The development of surgical techniques for the treatment of patients with cardiac arrhythmias was paralleled by the development of intraoperative electrophysiologic mapping systems to determine precisely where to place the lesions of conduction block in a surgical procedure (i.e., to “guide” the surgical procedure). Specific surgical procedures to treat patients with Wolff-Parkinson-White syndrome, elective His bundle ablation, automatic left atrial tachycardias, atrioventricular node reentry tachycardia, automatic right atrial tachycardias, ischemic ventricular tachcycardia, and nonischemic ventricular tachycardia were all “map-guided” surgical procedures.
Because of this long and successful experience with map-guided arrhythmia surgery, when it was decided to develop a surgical procedure to treat atrial fibrillation (AF) in the 1980s, it was assumed that whatever surgical procedure we might develop would also have to be map guided. However, extensive experimental and clinical mapping of AF confirmed that it would be necessary to interrupt multiple macro-reentrant circuits that were fleeting in nature and constantly changing locations. Thus it was obvious that AF surgery could not be map-guided. Efforts were refocused on the development of a surgical procedure based on anatomy that could be performed the same way in all patients with AF. This new approach culminated in the Maze procedure. Although intraoperative mapping was not used to guide the placement of lesions in the Maze procedure, we routinely mapped all patients who had AF surgery with a 156-channel computerized mapping system intraoperatively both before and after the Maze procedure. However, after a few dozen patients, it was apparent that the mapping was unnecessary, so we stopped performing intraoperative mapping. Even today, successful surgery for AF does not require intraoperative mapping.
The only two interventional procedures for the treatment of AF that have withstood the test of time over the past 40 years are pulmonary vein isolation (PVI) for paroxysmal AF and the Maze procedure for all types of AF. Both of these procedures are based on atrial anatomy, and neither of them requires electrophysiologic mapping to be successful. Nevertheless, sophisticated and expensive mapping systems are routinely used in all catheter ablation procedures for AF, though they are used primarily as a tool to confirm lesion integrity, not as a tool to guide the placement of catheter lesions in the atrium. Thus, the catheter ablation of AF is not actually “map-guided” because the procedures are all based on isolation of the pulmonary veins (PVI), which does not depend on AF mapping. In addition, the term “catheter ablation” itself is a misnomer because the objective of a PVI is not to ablate the AF but to isolate as many of its triggers as possible in hopes of decreasing the likelihood of having AF induced in the future.
Some interventional electrophysiologists do actually map AF during some catheter ablation procedures to locate and ablate extrapulmonary vein electrical and/or anatomic phenomena such as complex fragmented atrial electrogram spots, “rotors,” and ganglionic plexi. Unfortunately, the identification and ablation of these abnormalities have not significantly improved the results of catheter ablation for AF. Surgeons who treat AF also often use what is rather euphemistically called a “mapping system,” but again, this system is used primarily to confirm lesion integrity by simple pacing techniques, not to guide the placement of lesions in the surgical procedure.
When computerized mapping was first developed, reconstruction of activation maps was unusually time-consuming and laborious. In addition, data could be recorded only from the specific sites where electrodes were located. This was a particular problem during the mapping of ventricular tachycardia (VT) because it often originated in the mid-myocardium of the ventricular septum. Therefore, with only endocardial and epicardial electrode arrays in place, activation mapping could not identify the true earliest activity (“site of origin”) of such septal VT. As a result, “potential distribution mapping” (now called “voltage mapping”) of the amplitude of unipolar or bipolar deflections was developed in hopes of determining the site of VT origination more accurately. In addition, when three-dimensional (3D) mapping became available, 3D voltage maps of arrhythmias proved to be far easier to construct than were 3D activation maps. Few developments in all of cardiology compare with the monumental progress made in the development of arrhythmia mapping since that time.
Even though most surgeons are unfamiliar with preoperative or intraoperative mapping systems and techniques, it is important for them to understand some of the fundamentals of electrophysiologic mapping to optimally treat their patients with arrhythmia. A good starting point is to understand the difference between activation mapping and voltage mapping .
Activation Mapping
Electrophysiologic maps based on the sequence of electrical activation of the heart are called “activation time maps,” “sequential maps,” “isochronous maps,” or simply “activation maps.” Activation maps record dynamic electrical activity, that is, the generation of an electrical impulse and its subsequent propagation as a wavefront ( Fig. 6.1 ). Thus, activation maps are uniquely helpful in delineating the length and position of a line of conduction block. Even though a lesion is contiguous (has no “gaps”) and is uniformly transmural, if it is positioned incorrectly, electrical activity will simply be rerouted around the lesion ( Fig. 6.2 ). If the objective of an operation dependent on such a lesion is to isolate a portion of the atrium, say, the pulmonary veins, the procedure would fail. However, the ability to place such lesions in critical areas of the atrium is the basis of the Maze procedure because this is how one controls the direction of wavefront propagation postoperatively so that a sinus rhythm can activate both atria.
