Advanced mapping and navigation modalities

CARTO electroanatomical mapping system

The CARTO mapping system consists of an ultralow magnetic field emitter, a magnetic field generator locator pad, an external reference patch, location sensors inside the mapping-ablation catheter tip, a reference catheter, a data processing unit, and a graphic display unit to generate the electroanatomical model of the chamber being mapped.

The CARTO EAM 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 consists of three coils arranged as a triangle (in the locator pad mounted under the operating table) that generate a low-intensity magnetic field (5 × 10 ^–6 to 5 × 10 ^–5 Tesla), which is a very small fraction of the magnetic field intensity inside a magnetic resonance imaging (MRI) machine ( Fig. 7.1 ).

Three location sensors (positioned orthogonally to each other) are embedded proximal to the tip of a specialized mapping-ablation catheter. Additionally, six electrode patches positioned on the patient’s back and chest have magnetic sensors for the localization of the catheters. As the catheter tip moves through the three magnetic fields generated by the magnetic coils, the location sensors measure the strength of each magnetic field, which allows for determination of the distance of the location sensors 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 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. Catheters with bidirectional deflection capacity enable directing the catheter tip in either of two opposing directions, usually with different radius of curvature.

The latest version (CARTO-3) is based on 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 allows visualization of up to five EP catheters (with and without the magnetic sensors) simultaneously with clear distinction of all electrodes (see eFig. 13.8 ). In addition to the 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. Importantly, visualization of catheters is confined to a 3-D virtual area called the “matrix,” which can be built only by using a magnetic sensor-equipped manufacturer-specific catheter ( Fig. 7.1 ; eFig. 7.1 ).

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. The three back patches and the location pad are used as an anatomical reference, which allows the system to measure the catheter location relative to this anatomical reference for the compensation of patient and system motion. Compensation of the respiratory motion is achieved by monitoring the respiratory movement of the sensor-based catheter and impedance changes detected by the electrode patches positioned on the patient’s back and chest. For compensation of the cardiac motion, an electrical reference is used to match the catheter location with the time in a cardiac cycle.

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. Fast anatomical mapping is a feature that permits rapid creation of anatomical maps by movement of a sensor-based catheter throughout the cardiac chamber ( Video 7.1 ). Unlike point-by-point EAM, volume data can be collected with fast anatomical mapping ( Fig. 7.1 ).

Although CARTO-3 allows current-based visualization of EP catheters without magnetic sensors, visualization of catheters is confined to 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.

CARTO-Merge module

The CARTO-Merge Module allows for images from preacquired CT angiogram or MRI volume data sets to be integrated on the electroanatomical image of the cardiac chamber created with the CARTO system and simultaneously display them within the same coordinate system ( eFig. 7.2 ). This can be very valuable in guiding real-time catheter ablation using the detailed cardiac chamber anatomy acquired from the CT/MRI.

CARTO-Sound module

The CARTO-Sound Image Integration Module 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. ICE is performed using a phased-array transducer catheter incorporating a navigation sensor (SoundStar, Biosense Webster) that records individual 90° 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. 7.2 , Videos 7.2 and 7.3 ). Three-second segments of two-dimensional (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 late-expiration to midexpiration phase. Following optimizing each image by adjusting frequency (5–10 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 chamber deformity that often happens with contact mapping.

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 electroanatomical map, either as a stand-alone tool to guide navigation and ablation or as a facilitator of CT/MRI image integration. Additionally, studies have shown the feasibility of using CARTO-Sound to define scar boundaries in the left ventricle (LV, identified on ICE imaging by both 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.

CARTO-Coherence module

The Coherence module is a global vector-based algorithm that quantifies AT activation globally within the chamber and uses these data to modify annotation of individual electrograms to produce a physiologically plausible final map ( Video 7.4 ). The Coherence module is designed to overcome the limitations of standard electroanatomical activation mapping in complex substrates, such as inaccurate time annotation of complex multicomponent electrograms and passive diastolic activity recorded in scar unrelated to the tachycardia.

This novel mapping algorithm follows an integrative approach for annotation of complex multicomponent electrograms that examines the global pattern of activation in the chamber and forces the physiological constraints of propagation in atrial tissue to reconcile the most coherent activation pattern. The automated process of annotation employed in the algorithm captures all possible timing annotations in each individual bipolar electrogram and then selects the one which best reconciles global activation (i.e., provides the lowest spatial and temporal errors) and makes activation “coherent” by looking at surrounding electrogram points in an integrative approach.

