Introduction
Electromagnetic mapping has made possible the routine approach to complex arrhythmia mapping. Originally, it was developed to facilitate accessory pathway location, but it only complicated solitary pathway ablation. Especially, with the introduction of atrial fibrillation ablation, the electromagnetic mapping has become an irreplaceable component of this procedure. Latter, this 3-dimentional mapping method facilitated ventricular foci ablation in patients with ventricular tachycardia. During the last decade, the application of this mapping method is continuously extended and applied to a large number of complex arrhythmia cases. The electromagnetic mapping is based on minimal magnetic field generation around the tip of the catheter and several antennas, located in a special hardware, called location pad, detect this field. In the near future the location pad will be replaced by an intracardiac detection system. As multiple antennas record the signal, the location pad precisely localizes the catheter tip. By apposition of a location on the endocardial surface and the catheter tip, a virtual map of the cardiac chamber is obtained. Multiple points are represented in space and finally, a continuous wall is completed. Moreover, the catheter records the electrical activity in its vicinity, and the electrical flow can be superimposed on the location in the space. A color-coding localizes the starting point of the arrhythmia and the current flow from it. As of today, an advanced generation is used and data from other imaging systems are merged with the electromagnetic picture. First, magnetic resonance data was merged with the electroanatomic mapping, but today cardiac computed tomography and echo picture might also be merged. Using these merged pictures the anatomical presentation more accurately represents the anatomical structure.
A brief history of catheter ablation
The first type of energy used for catheter ablation to cure cardiac arrhythmias was the direct current (DC) shock delivered through the catheter tip in apposition to the desired site of ablation. By ablating the AV junction in supraventricular tachycardia, a reasonable rate control was achieved (Scheinman MM et al, 1982). The ablation of well-defined positioned accessory pathways could also be achieved by DC ablation (Morady F & Scheinman MM, 1984). The DC shock can be adjusted by delivering different amplitude of shocks, but as soon as it was delivered there was no more way to titrate or cancel it. To achieve a titratable energy, a different source is needed and the radiofrequency energy has fulfilled this condition. At the end of 1980′s the radiofrequency energy has been introduced into the clinical electrophysiology. With the advent of specific pathways ablation, the clinical electrophysiology has become a continuously extending brunch of the cardiology. The radiofrequency energy in opposite to the direct current shock, can be titrated, limited, suddenly discontinued and reapplied. For this reason, the RF energy is easily applied in clinical practice. The first accessory pathways ablations and AV Nodal pathways ablations were performed during the late 1980′s and published for the first time in 1991 (Jackman WM et al, 1991: Calkins H et al, 1991). Since than, a vast amount of literature was published on this subject and almost all type of arrhythmia has become feasible for ablation. In Table 2.1 a list of ablatable arrhythmic substrates is presented.
|
Arrhythmia Type |
Fist performed |
Publication |
|
WPW |
1989 |
Jackman WM et al 1991; Clakins H at al, 1991 |
|
ANRT |
1989 |
Lee MA at al, 1991 |
|
A Flutter |
1992 |
Feld GK et al, 1992; Cosio FG et al, 1993 |
|
A Tachycardia |
1992 |
Kall JG& Wilber DJ, 1992; Walsh EP et al, 1992 |
|
VT idiopathic |
1991-1992 |
Kuck KH et al, 1991; Klein LS et al, 1992 |
|
VT in Heart Disease |
1992 |
Morady F et al, 1993 |
|
A Fibrillation |
1996 |
Haissaguerre M et al, 1998 |
|
VF idiopathic |
2001 |
Takatsuki S et al, 2001, Haissaguerre M et al, 2002 |
Abbreviations: WPW-Wolf Parkinson White Syndrome; AVNRT- AV Nodal Reentrant Tachycardia; A Flutter-Atrial Flutter; A Tachycardia-Atrial Tachycardia; VT-Ventricular Tachycardia; A Fibrillation-Atrial fibrillation; VF- Ventricular Fibrillation
Table 2.1. The table summarizes the types of arrhythmia treated with catheter ablation.