Activation map. The small red circle represents the site of origin of an electrical impulse. The location of that impulse is designated every 20 ms as it propagates radially from the site of origin. These are isochronous lines (i.e., the tissue along each isochronous line is all activated at the same time in relation to the origin of the impulse). The tissue incorporated between successive 20-ms isochronous lines is color coded to show the sequential radial spread of the impulse away from its site of origin.
Effect of an incomplete line of conduction block as seen on an activation map. Note that the line of conduction block (heavy blue line) has no “gaps,” indicating that it is contiguous and uniformly transmural. This line of conduction block is not anchored on nonconductive tissue, so the wavefront is able to curl around both ends of the lesion. This disruption in the pattern of sequential tissue activation recorded on an activation map precisely identifies the location, shape, and length of the line of conduction block.
To attain complete isolation of a portion of the atrium with a lesion ( Fig. 6.3 ), it must be anchored to nonconducting tissue such as a valve annulus or the orifice of a cardiac structure like the superior vena cava. Surgical isolation can also be accomplished by enclosing tissue entirely within the lesion itself, as is done with the “box lesion” in the Maze procedure that encircles all four pulmonary vein orifices and the intervening posterior left atrial wall. In either case, an activation map defines the precise length and location of lesions of conduction block and also of areas of the atrium that are completely isolated.
Effect of a complete line of conduction block that is anchored at both ends to nonconductive tissue. Because the external extent of the tissue in this sketch is also nonconductive, the area beyond the line of conduction block is not activated and is therefore isolated from the rest of the atrium.
Voltage Mapping
Unlike activation maps in which isochronous lines are drawn at selected intervals of wavefront propagation, voltage maps are based on the amplitude of the local unipolar or bipolar complexes in which isopotential lines are drawn at selected amplitude intervals ( Fig. 6.4 ). Even though a lesion is contiguous (has no “gaps”) and is uniformly transmural, if it is positioned incorrectly, a voltage map can be quite misleading ( Fig. 6.5 ). Because atrial activation continues beyond the incomplete atrial lesion, there are still local electrograms in these regions, and they have amplitudes that can be measured. The presence of local electrograms on both sides of the incomplete lesion means that there is no way to identify the precise location, shape, or length of that lesion. In fact, a previously placed contiguous, uniformly transmural atrial lesion may be entirely invisible on a voltage map if it is incomplete and does not isolate a portion of the atrium. Thus, it is important to recognize whether one is looking at an activation map or a voltage map, and it is especially important if one is trying to determine the integrity of a previously placed lesion of conduction block. Fortunately, when lesions of conduction block are placed in the correct position to completely isolate a region of the atrium, a voltage map is as valuable as an activation map ( Fig. 6.6 ).
Voltage maps represent the difference in local electrogram amplitudes as an impulse propagates away from its site of origin (red circle). Propagation of the wavefront away from its site of origin is illustrated in this example by placing lines of the same voltage with every change of 1-mV amplitude in the electrograms. Thus these are isopotential lines (i.e., the tissue along each line all has the same local electrogram amplitude). The tissue incorporated between successive 1-mV isopotential lines is color coded and shows radial spreading of the impulse away from its site of origin.
A line of conduction block is placed in the same position on this voltage map as it was in the activation map shown in Fig. 6.2 . In this case, however, electrical activation of the tissue beyond the line of block results in the ability to measure the amplitudes of persistent local electrograms on both sides of the block. This makes it virtually impossible to detect the location, shape, or length of an incomplete line of conduction block with voltage mapping.
Voltage map of a linear line of conduction block that is anchored to nonconductive tissue. This demonstrates that voltage maps are equally as effective as activation maps in documenting areas that are completely isolated.
Comparison of Activation and Voltage Maps
The fundamental difference between an activation map and a voltage map is analogous to the difference between a terrain map and topographical map. For example, a terrain map of an active volcano would show the volcano and the flow of lava from its peak. If a large structure were blocking the lava from flowing down the mountainside in a certain area, the lava would be forced to skirt around the edges of the wall to continue its flow to the bottom of the mountain ( Fig. 6.7 , left upper panel ). On the contrary, a topographical map of that same active volcano would be unaffected by the presence of the large structure on the mountainside ( Fig. 6.7 , right upper panel ).
Comparison of activation maps and voltage maps when mapping incomplete lines of conduction block and completely isolating lesions. The most critical difference is that voltage maps are inadequate for determining the precise location of incomplete lesions. The differences are essentially the same as the differences in terrain maps and topographical maps. Left upper panel, A terrain map of an active volcano with lava (orange) flowing down the mountainside. The flow of the lava is diverted by the presence of a large wall on the mountainside, and because of that diversion, we know precisely where the wall is located. Right upper panel, A topographic map is unaffected by the presence the large wall on the mountainside and therefore is of no value in determining the location or completeness of the wall. Left middle panel, An activation map shows the precise location of a lesion and identifies any gaps in the lesion. Right middle panel, A voltage map is incapable of accurately determining the location of an incomplete lesion or where the gaps in such a lesion are located. Lower panels, The ability of activation maps and voltage maps to identify completely isolated areas are equivalent.
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