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Advisor high-definition HD Grid catheter for mapping accessory pathways in pediatrics

New technology that allows for bipolar orthogonal electrograms can be safely used in the pediatric population. Introduction Intracardiac mapping of accessory pathways has continued to evolve and contribute to high success rates for catheter ablations in pediatrics, ranging from 78% to 98%. With 3-dimensional electroanatomic mapping, increasing point density improves the accuracy of substrate prediction and modeling, particularly as the course of atrioventricular (AV) accessory pathways can be variable, with oblique atrial and ventricular insertion sites or multiple pathways that make accurate mapping challenging. The Advisor high-density HD Grid catheter (Abbott Laboratories, Abbott Park, IL) allows for bipolar recordings on 2 orthogonal planes among 18 electrodes to facilitate both the identification of smallamplitude high-frequency electrical impulses and the direction of impulse propagation. Although several adult studies have shown the utility of the HD Grid for mapping arrhythmia substrate including accessory pathways, to our knowledge there are no studies showing the utilization of the HD Grid for mapping accessory pathways in pediatrics. In this case series, we describe our initial experience utilizing the HD Grid for mapping accessory pathways in the pediatric population.


Introduction
Intracardiac mapping of accessory pathways has continued to evolve and contribute to high success rates for catheter ablations in pediatrics, ranging from 78% to 98%. 1-3 With 3-dimensional electroanatomic mapping, increasing point density improves the accuracy of substrate prediction and modeling, particularly as the course of atrioventricular (AV) accessory pathways can be variable, with oblique atrial and ventricular insertion sites or multiple pathways that make accurate mapping challenging. 4 The Advisor high-density HD Grid catheter (Abbott Laboratories, Abbott Park, IL) allows for bipolar recordings on 2 orthogonal planes among 18 electrodes to facilitate both the identification of smallamplitude high-frequency electrical impulses and the direction of impulse propagation. Although several adult studies have shown the utility of the HD Grid for mapping arrhythmia substrate [5][6][7][8] including accessory pathways, 9 to our knowledge there are no studies showing the utilization of the HD Grid for mapping accessory pathways in pediatrics. In this case series, we describe our initial experience utilizing the HD Grid for mapping accessory pathways in the pediatric population.

Advisor HD Grid catheter design
The Advisor HD Grid is a high-definition mapping catheter that has 18 electrodes and allows for assessment of voltage, activation, and directionality of conduction. This unique design consists of 4 parallel splines equally spaced 3 mm apart, each with four 1-mm electrodes that are spaced 3 mm apart (4 ! 4 electrode configuration), with 2 additional electrodes on the shaft. Figure 1 incorporates a depiction of the HD Grid catheter. When compared to conventional linear catheters, the HD Grid is able to utilize its electrodes in orthogonal orientations along and across adjacent splines to collect multiple bipolar electrograms (EGMs) at any given time. EnSite Precision (Abbott Laboratories) then compares the orthogonal bipolar EGMs in a single location by factoring in local activation timing, peak-to-peak voltage, and other algorithms to display the best point at that site. We utilized a high-density open-window mapping technique, where all local atrial, pathway, and ventricular signals are recorded. The absolute dV/dt detection algorithm identifies and annotates the highest absolute slope on the bipolar EGM, which directly corresponds to the largest local timing difference between the positive and negative electrode unipolar EGM. 10 The system calculates this sharpest, near-field location by calculating the derivative of the bipolar EGM for the entire mapping window, then annotates the point in that window with the highest positive or negative slope. The near-field annotation does not specifically differentiate between atrial and ventricular EGMs in a single point, but instead the mapping technique utilizes thousands of points to map the conduction across the pathway and guide ablation strategy. This technique differs from the conventional mapping approach, which maps the earliest activation site only in the chamber activated by the pathway and relies on subjective

KEY TEACHING POINTS
Pediatric accessory pathways are common and historically have had a high cure rate with ablation. There are new techniques and technologies being developed to potentially increase efficacy of ablation with improved mapping.
There may be a role in utilizing open-window mapping and the HD Grid catheter (Abbott Laboratories, Abbott Park, IL) to delineate difficult or refractory accessory pathways in pediatrics.
New technology that allows for bipolar orthogonal electrograms can be safely used in the pediatric population.
analysis for local timing when annotating fused annular signals.
Despite its larger mapping surface, the HD Grid is atraumatic and positioned with the 3D electroanatomical mapping system without the use of fluoroscopy. Of note, it is our institution's practice to anticoagulate when the HD Grid is deployed for both right-and left-sided procedures.
Mapping is conducted in pre-excited sinus rhythm, coronary sinus atrial paced rhythm, orthodromic AV reentrant tachycardia, or ventricular paced rhythm, as determined by the arrhythmia substrate and the discretion of the pediatric electrophysiologists. After mapping was performed with the HD Grid, limited micromovements of the ablation catheter allowed for confirmation of the pathway location and lesion(s) were subsequently placed.