In accessory pathway ablation, the presumed location is meticulously mapped with a special developed steerable tip catheter. The pathways are located on the mitral or tricuspid rings, or in the septum separating the left and right heart. When the recoding from this site is satisfactory, RF energy is delivered for certain duration. If the accessory pathway conducts electricity in both directions, or at least, in the antegrade direction (from the atrium to the ventricle), the resting ECG shows a typical preexcitation pattern. Short PR interval and a widening of the QRS characterize this preexcitation. A typical delta wave is preceding the QRS and its direction depends on the location of the pathway. Wolf, Parkinson and White described this type of ECG in 1930 and if it is associated with palpitation is called Wolff-Parkinson-White syndrome (Wolff L et al, 1930). During the RF energy delivery the preexcitation disappears and the ECG becomes normal. After the ablation, during temporary blockage of the AV node with intravenous adenosine injection, AB block is achieved, demonstrating the remaining conduction only through the AV junction. However, in 50% of the patients the resting ECG is normal, but still the patient can develop supraventricular tachycardia. In this case, the accessory pathway conducts the electricity only in the retrograde direction (from the ventricle to the atrium) and is called concealed Wolff-Parkinson-White syndrome. The mapping is completed either during tachycardia or ventricular pacing. RF energy application eliminates the retrograde conduction without affecting the resting ECG.
Patients with dual AV Nodal conduction may develop supraventricular tachycardia based on reentry circuit in the AV Node. One of the pathways has slow conduction of the electricity and the other has fast conduction. If the refractoriness of these pathways and the conduction times are appropriate, the electric impulse may be conducted in a circuit constituted by this two pathways and the clinical expression will be supraventricular tachycardia with short VA time. A premature beat during unidirectional block in one of the pathways initiates the tachycardia. Different duration of refractoriness of these pathways creates the unidirectional block. Blocking one of these pathways will prevent induction of the tachycardia. RF ablation is based on the same principle: selective elimination of the conduction on one of these pathways. In the late 1980′s the fast pathway was ablated (Lee MA et al, 1991). These fast pathways are located in the anterior site of the AV node very near to the proximal His Bundle. Although the success rate was very high, special attention was needed to avoid complete AV Block. After a year, the slow pathway ablation was described (Jackman WM et al, 1992). The slow pathway is located in the posterior AV nodal area, relatively far from the His Bundle. The ablation of these slow pathways is safer and today is routinely performed in most of the electrophysiology laboratories. After the ablation, the AV refractoriness prolongs and the AH interval (atrial-His conduction time) remains short immediately before the blocking.
The third classical ablation is that of typical atrial flutter (Feld GK et al, 1992, Cosio FG et al, 1993). In typical atrial flutter, a single electrical wavefront circulates in the perimeter of the right atrium. The area between the lower pole of the tricuspid valve and the inferior vena cava is called isthmus. A multipolar catheter on the lateral wall of the right atrium demonstrates the counter-clockwise or clockwise rotation during the flutter. Complete blocking of the isthmus, terminates the flutter. After the successful ablation, pacing at the coronary sinus osteum demonstrates the block, by conduction to the lateral wall only from above the tricuspid annulus.
In atrial tachycardia, idiopathic or peri-scar, and idiopathic ventricular tachycardia from right outflow tract or the basal left ventricle (fascicular), the atrial or the ventricular foci are ablated. The first three ablations, the accessory pathway (WPW) ablation, the selective or non selective AV Nodal pathways ablation and typical atrial flutter ablation are the classical ablations, which rarely if at all need other mapping methods, like the three-dimensional mapping methods. The atrial and ventricular tachycardia foci ablation is feasible in the majority of cases with "simple" electrophysiology mapping, like early activity, bracketing, entrainment, pace mapping, multi-pole catheter mapping, etc. However, occasionally, this mapping is not sufficient and three-dimensional mapping is needed (Marchlinski FE et al, 1998; Leonelli FM et al, 2001; Peichl P et al, 2003; Dong J et al, 2005). As we can notice, in the classical ablations, both the diagnostic test and the ablation procedure are focused on electrophysiology parameters. In atrial and ventricular tachycardia, although there are well defined electrophysiology maneuvers as previously mentioned, the more exact location of the focus and the demonstration of the ablation results demands the advanced electroanatomic mapping.
