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General Requirements of Cardiac Mapping Systems
At a minimum, a computer-based cardiac mapping system should be able to
- Accurately replicate the cardiac anatomy underlying an arrhythmia;
- Provide a plausible representation of activation of that chamber, as linked to the specific anatomic site of data acquisition;
- Readily capture and intelligibly display the other details of physiology; and
- Catalogue the site of interventions.
The first requirement provides the context for the arrhythmia. A mapping system should faithfully replicate the anatomy of a chamber under examination and those structures, both entering and exiting that chamber. In short, the geometry needs to look like the chamber under study. The extent to which this is accomplished is a matter of both “man vs machine.” The resolution of anatomy is a function of the number of points taken in the process of creating the surrogate geometry. The more points, the closer the image rendering comes to replicating the chamber under evaluation. This is an operator issue. The problem from the mapping system side comes from the algorithms used to graphically connect three or more points sampled by a roving catheter to create a surface segment and subsequent volume.
While interpolation between points along an uncomplicated surface readily produces a clear image of that surface, the process is strained at the junction between chambers and sites of entering or exiting “veins and valves,” or at areas of complex structures. The system must easily preserve the complexities of anatomy at those points, along with intervening acute and oblique angles between structures, without losing requisite detail. Some systems are more prone to “interpolation obliteration” of those junctions with smoothing over the angles defining the underlying structures. One way to minimize this is to treat multiple veins or neighboring structures as separate volumes or maps, which some systems readily allow.
Another challenge of mapping systems is created by the inherent difficulty in displaying three-dimensional (3D) structures on a monitor screen in two-dimensional views. Here, the ability to show multiple views simultaneously is of paramount importance to give the 3D perspective. The addition of virtual endoscopic views from within a chamber is enormously helpful. Transparency features incorporated into the system can be useful or create confusion. These challenges of anatomic rendering and display are not a trivial matter, since, under the best of circumstances, these features allow the user to clearly understand and navigate the underlying geometry. Under the worst of circumstances, bad geometry may be dangerous. It should be noted that the accuracy of the surrogate geometries would become increasingly important, as interventions are guided by those very images. Remarkably, very few validation studies addressing these issues in any mapping system are available. Additional investigations will therefore be required to insure that the surrogate mapping geometries accurately depict actual anatomy.
A second function of a mapping system is to catalogue local physiology as linked to the anatomic site of data acquisition. The system should readily “arrange” sequential sampling site activation times and voltages within the context of the entire surface geometry to provide the global indication of activation sequence. Furthermore, resulting depictions of chamber activation must be consistent with the first principles of cardiac electrophysiology. A map of sinus rhythm or any arrhythmia must be “plausible,” whether the activation sequence is displayed in terms of progressive activation times, voltage transients, or any other physiologic parameter. While mapping systems should make a case easier, the user still has the obligation of knowing when it is serving up “jewels” or “junk” without relegating this responsibility solely to an industry representative.
Again, success in this process is part operator and part machine. While large circuits can be dissected with relatively few mapping points, progressively more points at a sufficient density are required to resolve arrhythmia circuits of decreasing size or increasing complexity. A reasonable goal for the ideal system is to ultimately provide adequate resolution and mechanistic disclosure on the same order as found with optical mapping systems. The use of advanced computational capabilities should simplify this process by cataloging all sites of data acquisition with the accompanying electrograms without confusing interpolation across lines of block or other boundaries of the circuits. This should be extended to allow mapping of rapid tachycardias, nonsustained arrhythmias, or tachycardias with complex activation pathways involving several different chambers.
Third, the system must lend itself readily to capturing and intelligibly displaying other details or the “physiologic quirks” contributing to an arrhythmia. This implies system versatility in extracting relevant features from electrograms or other sensors and providing real-time parametric displays. Most electrophysiologists are familiar with unipolar or bipolar voltage mapping to reflect underlying tissue integrity and pathophysiology in patients with a prior myocardial infarction. “Scar mapping” has been used, for example, to identify the site of possible circuits in patients with unstable ventricular tachycardias (VT) that defy activation sequence mapping. An example of this approach is shown in Figure 1. Some investigators have used a 1.0 mV cut off to reflect scar and a 0.5 mV cutoff to reflect dense scar. It should be noted, however, that the presence of dense scar does not exclude the possibility of pathways within that scar that are incapable of generating a 0.5–1.0 mV signal. In atrial mapping, for example, scar cutoffs of 0.5 mV may better detect the underlying tissue pathophysiology and reflect relevant active circuit components. Any mapping system should also chronicle the voltage changes of repolarization, even if this would require alternative signal amplification and filtering.