The algorithm presents maps of vector propagation, which allows users to follow the propagation path of the activation wavefront ( Fig. 7.3 ). In macroreentry, it allows one to identify the circular path of the wave front throughout the tachycardia cycle and differentiate it from passive activation of dead-end pathways. In addition, it allows identification of those areas with slow conduction in the reentrant circuit. In focal tachycardias, it can differentiate a true focal source from a localized reentry: although both tachycardias are localized to a small area, in a true focal source, vectors spread away centrifugally from a point source, whereas in a localized reentry, highly curved vectors surround a small central core. The vector map displays a color spectrum (or isochrones) based on the relative propagation between areas, without predefined early and late activation, and is independent of a preset mapping window.

The algorithm was developed to manage complex electrograms, identify areas of electrical discontinuity, and integrate the entire electrical data set to identify the most coherent arrhythmia origin and location. The initial study showed a 92% diagnostic accuracy of the Coherence algorithm in identifying AT mechanism; a recent study showed a lower (67%) diagnostic accuracy, which was still superior to that of standard activation mapping. There was an inverse correlation between diagnostic accuracy of the Coherence activation map and the magnitude of voltage abnormality in mapped atria. Importantly, global vectors are sensitive to regions that are over- or undersampled, and so signal acquisition should be as spatially complete and uniform as possible. Also, by optimizing maps globally, localized patterns with little overall impact such as microreentry with a small, organized domain can potentially be overlooked.

Ensite electroanatomical mapping system

The EnSite system consists of a set of eight surface electrodes (three transthoracic pairs for three orthogonal axes and two patient reference sensors), a system reference patch, 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.

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 8 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 electrical current transmitted between the patches through the thorax will cause a drop in the voltage across the heart. 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 by use of 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, the system 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 EnSite 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. 7.4 ). 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.”

The EnSite 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 designated electrode at a rate of 96 points/sec. 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 ( Fig. 7.4 , Video 7.5 ).

To compensate for cardiac and respiratory motion artifact, the system uses either the reference patch on the patient’s body or an intracardiac electrode for the anatomical reference. It is recommended to use an intracardiac catheter for the anatomical reference that is not used for pacing because the EnSite system does not use an electrical reference for compensation of the cardiac motion artifact. 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 data are 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 shifts 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 ( eFig. 7.3 ). 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.

EnSite also has the capability to integrate images from a preacquired CT/MRI scan on the real-time electroanatomical image the cardiac chamber created with the mapping system to facilitate anatomically based ablation procedures. To allow local adjustment of the electroanatomical model, the registration module comes with dynamic registration. The system has the ability to mold the created geometry dynamically into the CT/MRI image.

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.

The latest system version (EnSite Precision) incorporates magnetic field–based localization technology to refine the impedance-based tracking in real-time. EnSite Precision requires an additional source of a magnetic field (EnSite Precision field frame) and two additional sensors on the patient (one on the back and the other on the chest). The field frame is attached under the patient’s table, and it generates a weak magnetic field (similar to the location pad of the CARTO system). The magnetic field technology, which requires the use of sensor-enabled catheters, helps refine the impedance-based location, especially in the peripheral areas. The magnetic field data help preserve the localization accuracy in case of gradual changes in the impedance field such as occurring in lengthy procedures.

One of the principal advantages of this system is its open platform. EnSite NavX enables the display in real time of 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. 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, which 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 largely 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.

Rhythmia electroanatomical mapping system

The Rhythmia EAM system uses a hybrid location technology that combines impedance and magnetic location. A special electrode is positioned as a patch on a patient’s back to serve as a reference electrode. For magnetic tracking, the system uses one sensor coil embedded back patch and magnetic field generator positioned under the patient’s table. For an impedance-based localization, the impedance field is generated by applying current to the back patch, patches for the ECG limb leads, and the V ₁ , V ₃ , and V ₆ precordial leads. The magnetic field is capable of locating catheters equipped 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.

The Rhythmia platform is paired to a proprietary 64-pole array catheter (Orion, Boston Scientific) and is capable of generating ultra-high-density electroanatomic maps. Orion is an 8.5 Fr bidirectional deflectable catheter with a minibasket electrode array containing 8 splines, each spline containing 8 flat, low-noise, iridium-oxide coated minielectrodes (see Fig. 5.3 ). 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 diameters (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. Other catheters are tracked by an impedance-based system.

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 ( Video 7.6 ). The iridium oxide-coated flat electrodes on the Orion catheter without sensing from the back side of the splines help avoid far-field signals and allow accurate detection of very low-amplitude local potentials, resulting in a very low (0.01 mV) noise floor. Low background noise uncovers signals that cannot be visualized using other mapping systems ( Fig. 7.5 ). Also, this system offers the ability to change the mapping window in retrospect ( Fig. 7.6 ).