The development of the electroanatomic mapping and the electroanatomic mapping guided ablation
In the early 1990′s, Ben Haim and his colleagues developed a new method of three-dimensional mapping of a tubular or spherical organ. First it was presented in the NASPE meeting in 1996 (Smeets J et al, 1996; Ben-Haim SA et al, 1996a; Hayam G et al, 1996; Gepstein L et al, 1996), than published as a new method evaluated in animal model and human subjects (Ben-Haim SA et al, 1996b; Gepstein L et al, 1997, Shpun S et al, 1997; Gepstein L et al, 1998). In this method, the locator pads, located beneath the operating table, generate a week magnetic field. The magnetic field strength is between 5×10-6 and 5×10-5 tesla. A miniature sensor embedded in the body of the distal catheter collects these magnetic fields. As the fields vectors have different directions, computer integration of the data resolves the location and orientation of the sensor in space. During the same time, the distal electrodes, located on the catheter tip, record the electrical activity behind it. By collecting multiple points, the heart chamber anatomy and the local electrical impulse in amplitude and timing are superimposed. With higher number of points added, higher precision is achieved. The resolution of the sensor location in space is < 1 mm. Finally, the electrical field propagation is presented on the virtual reconstruction of the heart chamber anatomy. If the amplitude is expressed, scar anatomy is obtained based on very low electrical activity on the scar. Moreover, a transitional region with increasing amplitude will interpose between the scar and normal tissue. In this type of mapping, invagination of conducting tissue in the scar, gaps and marginal zones may present the substrate of arrhythmias. Early in the development of the electroanatomic mapping, the scar mapping contributed to the understanding of the arrhythmia substrate and made possible non-specific scar ablation (Stevenson WG et al, 1998a; Stevenson WG et al, 1998b; Marchlinski FE et al, 2000; Sra J et al, 2001). However, the main development of the electroanatomic mapping happened with the use in atrial fibrillation ablation. After discovery of the triggers in the pulmonary veins, focal ablation was suggested, but the application of this ablation is limited and associated with an increased incidence of complication (Haissaguerre M et al, 1998). The use of electro-anatomic guided ablation offered ablation lines in the antrum of the veins and in this way the ablation is far from the vein itself and also significant debalking is added (Pappone C et al, 1999; Pappone C et al, 2000; Oral H et al, 2003). The ablation was extended to persistent atrial fibrillation and even to long-standing persistent cases (Nademanee K et al, 2004; Oral H et al, 2006; Takahashi Y et al, 2007). This addition contributed to development of this mapping. Additional mapping methods were developed and promoted in parallel with the original electromagnetic mapping, the CARTO mapping. Finally, the list of ablatable substrates has continuously increased and today includes beside atrial fibrillation and ventricular tachycardia in patients with or without structural heart disease, also atrial tachycardia, recurrent and refractory atrial flutter, idiopathic ventricular fibrillation (Knecht S et al, 2009) and more recently, the substrate of Brugada syndrome (Nademanee K et al, 2011). The following sections will describe in details the use of the mapping in each of these indications.
Right atrial tachycardia
The mechanism of right atrial tachycardia may be an autonomic focus or reentry around a physical obstacle, most commonly an iatrogenic scar or a post-infectious scar.
Right atrial tachycardia-reentrant type
For mapping the scar, bipolar voltage mapping is needed. Figure 4.1 shows two views obtained during atrial tachycardia ablation in a 32 year old man, who had a remote atrial septal closure surgery. During the surgery the lateral wall in the right atrium was opened and the septal defect was closed. On a recent echo, no interatrial shunt could be seen demonstrating lack of atrial septal defect and late success of the surgery. However the patient developed incessant and persistent atrial tachycardia. During the electrophysiology study, the right atrium was mapped using bipolar voltage mapping. This technique allows determination of the scar, as on the scar the electrical activity is practically non-existent. We defined everything bellow 0.04 V as scar and assigned with red color. The normal tissue is defined everything above 0.47V and assigned with purple. The tissue in between them is the transition zone. As evident from the picture, the two red areas were connected with a continuous ablation line obtained with a point-by-point ablation.