Mapping systems should identify and catalogue the presence and location of double potentials, fractionated electrograms, or the switching sequences of activation around a line of the fixed or physiologic block, and archive and display these “quirks” in real-time, 3D space. Ideally, it should also be possible to exploit any characteristic of the electrogram at each site to better explain an arrhythmia. For example, it would be highly useful if signal amplitude, width, fractionation, or evidence of specific patterns of temporal activation, such as repetitive firing could be translated into specific maps through straightforward signal processing.
In theory, the ideal advanced mapping system should also display mechanical events, such as motion, wall stress or tension, or any other fourth- or fifth-dimensional contraction parameter and allow easy visualization, understanding, and metrics to assess those processes. Activation mapping of several chambers may thereby give a clear-cut indication of inter- or intraventricular dysynchrony, while voltage or other mapping over the course of a single cardiac cycle, may disclose the possibility of intramural dysynchrony.
Finally, the ability to catalogue the site of an intervention (either performed or planned), such as a specific ablation, or marking the site of cellular or other factor injections is highly useful. While this annotation might be done with a marking pen on an acetate film taped over a fluoroscopic monitor, such an approach is eclipsed by the capability of CPUs with high computational speed.
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Integrated, Anatomy-Based Mapping
The last 5 years have also seen the rapid development of integrated, anatomy-based mapping and ablation. This has been driven by a realization of both the critical coupling and dependence of arrhythmias on their underlying anatomy and the limitations of surrogate geometries of contemporary mapping systems for reflecting that anatomy.
Over this same time frame intracardiac ultrasound and rapid CT and MR systems have emerged as the mainstays of imaging in the EP lab. Sixteen to sixty-four row helical CT and MR studies provide a broad “gestalt” or “anatomy library” of an individual patient at one point in time. Intracardiac ultrasound is highly useful to provide focused real-time images of the endocardial surfaces critical for positioning of catheters, establishment of catheter/tip tissue contact, and for monitoring energy delivery in the beating heart. Both changing tissue echogenicity and microbubbles reflect tissue heating, with the latter providing a signal for energy termination. In each of these tasks, ultrasound focuses on specific “books” and “shelves” within the “global library” established by prior CT on MR imaging.
3D Anatomy-Based Mapping
Each company is actively working on image incorporation into their systems. Segmented CT volumes can be downloaded on the Carto and NavX platforms, with similar integration work underway for the RPM system. With each of these approaches, the chamber of interest is segmented out of the entire CT axial image set, although this requires substantial user effort to sculpt or segment out the chamber of interest. Both systems display the CT image volume along with the surrogate geometries rendered from sequential mapping for side-by-side comparison. While this is useful in correlating electrophysiology and CT anatomies, manipulating image files slows down general map processing time to a noticeable level.
Additional work is also underway to fully “register” the surrogate map onto actual CT anatomy. At this point, the Carto system does accommodate merging the CT and electroanatomic map into one image, through matching 3–6 specific anatomic locations seen on both anatomic renderings. Displaying the ablation catheter and integrating its position onto the CT geometry is also possible.
True registration of mapping details onto the exact surface of the CT or MR anatomy, however, is not yet commercially available. This has been done in research studies, however. An example of full registration of an activation sequence generated by an APC, onto the underlying left atrial anatomy, as established in a patient with a left superior pulmonary vein AF focus by multirow CT scanning is shown in Figure 4. This kind of approach will be required for the creation of highly robust image-based system for guiding ablation. Obviously, substantial validation studies will be required to ensure a complete and appropriate match between the CT or MR anatomy and the surrogate geometries created by point-to-point anatomic mapping.
From the information listed in Table 1, recommendations can be made for choosing a specific mapping system for a particular interventional case. This choice will be shaped by the importance of a specific characteristic in the mapping process. In those cases where an undistorted anatomic rendering, with high spacial accuracy, is required, we use the Carto system. This has lesser problems with interstructure delineation and requires fewer fixed or snap points to preserve the “anatomy.”
The Carto XP, NavX, and RPM systems all work well for mapping sustained, stable arrhythmias.
Mapping nonsustained arrhythmias, APCs, or VPCs is admittedly tediously with each of these three approaches. With these arrhythmias, the noncontact mapping array works very well, although the maps can be filter frequency dependent. The noncontact approach does provide a quick snapshot of activation during unstable VTs, obviating the need for long periods of tachycardia. On the other hand, the precision of mapping can be limited in the setting of various cardiomyopathies with ventricular enlargement.
