42 Catheter ablation of VT in patients with structural heart disease based on conventional mapping techniques (during VT) is limited to patients with stable, tolerated VT. For this reason, considerable research efforts have been devoted to the localization of the specific areas of the VT substrate in the absence of an ongoing VT. Such an approach represents significant progress since it permits catheter ablative therapies in patients with noninducible or poorly tolerated VT. The theoretical base of this approach is as follows: VT in patients with structural heart disease has a reentrant mechanism that requires areas of slow conduction and arcs of block; areas of slow conduction and (maybe) arcs of block can be recognized by local EGM characteristics and pacing maneuvers in the absence of an ongoing VT; ablation of such areas may result in VT suppression. The studies showing the feasibility of this methodology can be divided into 3 phases according to the tools that were used and the outcome data analyzed (Table 42.1) and started in the 1980s, when a surgical approach to VT therapy was developed. Chronological phase 1980s Late 1990s, initial 2000 2000s Mapping type Activation during VT Activation + entrainment Activation + entrainment or NO VT mapping VT type Mappable Mappable Unmappable Tools EA mapping Fluoroscopy Fluoroscopy Fluoroscopy + Ablation No Yes Yes Outcome variable Compare SR with VT Compare SR with VT Clinical VT recurrence EA, electroanatomical; SR, sinus rhythm. In the first phase,1–3 it was found that abnormal bipolar EGMs with low amplitude and long duration could be recorded in patients with VT and structural heart disease, but not patients with normal hearts. When the site of origin of VT, as detected during activation mapping, was studied during sinus rhythm, it typically displayed abnormal EGMs. However, abnormal EGMs were also found at many other sites unrelated to VT. Certain EGM characteristics (fractionated, delayed) were more closely associated to the VT site of origin but lacked sensitivity for its detection. The second phase, adding pacing techniques and response to RF ablation to improve VT localization,4–6 described the so-called isolated potential during sinus rhythm (isoelectric line between 2 components) as a marker for the VT area of slow conduction, but again lacking sensitivity. With the hope that this lack of sensitivity could be overcome by a more detailed mapping with the aid of electroanatomic mapping tools, in recent years several groups have explored an ablation strategy based on targeting areas with abnormal EGMs suggestive of slow conduction7,8 (generally referred to as LPs), or suggestive of boundaries between conductive and nonconductive tissue (arcs of block, channels, represented by unexcitable tissue or voltage gradients).9–12 It should be mentioned that in all these studies pace mapping is added as a tool to select which EGMs are more likely to be related to VT. In general, results of these approaches have been encouraging although there is no controlled comparison with ablation based on conventional mapping during VT. In addition, it has been shown that there are important differences in the substrate of patients with ischemic and NICM, associated with a different prevalence and location of LPs.13–15 In all patients the preprocedure evaluation includes a detailed history and physical examination. It is always important to obtain and analyze 12-lead electrocardiograms (ECGs) during VT if they are available, since consideration of induced VT as clinical VT is based preferentially on the electrocardiogram. If this is not available or in addition to it, VTs documented in the stored EGMs of the ICD are also important references. In fact, with the increasing use of ICD for primary prevention, stored ICD EGMs tend to be the only documentation of clinical VT, and we pay attention not only to CL but also to EGM morphology. All the patients are evaluated with a 2-dimensional (2D) echocardiogram to quantify the left ventricular ejection fraction and to rule out the presence of a left ventricular thrombus. Other studies include exercise testing or coronary angiography. In some patients, a cardiac magnetic resonance imaging is of interest before the EP study for quantification of left ventricular volumes, function, and scar tissue. This image can be integrated with the electroanatomical map during the EP study, reducing the fluoroscopic and procedure time. In patients with elective ablation the antiarrhythmic drugs are discontinued several days prior to the scheduled procedure. After written, informed consent is obtained, the EP study is performed in the postabsorptive state, normally under conscious sedation or general anesthesia. A total of 2 or 3 5-Fr quadripolar catheters are introduced into the right femoral vein and positioned at the RA, HB area, and RVA. Having an atrial catheter is important to help distinguish between late ventricular potentials and atrial signals when the ablation catheter gets to annular positions, particularly during pacing. EGMs are filtered at 30 to 500 Hz and displayed simultaneously with 4 to 6 ECG leads and recorded and stored digitally (LabSystem™ PRO EP Recording System, Boston Scientific, Marlborough, MA). Our standard access to the left ventricle is via retrograde across