Case report
From January to March 2020, 5 patients had invasive electrophysiology studies for accessory pathways utilizing the HD Grid. All patients had normal heart structures, with the exception of 1 who had a small atrial septal defect. Pathway location and characteristics are presented in Table 1. Table 2 reports the specifics of the HD Grid maps for each patient. Of note, although the patients in this study were in the pediatric age range, they were all near adult sized (83, 48, 89, 45, and 80 kg).

Patient 1
A 16-year-old female patient (83 kg) was referred after 3 prior unsuccessful ablations for a para-Hisian accessory pathway with bidirectional properties. The HD Grid catheter was utilized to create 2 maps: 1 in pre-excited sinus rhythm and 1 in sustained orthodromic AV reentrant tachycardia. Both maps identified the same location for the accessory pathway, as seen in Figure 1. EGMs are displayed in Supplemental Figure S1. With the guidance of the maps created by the HD Grid, the first lesion was placed with a 6-mm-tip Freezor Xtra (Medtronic, Minneapolis, MN) cryoablation catheter in the para-Hisian position, slightly superior to the His bundle position. Of note, signals from the ablation catheter were low amplitude, suggesting possible scarring from prior ablation lesions. The accessory pathway disappeared as the ice ball formed, which was 20 seconds from cryoablation initiation.

Patients 2 and 3
Patients 2 and 3 are presented together, as both had HD Grid mapping in pre-excited sinus rhythm (Figure 2A and B).
Patient 2 was a 13-year-old female patient (48 kg) who had a prior successful ablation for 2 right-sided accessory pathways (right lateral and right anterior septal). She was referred because of recurrence of the right lateral pathway. This pathway conducted antegrade only and was able to sustain antidromic AV reentrant tachycardia. A TactiCath SE (Abbott Laboratories) radiofrequency irrigated catheter was used, and the first lesion placed resulted in prompt loss of the accessory pathway at 1.5 seconds.
Patient 3 was an 11-year-old female patient (89 kg) who had a para-Hisian pathway with bidirectional properties and a small secundum atrial septal defect. Mapping revealed the pathway location to be slightly anterior to the His bundle. Utilizing a 6-mm-tip Freezor Xtra cryoablation catheter, the first lesion was placed more superiorly than the map suggested, at the decision of the operator, and did not affect pathway conduction. The catheter was then moved to the location of the pathway as directed by the map. The time from cryoablation on to ice ball formation was 14 seconds and the accessory pathway disappeared 3.5 seconds after ice ball formation.

Patient 4
A 12-year-old female patient (45 kg) had an antegrade-only right posterior septal pathway. This pathway was mapped with the HD Grid in pre-excited sinus rhythm and coronary sinus atrial paced rhythm, as seen in Supplemental Figure S2. When the ablation catheter was advanced to the right AV groove and placed near but not on the site of the pathway, as guided by the HD Grid map, the ablation signal was assessed to be a good target for ablation. Initially, 2 lesions were placed with a Blazer II (Boston Scientific, Marlborough, MA) radiofrequency catheter and were unsuccessful in terminating the pathway. The ablation catheter was readjusted and brought to the precise site of the pathway in the inferior septal position, as guided by the HD Grid map. Ablation at this site resulted in accessory pathway block at 2.5 seconds after radiofrequency initiation.

Patient 5
A 14-year-old female patient (80 kg) had been referred after an unsuccessful ablation for a retrograde para-Hisian pathway. Four separate HD Grid maps were created: (1) right ventricular apical pacing, (2) right ventricular mid-septal pacing, (3) orthodromic AV reentrant tachycardia (cycle length 300 ms), and (4) orthodromic AV reentrant tachycardia  (320 ms). All 4 maps showed that the pathway consistently coursed just below the His bundle, as seen in Figure 3. Using a Freezor Xtra catheter, cryoablation lesions placed in this region repeatedly resulted in transient A-H prolongation and 2:1 AV node block. Mapping and ablation from the noncoronary aortic cusp was not performed secondary to family preference. Owing to risk of permanent AV nodal damage, no further attempts were made.

Discussion
To our knowledge, this is the first study demonstrating utilization of the Advisor HD Grid in pediatric patients for intracardiac electroanatomic mapping of accessory pathways. This initial series shows that the HD Grid can be used safely and effectively in this patient population. There were no complications from the use of the HD Grid, including vascular access or anticoagulation complications. Of note, for these right-sided procedures, only the 3D EnSite system was utilized to track catheter movement. No fluoroscopy was necessary. Further studies are needed to delineate the utility of the HD Grid in all pathway locations.