Similar to the EnSite system, Rhythmia can acquire electroanatomic data from magnetically enabled catheters (Orion and ablation catheters) as well as catheter without magnetic sensors. Data acquisition from catheters without magnetic sensors is possible 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, 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.

LUMIPOINT module

The Rhythmia EAM combines ultra-high-resolution data acquisition with powerful software for data analysis. The LUMIPOINT software offers a set of automatic tools to provide a comprehensive overview of the mapping findings and simplify analysis of complex electrograms ( Video 7.7 ). These features include the “Skyline” graph, “Trend” tool, “Activation Search” tool, “Complex Activation” tool, and “Group Reannotation” tool.

“Skyline” is a tool to display full chamber activation, converting 3-D spatial and temporal activation data into a 2-D trace. The “Skyline” graph plots the relative proportion of the cardiac chamber surface area that is activated at each moment in time throughout the entire mapping window as a fraction of the total surface area of the map. Importantly, this plot considers chamber surface area and not the number of acquired mapping points; hence, it is not vulnerable to influence of varying density of mapping points acquired in different myocardial regions. The “Skyline” histogram is displayed with a normalized value, ranging from 0 to 1.0 (referred to as the “GAH-Score”), in which peaks are seen when a large part of the mapped cardiac chamber is being activated, while valleys correspond to a smaller surface of activation. Zero on the histogram indicates the absence of electrical activity anywhere in the mapped myocardium. In macroreentrant tachycardias, peaks of the histogram correspond to electrical systole (P wave or QRS complex on the surface ECG), whereas valleys likely contain the diastolic pathway (critical isthmus) of the macroreentry circuit (as well as other bystander regions), which typically involve myocardial tissue that is too small to be detected on the surface ECG ( Fig. 7.7 ; see Fig. 14.11 ). Given that comprehensive mapping is achieved, zero values (no electrical activity) on the histogram are expected to occupy less than 30% of the tachycardia cycle length (TCL). A long plateau of zero value on the histogram during macroreentry suggests incomplete map of the reentry circuit and should prompt additional mapping. In contrast, focal tachycardias are characterized by absence of any activity between two consecutive activations, displayed as a value of zero on the histogram, which typically occupy >30% of the TCL. The “Trend” tool displays pooled areas with the same electrogram timing within a selected small region of the mapped chamber.

In addition to voltage and activation time annotation, LUMIPOINT software processes each electrogram to detect all electrical activity and trace the presence of deflections for each time point. This tool allows the operator to select a portion of the mapping window within which the software highlights regions with electrical activity. The “Activation Search” feature uses an adjustable time-of-interest period within the mapping window. Superimposed onto the voltage or activation map, the tool highlights regions of the map that contain electrograms that show activity in the time of interest (irrespective of the system annotation of the electrogram’s activation time). This feature is of particular value when mapping diseased myocardium where complex electrograms often contain information related to far-field potentials from adjacent tissue, which can be larger in amplitude than local activity and, hence, are preferentially selected for automatic local activation timing, introducing errors in the electroanatomical maps. This is a 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 this setting, adjusting the time-of-interest period within the mapping window to the period of electrical diastole (i.e., between P waves or QRS complexes on the surface ECG) excludes far-field signals resulting from activation of nearby tissue and unmasks local diastolic electrical activity (that was not annotated due to its smaller amplitude). During substrate mapping, unmasking diastolic activity (late potentials) can help delineate potential arrhythmogenic substrate. During macroreentrant tachycardia mapping, the “Activation Search” can be used to scan diastole for electrograms to delineate the diastolic pathway (critical isthmus) of the reentry circuit ( Fig. 7.7 ). Since the isthmus of the macroreentry circuit will colocalize with the region of the map with the smallest activation region, the window of interest of the activation search tool is placed over the trough of the “Skyline” graph. Once the proper timeframe has been selected by the operator, the “Group Reannotation” tool can be used to automatically reset the annotation of the selected near-field electrograms (instead of annotating the higher-amplitude far-field electrograms).

