensei robotic catheter system (Hansen Medical, Mountain View, CA, United States), allowing completely remote catheter navigation.

Also, unlike with the CARTO and Rhythmia, anatomic and electrical data (voltage or activation) can be acquired simultaneously by the EnSite NavX system from multiple poles on all catheters utilized during the study (and not just catheters with magnetic sensors). Data acquisition can be augmented by the addition of a multielectrode array (MEA) for noncontact mapping.

A unique advantage of the EnSite system is that it can locate the position of the catheters from the puncture site to the final destination in the heart. Therefore all catheters can be navigated to the heart under guidance of the EnSite NavX system, and the use of fluoroscopy can be minimized for preliminary catheter positioning. This contrasts with CARTO-3 that enables visualization of EP catheters without magnetic sensors only when positioned within the 3-D matrix built only by using a magnetic sensor-equipped catheter.

Furthermore, NavX technology is partially insensitive to potential patient movements, as the coordinate system (patches) are linked to the patient, and therefore they move simultaneously with the patient, preventing map shifts.

On the other hand, the impedance-based localization system used by EnSite is subject to changes in tissue properties. Changes in the respiration pattern and volume shifts during the course of the procedure (e.g., secondary to saline infusion when using an irrigated ablation) can cause impedance changes inside the body and potentially impact the localization accuracy of the EnSite system. The newer version of EnSite (EnSite Precision) now uniquely combines impedance and magnetics, which can enhance navigation and model creation.

Rhythmia

The main advantage of Rhythmia is the ability to create ultra–high-resolution activation and voltage maps using rapid and accurate automated data acquisition and annotation. The very small size of the electrodes on the Orion catheter minimizes far-field signals and background noise and allows accurate detection of very small amplitude signals. Also, this system offers the ability to change the mapping window in retrospect (see Fig. 6.10 ).

Similar to the EnSite system, Rhythmia can acquire electroanatomical data from magnetically enabled catheters (Orion and ablation catheters) as well as catheter without magnetic sensors. Data acquisition from catheters without magnetic sensors can be acquired only after construction of the electromagnetic field using magnetically enabled catheters. Notably, Rhythmia does not allow for integration with CT or MRI.

This mapping technology is preferentially designed for complex cardiac arrhythmias like AF, macroreentrant atrial tachycardia (MRAT), and scar-related VT, especially for substrate analysis (voltage maps), and analysis of conduction pattern (activation maps), but also for maps promoting effective catheter ablation (e.g., mapping gap in ablation lines). However, further studies are required to explore the potential clinical benefits gained from such ultra–high-density mapping.