Electroanatomic mapping and magnetic guidance systems

Electroanatomic mapping

Theoretical considerations

During conventional electrophysiology (EP) procedures, catheters are manually navigated with the use of single or bi-plane fluoroscopy. An inherent limitation of fluoroscopic navigation is that orientation of the catheter relative to the cardiac anatomy can only be appreciated in two dimensions. Thus it may make complex procedures challenging and is associated with radiation exposure for both patient and physician. In 1996 Ben-Haim and Josephson published a report of a new technology referred to as nonfluoroscopic in vivo navigation and mapping ( Fig. 7.1 ). Navigation was achieved by a magnetic sensor inserted in an EP recording catheter and magnetic field radiators placed below the operating bed. This discovery led to an era of innovation with several three-dimensional (3D) electroanatomic mapping (EAM) systems being developed for clinical use in the EP laboratory.

A fundamental principle of 3D EAM is the ability to collect anatomic (location) and electrical (electrograms) data simultaneously. This means that as the catheter connected to the 3D EAM system is moved in a cardiac chamber, serial recording of location data allows replication of the catheter tip’s location in real time on a computer monitor. When location data are acquired at specific anatomic locations, a meshlike geometry can be created that replicates the chamber’s anatomy. As location data are being collected, the catheters also record electrograms. Acquired electrograms can be displayed on the 3D chamber anatomy at their collected location coordinates. Each electrogram can be annotated for local activation time (LAT) or amplitude (voltage) and the annotation displayed, following a color code, on the map. Therefore, during this mapping process, the operator creates a 3D electroanatomic map of the chamber of interest ( Fig. 7.2 A).

Impedance-based location measurements (or a combination with magnetic field–based technology) are also used by some 3D EAM systems. This is performed by emitting a small current from the catheter electrodes and sensor patches placed on the patient. The resulting current ratio recorded at each sensor, per electrode, allows calculation of location data and consequent catheter visualization and mapping.

EAM has rendered mapping and ablation of complex arrhythmias, such as postoperative right atrial flutters and ventricular tachycardia (VT), feasible when previously this was a challenge. ^, Randomized ablation trials using EAM in patients with supraventricular tachycardia have demonstrated similar acute procedural success rates and reduced fluoroscopy time. ^, EAM is also useful for activation and voltage mapping and for tagging locations of specific electrograms of interest that allow substrate-modification ablation approaches (see Chapter 16 ).

The three systems used in clinical practice for EAM are the CARTO mapping platform (currently in its CARTO 3 version 7), the EnSite Precision platform, and the Rhythmia HDx system, which has the capability to record and map from 64 electrodes simultaneously.

Activation, voltage, and propagation mapping

Electrograms are acquired at specific anatomic locations, as the catheter moves across the chamber of interest, and is assigned to a local activation timing. The local activation time (LAT) of each electrogram is then compared with an automatically selected but often user-modified stable fiducial point. This is a surface electrocardiography (ECG) lead, or an intracardiac electrogram recorded by a diagnostic catheter in a stable position within the heart. The comparison occurs within in a fixed period, the window of interest (WOI), encompassing only one reference signal. If, for example, sinus rhythm is being mapped in the right atrium, the reference could be a clear surface P wave (V1 is usually convenient), and the WOI would have to span a period before the P wave and a period after it. With present systems, regions of red color indicate sites of “early activation” and activation becomes progressively later proceeding through the colors of the rainbow to yellow-green and finally the blue and purple hues that define the sites of late activation relative to the reference point. These colors are displayed as a time bar adjacent to the 3D map. Thus, if the annotated LAT of an electrogram is “early” in the window compared with the reference signal (i.e., occurs before it), it will be assigned a red color to be displayed as on the 3D map. As signals get later compared with the reference, the color eventually will change to purple. If colors are missing, then, a portion of the window has not been mapped with EGMs, either because the window has been set up incorrectly, mapping has been insufficient, or activation encompasses more time than the window covers. A common example of this is a perimitral flutter with epicardial conduction over the coronary sinus (CS).

Two other concepts pertain to activation mapping: isochrones and propagation. Isochronal mapping is a type of LAT mapping whereby color coding is done based on groups of EGMs with the same LAT (isochrones) as opposed to depicting and annotating each EGM distinctly. The maps appear similar, but crucially the area of a color (i.e., how much tissue is activated simultaneously) is purported to correlate to conduction speed ( Fig. 7.2 C and D).

Voltage maps of the recorded electrograms can be created with colors representing the maximal bipolar/unipolar voltage amplitude. These maps provide a method by which to quantify cardiac scarring and have significant value in modern cardiac EP procedures ( Fig. 7.2 B).

Mapping during arrhythmia also allows the creation of an actual propagation map of the tachycardia. This is a “moving” version of LAT that displays spread of activation wave front throughout a cardiac cycle as a live wave front traversing the cardiac chamber. It allows visual appreciation of the mapped arrhythmia mechanism and a visual estimation of relative conduction velocity through various sites.

Practical points of 3D mapping

Several problems can influence accuracy and should be eliminated as much as possible during the mapping process. They include the inherent noise of the location system, the reproducibility of the fiducial point on the ECG, the reproducibility of cardiac mechanics on a beat-to-beat basis, and gating of image acquisition to the cardiac cycle and respiratory phase. Maps created using EAM systems are also subject to additional variability depending on accurate annotation of electrogram qualities, consistent catheter contact with tissue, distributed sampling of the entire structure of interest, density of location “points” in the map, type of rhythm being mapped, direction of activation wave front propagation, and the size and spacing of the electrodes used to acquire the data.