Introduction

The development of intracardiac mapping was based on the use of individual contact catheters. Despite the ability to record information from different sites with multi-polar catheters, complex arrhythmias and arrhythmias that occur in complex substrates can be difficult to characterize with these techniques. A system that could provide information from a whole cardiac chamber or chambers was required.

Following the development of epicardial mapping techniques used during surgery, basket catheters were developed for endocardial use. These catheters are no longer in frequent use. Later, electroanatomic mapping systems were developed that could record electrophysiological data and also measure the three-dimensional location of the recording electrode. Chamber geometries could be created from points recorded along the endocardial surface. The latest systems are significantly more accurate and also offer the electrophysiologist many more features to help facilitate the mapping of complex arrhythmias. Voltage mapping, propagation maps, rapid mapping with multi-polar catheters, and mapping of fractionated electrograms are all now possible, and datasets can be merged with preoperative computed tomography (CT) or magnetic resonance imaging (MRI) scans of the heart or rotational angiograms. Different approaches to complex mapping are noncontact mapping and the very new “ripple mapping.”

Electroanatomic Mapping Systems

Electroanatomic mapping systems build a three-dimensional image of a cardiac chamber by incorporating sequential electrogram data from an entire cardiac chamber or chambers if necessary. An electrophysiological catheter in contact with the endocardium or the epicardium records voltage (amplitude of the electrical signal) and timing (relation of the recorded electrogram to a fixed-reference electrode electrogram). Low-voltage areas and scar can be easily identified, which makes it possible to elucidate the overall underlying substrate through which an arrhythmia persists. Recording electrogram timings throughout the chamber enables an activation map to be created, which is color coded to differentiate between “early” and “late” signals with respect to the reference electrode (Figure 92-1). These data can also be viewed in the form of a propagation map, in which a representation of the excitatory wavefront of activation advances across the chamber geometry (see Video 92-1 on the Expert Consult site for this text). Maps can be rotated on the monitor or viewed in multiple orientations simultaneously so the wavefront can be followed throughout the cardiac cycle. Electroanatomic mapping systems are not particularly suitable for activation mapping during unstable rhythms, although mapping in discrete areas can be performed during separate tachycardia episodes, thereby building a map over a longer period. Data acquired during sinus rhythm can also be used to target ablation, which is particularly useful when mapping ventricular tachycardia (VT) in abnormal hearts. This method obviates the need for mapping during tachycardia by defining the arrhythmogenic substrate (Figure 92-2).

FIGURE 92-1 Right atrial geometry collected on CARTO XP (Biosense Webster), with local activation map displaying color-coded timings of each activation point. This demonstrates a re-entrant tachycardia mechanism because the early and late signals are adjacent to each other. The tachycardia displayed is a clockwise typical right atrial flutter.

FIGURE 92-2 CARTO XP (Biosense Webster) geometry of the left ventricle displaying a map of ventricular voltage, with areas of healthy myocardium in pink and colored areas depicting areas of scar.

Once generated, the combination of voltage and activation maps can be very useful in guiding the electrophysiologist in placing focal ablation lesions or creating lines of ablation. This may be between important scar boundaries (e.g., lines of ablation transecting the diastolic pathway in VT circuits) or between inert structures (e.g., linear ablation between the tricuspid valve annulus and the inferior vena cava in isthmus-dependent atrial flutter [AFL]). In addition, these systems facilitate the encirclement of cardiac structures, the best example of which is the encirclement of pulmonary veins as part of the treatment for ablation of atrial fibrillation (AF). Maps do not have to be viewed exclusively from the outside. Sagittal, coronal, or transverse sections, or any other user-defined plane, can be mapped so the operator can view the chamber from the inside (Figure 92-3).

FIGURE 92-3 CARTO Merge (Biosense Webster) image with a sagittal clipping plane to allow visualization of the antrum of left-sided pulmonary veins.

Over the past 10 years, there has been an escalation in the use of three-dimensional mapping systems, particularly for the ablation of AF. The advent of electroanatomic mapping has enabled the deployment of complex linear lesions within the left and right atria—most notably the roof line between left and right superior pulmonary veins and the mitral isthmus line, between the left inferior pulmonary vein and the mitral valve annulus. Although these lines are often performed by using conventional techniques alone, three-dimensional mapping systems truly allowed electrophysiologists to think in three dimensions within the left atrium.

Another particular advantage of these systems is that they are nonfluoroscopic; that is, x-ray radiation is not required to visualize catheters, which can be positioned and moved within a cardiac chamber or chambers solely by using the electroanatomic system. At present, electrodes of each catheter and their positions can be visualized. In the future, however, incorporation of electrodes into the shaft of catheters and long sheaths would offer a more complete view of the equipment within in the heart and would assist electrophysiologists still further in catheter positioning without the use of x-ray radiation.

The CARTO System

The CARTO system (Biosense Webster, Diamond Bar, CA) works on the principle that a conducting coil placed in a changing magnetic field will generate an electrical current. In the XP version of this system, an attachment is fixed to the operating table. This generates magnetic fields from three different locations that can be distinguished because they are generated with different frequencies. A purpose-built catheter that has three small coils implanted in the tip at different orientations is used. The magnetic fields induce currents in these sensing coils, which are measured to calculate the distance from the source of each magnetic field. The position of the catheter tip is then calculated by trilateration, and the orientation of the catheter tip is also estimated. The position information from the catheter is gated with the electrocardiogram to reduce the effects of cardiac motion. In addition, a reference patch is attached to the patient’s back, which can be used to detect and compensate for horizontal movement of the patient (but not rotation or rolling). Navigation is accurate to a resolution of 1 mm, and maps can be enlarged and reduced to aid catheter placement.

The latest iteration of this technology is the CARTO 3 system. In addition to the use of magnetic location technology, this system also uses six additional patches on the patient’s skin. Integration of information from current measurements between the catheters and these patches with information from the magnetic location technology has a number of advantages. Multiple catheters can be tracked, and the system is able to compensate better for patient movement (Figure 92-4). Maps can be generated rapidly by collecting data from all the electrodes of a circular mapping catheter simultaneously. It is also possible to alter the degree of interpolation between points, thereby reducing the potential to create “false space” within geometries and reproduce the true endocardial surface more accurately. Increased mapping technology accuracy reduces the requirement for fluoroscopy and also for contrast angiography, which has been the gold standard for delineation of endocardial shape and is used in many centers prior to geometry creation. From initial experience, CARTO 3 may reach this degree of accuracy and may mitigate the need for x-ray exposure even more than other available systems. In addition, the system produces higher quality recordings of the electrograms, removing the problems from 50 Hz noise that were present on earlier systems.