We use mapping of an alternative characteristic, such as scar or voltage, as a very useful alternative to noncontact mapping. Carto performs very well in this regard. NavX also works reasonably well, with its dynamic substrate mapping capabilities. While each of these systems run in the same range for cost, the NavX system wins out in disposable cost, since any catheter can be used for the creation of the anatomy and voltage or activation maps. The same is true of the Local Lisa system, although its capabilities are limited to cataloguing the sites of ablation or other specifically marked structures. An ongoing added expense is required for catheter purchase with the RPM, Noncontact, and Carto systems.
In some cases, the choice of a mapping system depends on the skill and experience of the operators. The user interface of the Carto and NavX systems are acceptably straightforward. The Noncontact system requires more steps in the creation of a user-friendly working geometry. Obviously, each of these systems is in the development stage and the various capabilities could change substantially over the next couple of years.
Carto XP Mapping System
Each system has considerable strengths and some limitations. The Carto system provides a highly accurate geometric rendering of a cardiac chamber with a straightforward geometric display. The position of the mapping tip at any point in time is readily apparent from a tip icon, providing that the tip is at or beyond the rendered chamber geometry. This system allows a straightforward annotation of cardiac valves and veins on the surface of a cardiac chamber. It is possible to create an individual map of each pulmonary vein, SVC or IVC as a more robust reflection of structure geometry. Activation maps are straightforward, to the extent that the underlying circuit is appropriately sampled by a sufficient number of acquisition sites to resolve the underlying process. Respiratory artifact is limited.
This approach has several limitations, however. Since the acquisition of anatomic and physiologic points is based on sequential site sampling, an arrhythmia must be inducible and hemodynamically stable. Single ventricular premature contractions (VPCs) or atrial premature contractions (APCs) or nonsustained events can be mapped, although at the expense of an appreciable amount of time. Since only sequential events are used to establish the maps, data acquisition during rapid VT, for example, is not possible. Nevertheless, mapping based on underlying voltage is very useful for mapping scar borders as described elsewhere. Annotation of ablative sites is straightforward, although the “red” tip displayed during ablation is counterproductive, in that it is lost in the red background of the earliest site of activation of an arrhythmia or among previously annotated ablative sites. Another limitation of the Carto XP system is the requirement of a separate Biosense Webster catheter for each case. No other catheter types can be used with this system, and bidirectional steerable catheters are not available. The magnetic signal also creates interference with other EP lab recording systems. The QWIKMAP mapping approach has been forwarded as a means of mapping multiple sites simultaneously. The surface geometries so created can be somewhat distorted if the chamber under examination distends with the mapping catheter. When including both tip points and shaft points, the chamber geometries tend to be artificially large. Annotation of the location of specific ablative sites on the surface geometry with this new software can be a laborious process.
The noncontact mapping approach does permit a straightforward means of assessing activation, as seen in the real-time voltage excursions. Given the underlying technology, it is possible to map out an entire cardiac cycle of an entire chamber, without requiring sequential point-to-point acquisitions. This is particularly well suited for localizing the origin of nonsustained arrhythmias, APCs or VPCs, or very rapid VT with hemodynamic compromise. The system can also map multiple cardiac cycles in real time, which disclose changes in activation sequence from one beat to the next. Obviously, the chamber geometry must first be established through current injection from the ablation or mapping catheter at a variety of different sites. An additional advantage of this system is that any catheter from any manufacturer can be used in conjunction with this mapping platform.
Although much progress in the development of this system has been made, some limitations remain. Activation maps, expressed in terms of isochronal activation times are not as easily produced. In addition, the acquired geometry with the current version of software is somewhat distorted, requiring multiple set points to clearly establish the origin and shape of complicated structures such as the left atrial appendage or pulmonary veins. Otherwise, these structures can be lost in the interpolation between several neighboring points. Some loss of accuracy of localization can occur with mapping at endocardial sites more than 4 cm away from the balloon surface. Positioning the noncontact balloon array into the left atrium also makes it more difficult to manipulate an ablation catheter around the outside of the balloon. Low voltage generating acquisition sites may also be missed. Synchronized mapping of multiple chambers requires multiple systems, and maps are highly sensitive to changes in filtering frequencies.