The “Complex Activation” tool allows the system to highlight specific subsets of electrograms (continuous activity, double potentials, and fragmented) and show their distribution on the map within the time-of-interest period. This is in contrast to other systems, where the operator has to actively inspect all acquired mapping data and tag the sites of specific electrogram characteristics (e.g., double potentials, fractionated electrograms, late potentials). The number of components necessary to cause an electrogram to be highlighted is adjustable (e.g., double potentials, fragmented electrograms with certain number of peaks). The search window can be adjusted to examine the entire mapping window, only electrical diastole, or any specific interval during the mapping window. Highlighting areas with double potentials can be valuable to identify lines of conduction block, which can be a central component of the arrhythmogenic substrate or a macroreentry circuit ( Fig. 7.7 ; see Fig. 14.11 ). Additionally, it can be used to verify complete block across a line of ablation. Highlighting areas with fragmented potentials or continuous activity can help identify diseased myocardium as well as areas of residual slow conduction (e.g., conduction gaps in a line of ablation).

The “Group Reannotation” feature automatically annotates a deflection (even when it is not the highest amplitude component of the recorded electrogram) within the time-of-interest period as the activation time for highlighted electrograms. This is in contrast to other systems, where the operator has to actively inspect all acquired mapping data, discern local activity from far-field signals, and manually reannotate local activation timing, which is time-consuming and impractical in the setting of high-density maps with a huge amount of acquired signals.

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 electroanatomical 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.

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 cycle ( Video 7.8 ). 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. Mapping systems offer 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, coronary sinus (CS) electrograms are usually chosen for mapping supraventricular rhythms, and a right ventricular (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 of 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 ( eFig. 7.4 ). Within this window, activation is considered early or late relative to the electrical 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 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 window of interest 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 ( eFig. 7.4 ; Fig. 7.6 , Videos 7.9 and 7.10 ). 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. 7.8 ).

Local activation time

Once the reference electrogram, anatomical reference, and window of interest have been selected, 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. 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 (as discussed in detail in Chapter 6 ).

For bipolar electrograms, the initial peak of a filtered (30–300 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, 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. It is important to recognize that automated annotation methods differ among the systems. The CARTO CONFIDENSE module uses the maximum negative dV/dt of the distal unipolar signal to set the timing of the annotation, and the annotation is displayed on the corresponding bipolar signal. The EnSite AutoMap module system allows the user to select which parameter will be used for the annotation including the peak positive/negative voltage, negative/positive slope, absolute slope (steepest slope, either positive or negative), and absolute voltage (largest voltage, either positive or negative). The grid mapping catheter with the EnSite system uses a duplicate algorithm (the bipolar electrogram in both directions, along the splines and across the splines) and displays the largest bipolar voltage at the positive electrode. The Rhythmia system annotates the greatest peak-to-peak amplitude of the bipolar signals with consideration of the maximum negative dV/dt of the unipolar electrogram to reduce the far-field components. When automated timing annotation is used, it is important to utilize the same parameters for timing of local activation when additional points are acquired manually or during 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, EnSite-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 EnSite 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 system, and the use of fluoroscopy can be minimized for preliminary catheter positioning.

The mapping catheter is initially positioned (using fluoroscopy or ICE) at known anatomical points that serve as landmarks in the chamber of interest for the electroanatomical map. For example, to map the right atrium (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.

Points are selected only when the catheter is in stable contact with the wall. The system continuously monitors the quality of catheter-tissue contact 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 superimposing of the intracardiac electrogram recorded on two consecutive beats; and (4) cycle length (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. These algorithms help streamline the mapping and validation process and reduce overall mapping and manual annotation time. The algorithm automatically accepts and annotates activation times and voltages for points that fulfill an operator-defined set of acceptance criteria, which often include: (1) CL stability, (2) time stability of electrograms recorded from the reference catheter, (3) beat-to-beat electrogram stability (i.e., consistent morphology and timing of the recorded electrogram), (4) catheter stability, and (5) respiration gating, allowing data acquisition at a constant respiratory phase. Additionally, since tissue contact is unlikely to be uniform across all electrodes, and in many cases, individual electrodes are intracavitary, completely devoid of contact with the myocardial surface, EAMs calculate the relationship of the information obtained with reference to the most outer aspects of the geometric chamber obtained, and disregard points that are “internal” to the outer margin samples.

Each selected point is tagged on the 3-D map. Points of interest (e.g., sites with double potentials or complex electrograms) can be tagged for easy identification. Electrically silent areas are tagged as “scar” and therefore appear in gray on the 3-D maps and are not assigned an activation time. 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. The use of multielectrode mapping catheters can 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. The acquired local activation times are then color-coded and superimposed on the anatomical map ( Fig. 7.4 , Fig. 7.8 ). 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 high-density mapping 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.

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.

The electroanatomical model, which can be viewed in a single view or in multiple views simultaneously and freely rotated in any direction, forms a 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

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 overlaid on the reconstructed 3-D geometry (see Figs. 12.18 and 13.11 ). 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 electroanatomical activation 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 ( eFig. 7.1 ). 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 ( Fig. 7.8 ). 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. This zone can shift in location and timing in response to shifts in the window of interest or electrical reference ( Fig. 7.8 ; see Fig. 13.11 ).