Fig. 4.1. The picture shows the voltage mapping of the right atrium with two scars on the lateral wall, a large (Scar A) and a small one (Scar B). The small one (Scar B) is connected to the Inferior Vena Cava. A small strip on normal myocardium separates the scars and this strip was transected with the ablation line. Annotations: AP- antero-posterior (the left picture); RL- Right lateral (the right picture). For further explanation see the text.
Fig. 4.2. Schematic presentation of picture 4.1; the picture shows the two scars, a large and a small one. The IVC ring limits the small one. To interrupt to reentry circle the ablation line connects the two scars.
During two-year follow-up the patients is free of any arrhythmia. This case demonstrates the of electro-anatomic mapping used for scar dependent atrial tachycardia ablation.
Right atrial tachycardia-automatic focus type
The next case will exemplify the ablation of automatic paroxysmal atrial tachycardia. The patient is a 17-year-old male patient with a history of palpitation and documented supraventricular tachycardia. Electrophysiology study demonstrated supraventricular tachycardia with long VA time and the ventricular rate was dictated by the atrial rate. Occasionally 2:1 AV conduction and even Wenkebach conduction was documented. The final diagnosis was right atrial tachycardia originating in the inferior third of the crista terminalis. Ablation was focused on the earliest atrial activity using a duodecapolar catheter. The tachycardia terminated during the RF application and has become noninducible. However, after 2 weeks the patient again experienced palpitation and again a similar arrhythmia was documented. The patient was referred for a second ablation this time CARTO guided. The tachycardia was again easily induced with atrial overdrive pacing, but not with atrial premature beats. Figure 4.3 shows the CARTO mapping.
Fig. 4.3. Right Atria Tachycardia with the origin at the Inferior Vena Cava (IVC) area and the activation terminating at the Superior Vena Cava (SVC) area. This is a propagation map and the colors define the timing at each area. The red is the earliest time (-88 msec) and the purple the latest time (+23 msec) compared to the reference catheter. The white line with the arrow shows the activation direction. The red points in the origin area show the Radiofrequency application sites, which terminated the tachycardia rendering it non-inducible.
In this case, voltage mapping cannot be used as no scar could be mapped and the patient had no any previous cardiac procedure except the ablation procedure. For this reason, a propagation mapping was used. During this mapping the virtual reconstruction of the right atrium is completed parallel with accurate timing detection by comparing the activation time with the timing on a reference catheter, in this case in the coronary sinus osteum. The timing of the points located with the mapping catheter is compared with the reference catheter and is assigned with red color if it is early and purple if it is late. In between are the other colors like yellow, green and blue. Finally, a cine presentation shows the flow of the electrical activity, starting at the red point and terminating at the blue points. The red points are the area focused for ablation. As evident in Figure 4.3, the origin is well determined and ablation at this point again terminated the tachycardia leaving it noninducible. During the follow-up the patient is asymptomatic, practicing non-professional sportive activity.
These two cases exemplify not only two types of right atrial tachycardia, but also two methods of mapping. If a scar is to be mapped, the amplitude of the electrical activity will discriminate it from the normal tissue with clear demarcation. The reentry can be completed around a scar and by connecting the scar to a non-conducting tissue will prevent the reentry. This map is called, voltage-map, or scar mapping. In contrary, if the tachycardia is generated by an automatic focus, the earliest activation will reveal the origin of the tachycardia and the ablation may be focused to it. This is achieved by determining the timing at each point and the map is called, flow-map.
These two cases are presenting the classical types of atrial tachycardias. However, occasionally the diagnosis is not so simple and the first impression may be misleading. The electro anatomic mapping may elucidate the diagnosis and make possible the correct ablation. In the next subsection such a complex case is presented.