NavX Mapping System
The NavX approach likewise establishes a straightforward geometry. With this approach, the locations of veins or even the esophagus are established and readily displayed by the juxtaposition of multiple mapping marker, or “Sphere Stacking,” spheres along the length of that vein, as seen in Figure 2. The size of each marker is based on size “selection” rather than actual vessel dimensions. A recent software upgrade allowing point-to-point activation mapping for the NavX system has also been released. This is a substantial improvement, permitting the same kind of activation mapping and display as possible with other systems, with the similar advantage of specified voltage mapping as well. This point-to-point mapping, however, is only suited for sustained arrhythmias or frequently recurrent APCs, VPCs, or nonsustained arrhythmia. This can be augmented by the addition of noncontact mapping to the procedure. With the NavX system, any brand of catheter can be utilized. Respiratory motion has been radically reduced in recent software versions.
Some limitations exist. With individual interpolation schemes, significant anatomic distortions in complex structures can occur unless a family of fixed points is incorporated into the geometry to preserve critical junctions between those structures. Appropriate filtering has decreased this problem. The “flashlight” approach to identifying and displaying the specific location of a catheter at the geometry surface with the NavX system can be arduous, again requiring a meticulous rendering of anatomy in the first place, and oversizing of the anticipated lesion size to register the catheter tip position on the map surface. In this regard, the use of the ESI locator line is substantially easier for tracking catheter tip position.
RMP Mapping System
The RPM system remains in evolution. A reasonable geometry is established with point-to-point mapping, although the operator must be cautious in obtaining multiple roving and “snap” points to clearly establish the limits of chamber geometry. This is as seen at the accompanying intersection with other complex structures such as pulmonary veins in Figure 3. Ablative lesions on the surface of this geometry are readily catalogued, although deformation of the surface geometry or “learning” in the process of point-to-point ablation is possible, but not automatic and ablation locations can be lost to the internal side of the geometry. There is a transparency function that allows one to see through the walls and identify these sites, but the application of this utility proceeds along a steep learning curve. A downside of the RPM system is the requirement of specified 7 F catheters fitted with the ultrasound transducers. Substantial ongoing work is being done with each system, to overcome some of these specific limitations.
The Future of Cardiac Mapping
Automatic segmentation, registration, and the capability of fusing multimodality imaging technologies into a single coherent display will undoubtedly evolve over the next 5 years. It is not highly likely, but probable that the ablative interventions within this time frame will be based on these kinds of multimodality image fusion. With the advent of ultra rapid CT imaging, multiple image sets over the course of a single cardiac cycle will also be available for integration into the overall mapping system. When synchronized in both spatial and temporal domain, this approach will solve some of the vexing problems of image registration, and will also allow higher anatomic resolution down to the submillimeter level.
Systems of the future will also display a wide variety of physiologic parameters beyond activation times and voltage. Future systems will be able to display any parametric process, including vectors, strains, contraction patterns, or will even be based on the characteristics of an electrogram. Mapping capabilities, demonstrating fractionation of electrograms are already being developed to disclose dominant frequencies. Any other parametric display will be limited only by whether that parameter can be measured with a sampling system.
The next task will be for the development of clear capabilities for intervening within the context of these multiparameter images. Already, the Stereotaxis system is available for controlled positioning of a catheter as guided by an individual mapping system. The Hansen catheter manipulation system is being developed as well, as an alternative means of precise catheter manipulation within an established 3D geometric framework. Similar approaches using more traditional robotics systems will undoubtedly become commonplace. With fully integrated CT, ultrasound, and any other physiologic data set, it will be possible to plan an ablative session by marking the exact locations of the desired intervention on the surface anatomy. The system will then semiautomatically drive ablation or other intervention, according to those preset locations.
Limitations of Technology
The obvious downside to this technological explosion is cost. The question also arises as to whether or not this technological “tour de force” is necessary in an individual EP lab. Certainly, standard EP mapping systems have already been highly useful for facilitating the successful completion of a wide variety of ablative interventions. Whereas ablation in the early 1990s was limited to the atrioventricular (AV) nodal reentrant tachycardia, accessory pathways, and atrial flutter, the specific cataloging of both anatomy and physiology has contributed substantially to the expansion of ablation to atypical flutters, VT, congenital heart-disease-related atrial flutters, and atrial fibrillation. In this sense, the technology is already facilitating, if not enabling. Subsequent studies will be needed, however, to show that this translates into improved outcomes at a cost savings.
In this regard, it may be that these approaches will be the “great equalizer” for facilitating complicated studies that otherwise would require an insurmountably steep learning curve. Nevertheless, future generations of systems will have to be affordable within the context of prevailing reimbursement paradigms. This would be facilitated by multiple-use catheters, inexpensive higher resolution imaging and more automatic segmentation, and robust registration software, all designed to reduce procedure times and therefore increase the number of procedures that can be safely performed in a day. Undoubtedly, this will occur and will lead to even better, expedited care for patients with substantial arrhythmia burdens.
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