the aortic valve, but in patients with significant atherosclerosis of the aorta or peripheral arteries, or in older people, we tend to increasingly choose the antegrade transseptal approach ( When the left heart is instrumented, anticoagulation with intravenous heparin is maintained throughout the procedure, maintaining the activated clotting time over 250 to 300 seconds. We tend to perform a full endocardial procedure as first-line study, and consider an epicardial approach only if the endocardial procedure fails, except in cases of previous failure at another institution, and also considering the underlying structural heart disease. Epicardial access is achieved introducing the ablation catheter through an 8-Fr sheath into the pericardial space, using a subxiphoid approach, before the initiation of anticoagulation. Special attention is made to delimitate the course of the coronary arteries by coronary angiography or merged CT images (Figure 42.2). Electroanatomical mapping has become the standard for any type of substrate mapping. A detailed 3-dimensional (3D) voltage map of the ventricle is created using CARTO (Biosense Webster, Diamond Bar, CA) or NavX System (Ensite, St. Jude Medical, St. Paul, MN). We prefer to use catheters with small electrodes to obtain a more local signal (3.5- or 4-mm-tip electrodes rather than 8-mm-tip electrodes). With CARTO, the bipolar EGMs are obtained normally using a 3.5-mm-tip open-irrigated ablation catheter (Navistar THERMOCOOL, 2-5-2 mm interelectrode spacing), with a fill threshold of 20 mm to ensure the representation of the entire endocardial surface of the ventricle. With the Ensite NavX 3D mapping system (St. Jude Medical), any catheter can be used. Irrigated-tip ablation catheters tend to produce noisy signals in some laboratories, and in such cases, the use of a different catheter for mapping than for ablation can be considered. Mapping is made using bipolar mode (filter 10–400 Hz) trying to identify areas of isolated delayed potentials, but voltage mapping is simultaneously performed. The peak-to-peak signal amplitude of the bipolar EGM is measured automatically to the largest component, and if an activation map is performed, we reannotate the local activation time to the EGM onset. The color display on the voltage map is set to a color range of 0.5 to 1.5 mV to distinguish the limits of the scar. Signals with an amplitude higher than 1.5 mV represent normal tissue, whereas the abnormal endocardium is defined as “dense scar” when the EGMs have amplitudes between 0.1 and 0.5 mV, and “complete scar” when the EGM voltage amplitude is lower than 0.1 mV. For the epicardial maps, the normal tissue voltage limit is changed to 1 mV, maintaining the limit of dense scar at < 0.5 mV. Endocardial EGMs are recorded with the intention of fully defining the border zones and the scar region(s), trying to obtain a distance of < 1 cm between the mapped sites. A detailed mapping of the whole ventricular shape is required even if one scar area is detected because there may be more than one. Our emphasis is in the identification of sites with abnormal signals, especially LPs that reveal local delayed activity in small isolated myocardial bundles within the scar. Isolated delayed potentials are defined as EGMs with some delayed low-voltage components separated from the early and usually larger components by an isoelectric segment of 20 to 50 ms (Figures 42.1 and 42.2). In general the delayed components are recorded after the QRS offset, but in our opinion this is not critical. Once a site with LPs is identified it is important to map adjacent sites in as detailed a manner as possible. We believe LPs are most significant when they are recorded over an area, not just a single point, although the area may be small. Some authors have differentiated between “moderate late potentials” and “very late potentials” if the onset of the EGMs after the QRS is shorter than 100 ms or longer, respectively,10 and they consider more significant the latter. Figure 42.1 Endocardial electroanatomical map of the left ventricle from a patient with previous anterior myocardial infarction. Panel A: The anteroposterior view shows the apical dense scar; the pink tags correspond with LPs recorded within the scar mapping during sinus rhythm, and the gray tags represent areas with voltage lower than 0.1 mV. Panel B: An LP with an isoelectric segment of 70 ms between the ventricular EGM and the LP (red arrow) is shown. The amplitude of the LP is 0.10 mV, and the interval between the end of QRS and the LP is 20 ms. Panel C: Another LP was recorded in the same area 78 ms after the end of the QRS (yellow arrow), with an amplitude of 0.12 mV. Two different ventricular tachycardias (VTs) were induced. After the ablation (see lesions in
How to Map and Ablate Ventricular Tachycardia Using Delayed Potential in Sinus Rhythm
Eduardo Castellanos, MD, PhD; Jesús Almendral, MD, PhD; Carlos De Diego, MD
Introduction
Preprocedural Planning
Procedure
Electrophysiologic Study
Videos 42.1 and 42.2). We use deflectable sheaths for LV access through a transseptal approach since they help in catheter control.
Mapping Protocol
Video 42.6) VTs were not inducible. M1-M2 distal electrode pair of the ablation catheter.
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