It is also important to recognize that a focal tachycardia can produce an electroanatomical 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 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 CS os in a patient with prior cavotricuspid isthmus (CTI) ablation can produce an electroanatomical map mimicking that of peritricuspid macroreentry (counterclockwise typical AFL). 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 a more detailed mapping should be performed before concluding macroreentry as the mechanism of the tachycardia.

On the other hand, a macroreentrant tachycardia can produce an electroanatomical 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. 14.10 ). 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 to 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. Usually, this results in a relatively large area of similarly early timing, unlike actual focal sources.

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 or 20 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 ( Video 7.11 ).

As isochrones are all assigned the same unit of time, the width of an isochrone is a graphical representation of conduction velocity (distance/time). Faster propagation traverses a greater distance over the same amount of time (wider isochrone) and slower propagation traverses a shorter distance for the same amount of time (thinner isochrone) (see Figs. 26.25 and 26.27 ). Conduction velocity can be calculated based on the activation time and the known distance between data points along a vector that has an origin and direction. Isochronal crowding (i.e., multiple colors evident over a small distance) indicating a conduction velocity of 0.033 cm/millisecond (slower than 0.05 cm/millisecond) 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.

Propagation map

Activation mapping data can be displayed in a color-coded animated dynamic map of activation wavefront (propagation map) (see Fig. 12.18 , Video 7.12 ). 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 ( Fig. 7.9 ).

Entrainment map

A graphical representation of entrainment mapping can be constructed by plotting values of the differences between the PPIs (postpacing intervals) and the TCLs (PPI–TCL) on the EAM system to generate color-coded 3-D entrainment maps ( Fig. 7.10 ). 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 EAM systems contains 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 EAM system, entrainment stimulation is performed, and the difference between PPI and TCL is calculated and associated with that site on the EAM 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).

Color-coded 3-D entrainment mapping allows determination of the full active reentrant circuit (versus 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 ( Fig. 7.10 ). However, ablation will not terminate reentry at all these sites (just as, although the circuit in orthodromic AVRT includes the ventricle, ablation at one or two sites in that ventricle will not eliminate reentry); the final choice is determined by 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 electroanatomical 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 time-consuming. 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 a complete activation map. 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 electroanatomical 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 to address many of these issues.

One difficulty with current methods is that incorrect assignment of activation for even a few electrograms can invalidate the entire activation map, and manual adjustment is often required to achieve the optimal representation, especially in the setting of low-density mapping. In particular, complex electrograms often contain information related to far-field potentials from adjacent tissue, which can be larger in amplitude than local activity, creating challenges in assigning accurate activation times and introducing sources of error in the electroanatomical maps.

Additionally, 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 as they are based on the assumption that activation is uniform and predictable between collected points. While this may be acceptable in healthy myocardium, in scarred tissue it will create a false map. In propagation maps, interpolation of colors between the apparent early and latest sites on the map can lead to “backward wavefront” at sites where “early meets late” do not quite meet, whereby the wavefront appears to move slowly in reverse to the true direction of activation.

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 electrograms within areas of slow conduction. Furthermore, 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. Tagging regions with fractionated electrograms and manual reannotation of those points can improve map quality, but in high-density mapping, this can be labor intensive and time consuming.

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, the angle at which the electrode engages the tissue, and signal filtering, among others.

Electrode size and interelectrode spacing

The resolution of voltage mapping is influenced by 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 amid 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 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 bundle channels, otherwise considered dense scar by standard linear catheters ( eFig. 7.5 ).

Conventional definitions of normal (>1.5 mV), border-zone (0.5–1.5 mV), and densely abnormal (<0.5 mV) ventricular endocardium were developed in the setting of healed myocardial infarction (MI) and with use of a specific catheter with a 4-mm tip electrode, 2-mm ring electrode, and 1-mm interelectrode spacing. 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. Specific bipolar endocardial voltage thresholds have been proposed for each dedicated mapping catheter: PentaRay: 0.2 to 1.0 mV or 1.5 mV; Orion: 0.1 to 1.0 mV; and LiveWire: 0.5 to 1.5 mV.

Limitations of electroanatomical 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 ( eFig. 7.6 ). Manual tagging of abnormal potentials or manual annotation of near-field electrogram voltage can help improve the map accuracy, but this can be challenging and time consuming with high-density point acquisition.

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 midmyocardial substrates can be challenging.